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The Embryology Anatomy and Histology of the Eye
 
 
BY EARL J. BROWN, M. D.
Professor of Histology of the Eye, at the Chicago Eye, Ear, Nose and Throat College
 
 
WITH ILLUSTRATIONS MADE FROM TRANSVERSE SECTIONS OF THE HUMAN EYE ENLARGED BY MICRO-PHOTOGRAPHY
 
 
 
 
CHICAGO:  Hazlitt Sc Walker, Publishers.  1906
 
 
Copyrighted  1905  BY HAZLITT & WALKER
 
 
 
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FOREWORD.
 
 
In approaching this work, perhaps a word of explanation to the reader may be desirable. It is not undertaken because the author thinks there is a lack of knowledge about the eye; neither have there been any new facts discovered which would merit the production of these articles. It is therefore not the intention to bring out any new facts, but to put the known and widely scattered facts in a more comprehensible form and to illustrate the subject so thoroughly and completely that it will be made more easy for the beginner and more interesting to those who find it necessary to review the subject.
 
All the illustrations of the structures of the eyeball and smaller structures will be microphotographs taken from microscopic slides in the author's possession, while the coarser structures of the orbit will be illustrated by drawings, as these structures are too large for the tissues to be mounted on microscopic slides.
The microscopic slides used to photograph the foetal eye are from the pig and were procured at the Armour packing house by collecting the foetal pigs at the gutting table. These foetesis ran from two millimeters to forty millimeters in length, and the mounting of the slides was done by Dr. Slonaker, at the Chicago University.
 
The slides used in photographing the adult eye were made by Dr. Slonaker when he wrote his thesis on the acute area of vision. These slides have been used in my illustrated lectures before optical and medical societies for several years, and they have been enjoyed so much by my hearers and I have received so many requests for them in a i)ermanent form, that it is in response to these wishes that the author lias determined to perpetuate these pictures and place them in the reach of every one who is interested in the eye ; otherwise these articles would never have appeared.
A few words about the physical development of the foetus might be of benefit before the illustrations are studied. The foetus is first represented by one cell, the ovum. This is fertilized by the spermatozoa; then there is a multiplication of cells. These increase very rapidly, and the first definite form assumed is a tube, representing the worm, and this tube has two walls ; one is the outer covering and the other lines the inside. The outer is known as the cpiblast (meaning above) and the inner the hypoblast (meaning below). Then there is a layer developed between these two layers. This layer is known as the mesablast (meaning the middle). From the epiblast is developed the skin and nervous system. From the hypoblast is developed the alimentary canal and all the internal organs which communicate with the alimentary canal. From the middle layer, or mesoblast, is developed the connective tissue, blood vessels, muscles, bones, etc.
 
From the foregoing we see that in the study of the eye we are most especially concerned in the epiblast, as it forms the nervous system and therefore the brain, and the inner seat or sensory coat of the eye, and some one has well said that the eye is a part of the brain placed near the surface, back of an opening, where it may receive impressions from the external world and communicate these impressions to the main portion of the brain.
 
The first indication of the nervous system commences by the development of two ridges along the dorsum, or back, of the foetus during its tubular development. These are known as the neural ridge$. The cells composing these ridges multiply and they rise higher and higher and finally meet al)Ovc, at the center, and coalesce, or grow together, leaving an opening. Tliis is knowm as the neural
 
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FOREWORD. 7
tube, and the whole nervous system is developed from the cells which line this tube.
Soon after the neural tube is formed, the anterior half of the foetus folds on itself, and this portion forms the brain, while the posterior, or unfolded portion, forms the spinal cord. From the anterior portion of the brain, two tubes grow out and toward the surface. These are known as the optic stalks, and they form the first step in the development of the eye.
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CHAPTER I. EMBRYOLOGY.
It might be well to explain that the major part of the embryology of the eye has been worked out from the eye of the chick and rabbit, as it is almost impossible to get fresh material in human embryos. The writer conceived the idea of going to a large packing house, where hundreds of pregnant sows were gutted every day and material could be obtained fresh and in all the stages of development. This was suggested to Dr. J. Rollin Slonaker of Chicago University, and he, acting on the suggestion, procured the material and prepared the microscopic slides from which the following illustrations were made.
 
 
 
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Fig. 1. Horizontal Section through Head of Foetal Pig, 2 mm. long. Magnified 3,000 times.
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The first manifestation of the development of the eye is a hollow protrusion from that part of the neural tube which forms the anterior cerebral vesicle. A vesicle is an enclosed cavity, between two layers of tissue and filled with fluid, like a water blister on the hand. The neural tube, as explained before, is a tube developed along the dorsum or back of the foetus, during the tubular stage of development, and the whole nervous system is developed from the cells lining this tube. This hollow protrusion is known as the primary optic stalk. (See A, Fig. i.)
 
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Fig. 2. Horizontal Section through Head of Unhatchhjd Chick, 2 ihtn. long. Magnified 3,000 times.
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As this stalk grows outward, the anterior portion rises upward, as shown in vertical section at A, Fig. 3. When the optic stalk comes near to the surface, the anterior portion enlarges, as shown at C, Fig. i. Also when the optic stalk encroaches on the surface, it stimulates the epithelial cells forming- the skin and they multiply rapidly (see B, Figs. I and 3), and the anterior wall of the primary optic
 
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Fig. 3. Vertical Section through Head of Foetal Pig, 2 mm. long. Magnified 2,500 times.
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Fig. 4. Horizontal Section through Head of Foetal Pig, 3 mm. long. Magnified about 1,200 times.
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vesicle invaginates and passes inside of the vesicle. (See A, Fig. 2, and C, Fig. 3.) This invagination might be likened to the denting of a hollow rubber ball.
This invaginated portion forms the secondary optic vesicle and it is from this that the nine innermost layers of the retina are eventually formed, while the primary optic vesicle only forms the outer or pigment layer. As the secondary
 
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Fig. 5. Vertical Section thr()u.i^li Head of.Foetal Pi^, 3 mm. long. Magnified about 2,000 times. +++++++++++++++++++++++++++++++++++++
 
Optic vesicle is passing into the primary optic vesicle there appears in front of the secondary optic vesicle a depression on the surface at the point of activity of the epithelial cells. (See A, Fig. 4, and A, Fig. 5.) This depression becomes deeper and deeper and the mouth is finally closed by the rapid formation of cells around the depression, as shown at B and C, Figs. 4 and 5.
 
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Fig. 6.
 
 
Vertical Section through Head of Foetal Pig, 4 mm. long. Magnified about 1,000 times-: +++++++++++++++++++++++++++++++++++++
 
 
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Fig. 7. Horizontal Section through Head of Foetal Pig, 7 mm. long. Magnified about 600 times. +++++++++++++++++++++++++++++++++++++
 
Til US a vesicle is formed which is known as the lens vesicle, as shown at A, Fig. 6. Then this vesicle hecomes separated from the surface and passes into the secondary optic vesicle (see A, Fig. 7), and eventually forms the lens, which will be described later.
 
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Fig. 8. Vertical section through head of pig, 8 mm. long. Magnified
460 times." +++++++++++++++++++++++++++++++++++++
 
As the lens vesicle passes into the secondary optic vesicle, some of the mesoblastic cells pass upward from below, behind it (see B, Fig. 6), and it is these mesoblastic cells which will eventually multiply and form the vitreous body. It will be remembered that the mesoblast is the middle layer of the three primary layers, first formed in the foetus. The surface of the skin from which the lens vesicle was cut away, remains and forms the cornea and some students of the embryology of the eye believe that the cornea owes its transparency to the changes that take place in the nature of the cpitheHal cells during the formation of the lens vesicle from this immediate point. The lids are formed by an external fold, growing downward from above and upward from below the eyeball and the first indication of this growth is shown at B, Fig. 7.
 
 
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Fig. 9. Horizontal section through head of pig, 9
460 times.
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The further development of these folds is shown at A Figs. 8 and 9, also, there is a groove running from the inner side of the eye to the nasal cleft of the foetus and the edges of this groove come together and cover in the cells at the bottom of this groove and a cord is formed from the nasal cleft to the palpebral fissure. (The palpebral fissure is the opening between the lids.) This cord divides and one branch goes to the upper lid and the other to the lower. Later there is a tube formed from this cord and this tube so formed is the lachrymal or tear duct, which runs from the palpebral fissure to the nose, and it is through this duct that the tears are pumped from the conjunctival sack to the nose. This process will be explained later.
 
In Figs. 7 and 8 the lens vesicle will be seen to have been entirely separated from the surface, and at B, Fig. 8, is seen the epithelial cells which will form the outer layer of the cornea, and at C, Fig. 8, is seen the mesoblastic cells which will multiply and eventually form the four innermost layers of the cornea, the iris and other structures anterior to the lens.
 
At D, Fig. 8, is seen the commencement of the formation of the lens substance. This formation is accomplished by the cells of the posterior portion of the lens vesicle wall elongating and forming long spindle cells. These grow forward and fill the whole cavity of the lens vesicle, as shown at D, Fig. 9, and these extend from the anterior to the posterior limits of the cavity and are known as the lens fibers. At E, Fig. 8, is seen the opening at the posterior pole of the eye ball, where the axis cylinder processes make their escape from the eye ball, as shown at E, Fig. 9, to pass into the optic nerve as they grow from the retina toward the brain. This opening through which the optic fibers leave the eye ball is known as the choroidal fissure in the adult eye.
 
At F, Figs. 8 and 9, are shown the mesoblastic cells which have passed into the space between the retina and the lens. As shown at B, Fig. 6, they are just commencing to form the vitreous body, and this cavity so filled is known as the vitreous cavity in the adult eye. At G, Figs. 8 and 9, the primary optic vesicle is shown, which has become quite thin, and in the cells forming it there is being deposited pigment granules, and it will be remembered that this eventually forms the outer or pigment layer of the retina. At H, Figs. 8 and 9, will be seen the first indication of the formation of the nine innermost layers of the retina, and these nine layers are all formed from the walls of the secondary optic vesicle. There is a folding over of the optic stalk and optic vesicles, which is well illustrated by the accompanying diagrammatic drawing, Fig. 10.
 
A represents the primary optic vesicle; B, tlic secondary optic vesicle; C, the walls of the primary optic stalk, and D, the groove below the optic stalk. The lower edges of
 
 
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Fig. 10. +++++++++++++++++++++++++++++++++++++
 
the optic stalk come together at E. and coalesce, thus forming a tube with a double wall which extends from the eye ball to the cranial cavity. This joining together takes place clear forward, along the lower part of the primary and secondary optic vesicles to F. Thus an eyeball is formed and the fissure closed is known as the choroidal fissure of the foetus. However, at the posterior of the eyeball there is an opening left, through which the axis cylinder processes leave the eyeball, E, Figs. 8 and 9. This opening is known as the choroidal fissure in the adult and corresponds to the optic disc as seen with the ophthalmoscope. It is this folding over of the embryonic structures of the eye which makes it possible for the incorporation of the arteria centralis retina (central artery of the retina) and its accompanying vein within the optic nerve for some distance back of the eye, in the adult, as this artery was already developed in the groove below the optic stalk. H, Fig. 10, represents the lens vesicle within the secondary optic vesicle. Fig. 11 represents a vertical cross section of the primary and secondary optic vesicles at about the line marked G, in Fig. 10, and A in Fig. 11 shows the primary optic vesicle wall.
B, Fig. II, shows the secondary optic vesicle wall and C shows the choroidal fissure at the bottom of the foetal eye. D, Fig. II, is the vitreous cavity. F, Fig. ii, is the lens vesicle cut through, and the two edges which come together and close the choroidal fissure are shown at E. When this union fails to take place we have an anomaly known as
 
 
 
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Fig. 11.
colobom^a of the fundus in the adult and means a lack of development.
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At A, Fig. 12, will be seen the further development of the lids; B, Fig. 12, the anterior epithehal layer of the cornea, and just beneath it is seen a lighter colored Hne. This is the anterior homogeneous (structureless) layer of the cornea, also known as Bowman's membrane, as he was the first to describe it. C, Fig. 12, shows the lamina propria (proper layer) of the cornea. D, Figs. 12 and 13, shows the lens fibers extending from the front to the back of the lens. These fibers are simply long spindle cells and each one has a nucleus. These form a crescent-shaped line of dots, as seen at K, Figs. 12 and 13, running from one side of the lens to the other. At J, Figs. 12 and 13, is seen the transitional (transformation) zone, and it is at this point that the lens fibers are formed, and this formation is simply the multiplication of the columnar epithelial cells, which first formed the wall of the lens vescicle and their
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Fig. 12. Horizontal sectioti through ej'e of a pig. Magnified 730 tinres.
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Fig. 13. Human embryo eye, 2 months. Magnified 1,080 times.
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eluiii4atioii into spindle cells. These spindle cells are known as the lens fibers. These fibers are especially well illustrated at D, Fig. 15, and anterior to this transitional zone where the lens fibers are formed in the adult eye will be found a single layer of the columnar epithelial cells. Underneath the capsule L, Figs. 12 and 13, and J, Fig. 16,
 
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Fig. 14. Eye of embryo pig, 10 mm. long. Magnified 1,600 times.
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while posterior to the transitional zone, no such cells will be found, they having elongated to form the lens fibers, as shown at D, Figs. 8 to 15.
After the lens vesicle is completely filled by fibers, extending from the front to the back of the lens in an anteriorposterior direction, as shown by the lens in Figs. 9 to 13, there is a continuation of growth of the lens by the multiplication and elongation of the columnar cells at the transitional zone, J, Figs. 12, 13 and 14. These grow forward and backward toward the anterior and posterior poles of the lens, around the ends of the first formed fibers, and these latterly developed fibers form the soft outer or cortical portion of the lens, B, Fig. 15. While the first formed fibers constitute the nuclear or central denser portion of
 
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Fig. 15. Highly uiaguitied deaiii from the anterior of a liumaii euibrj'o,
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the lens, C, Fig. 14, these last formed fibers grow in such a way that when their ends come into apposition at the front and back of the lens, there are formed seams, as shown at A, Figs. 14 and 15. A, Fig. 14, is at the posterior surface of an embryo pig, and A, Fig. 15, is a more highly magnified seam from the anterior of a human embryo. These seams have a star or stilate shape, the central part being at the anterior and posterior poles and the points run outward toward the equator of the lens, and it is these seams, where the ends of the fibers in the cortical portion of the lens abut against each other, that forms the so-called lens stars. These fibers of the lens are long, diamond-shaped, spindle cells, and these are arranged in lamella or layers and all held together by a matrix of jellatinous cement substance, and when a lens is macerated (soaked) in an alkaline solution, which will dissolve this cement substance, these lamella of the lens may be peeled ofif, and the best illustration is the peeling of the layers of an onion. At E, Fig. 12, is seen the commencement of the growth of the third eye lid, known as the membrana nictatans (winking membrane) in the lower animals, especially birds. This develops in man up to a certain stage, then ceases and remains as a vestige in a crescent-shaped fold near the inner side of the eye, and is called the plica semilunaris (half moon fold). At F, Fig. 12, is seen the developing vitreous body and the dark spots are the small blood vessels which furnish this body its nutrition during development. These are from the hyaloid artery, which will be described later, and these atrophy before birth. At G, Fig. 12, it will be seen that the brim or fornix at the anterior margin of the primary and secondary optic vesicles are in apposition to the capsule of the lens at its equator, and this enables some of the connective tissue, which binds the retina together, which is known as the fibers of Mueller (he being the first to discover them) to become attached to the capsule of the lens, and as the eye enlarges and the retina settles farther backward, these attached fibers elongate and thus the suspensory ligament (also known as the zonule of Zinn) is formed, and this accounts for this connection between the retina and the lens.
At H, Fig. 12, will be seen the farther development of the layers of the retina. At I, Fig. 12, are seen some cells, which are showing signs of activity. This is the first sign of the development of the choroid and sclerotic coats.
At A, Fig. 16, it will be seen that the lids are gradually covering the cornea and the membrana nictatans. E, Fig. 16, is not any farther developed than seen in Fig. 12 at E.
At B, Fig. 1 6, will be seen a portion of the hyaloid artery. This is an artery given off by the arteria centralis retina at the head of the optic nerve and only exists during foetal life lor it atrophies before birth. It supplies the nutrition necessary for the development of the vitreous body and the lens and when these are fully matured it atrophies and the canal through which it passed remains as a lymph channel and is known as the hyaloid canal, or the canal of Stilling, in the adult eye. The hyaloid artery, as before stated, is a branch of the arteria centralis retina and runs from the head of the optic nerve to the posterior of the lens, giving off small twigs to the developing vitreous body. At the posterior surface of the lens it breaks up into several branches. These pass around the lens to the front and there come together, forming anastomoses (an anastomosis is where one vessel runs into another and continues by continuity of tissue), and the connective tissue which these vessels are imbedded in forms the pupilary membrane, which will be described later. There is a connection of these hyaloid arteries by anastomosing vessels from the front of the iris, near its free margin, with the branches of the blood vessels of the iris. This connection only exists during foetal life.
 
 
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Fig. 16. Horiaontal section through head of pig, 20 inni. long. Magnified
670 times.
 
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Fig 17. Horizontal section through head of a pig, 25- mm. long. Magnified 85 times.
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Fig. 18. Horizontal section through head of pig, 40 mm. long. Magnified
225 times.
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At F, Fig. 16, is shown a band of tissue connecting the lens with the retina. This will eventually form the suspensory ligament or the Zonule of Zinn of the older writers. At H, Fig. 16, is shown the farther development of the cells which will eventually form the choroid and sclerotic and it will be noted that they may be traced well into the optic nerve (C, Fig. i6), at either side at the choroidal fissure,
and it is these cells which will form the lamina cribrosa (seive layer), which strengthens the eye ball at this point in the adult eye. Also C, Fig. i6, shows the first growth of the axis cylinder processes through the choroidal fissure to form the optic nerve.
 
 
 
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Fig. 19. Vertical section through head of pig, 40 mm. long. Magnified 320 times.
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Fig. 20. Horizontal section through eye of a pig, 50 nified 150 times.
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It must be remembered that the fibers which transmit impulses of the sight from the retina to the brain, grow from the cells in the ganglionic layer of the retina, back toward the brain and not from the brain to the retina. At I, Fig. i6, is shown the activity of the cells, which are just commencing to form the recti (straight) or extrinsic muscles of the eye. At K, Fig. i6, will be seen a line of small openings. This is the commencement of the space of Tenon. Fig. 17 is a horizontal section through the head of a pig, 25 M. M. long, and is shown to illustrate the rapid development of the eyes in the growth from 9 to 25 M. M. in length. A, Fig. 17, shows the nasal cavities. C, Fig. 17, shows the developing brain, and D shows the developing bone. At E is seen the farther development of the extrinsic muscles, at F is shown the farther development of the choroid and sclerotic and at G is seen the plica semilunaris and at H the lids.
 
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Fig. 21. Vertical section through human foetal eye at five months Magnified 120 times.
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At A, Figs. 18 and 19, are shown the lids entirely covering the front of the eye ball and just back of it the cornea B, and the space between the two, C, is the conjunctival sack.
 
The conjunctiva lines the inner surface of the lids, then folds on itself as shown at D. This is known as the Fornix (arch) conjunctiva. Then it covers all the front exposed portion of the eye ball except the cornea. The epithelial layer of the conjunctiva, continues over the cornea and forms the outermost or stratified epithelial layer of this structure, E, Figs. i8 and 19. That portion of the conjunctiva lining the lids is known as the palpebral conjunctiva, F. Figs. 18 and 19, and that portion covering the exposed scleral portion of the eye ball is known as the ocular conjunctiva, as shown at G. At H, Fig. 18, is shown the plica semilunaris, and it will be noted that it is gradually becoming smaller. At I, Figs. 18 and 19, is shown the choroid, which is just forming; at J, is shown the sclerotic and at K, Figs. 18 and 19, is shown the farther development of the extrinsic muscles.
 
When the twt* lids come into apposition in front of the eye ball they become cemented together, as shown at h. Fig. 19. In all animals in which the retina is completely developed before birth the lids are separated at birth, but in those animals whose retina is not fully developed at birth, such as the kitten and puppy, the lids do not separate for some days after birth, or until the retina is sufficiently developed so as to withstand the effects of light.
 
At A, Fig. 20, is shown the conjunctival sack, at B the shrinking plica semilunaris and at C the tendon of the external rectus muscle and its attachment to the eye ball in front and the belly of the muscle posteriorly. At D, Fig. 20, is shown the sheath of the optic nerve and the farther development of the nerve itself, at E the vitreous body and at F it will be seen that the retina is farther developed and about four layers may be made out.
 
At A, Fig. 21, is shown the developing fibers of the orbicularis (circular) palpebrarum muscle, at B is shown the margins of the lids and the developing cilia (hairs) or eye lashes, and at C is shown a developing hair in the lid. D, Fig. 21, shows the commencement jf the development of the ciliary body and the iris. These are the last structures to be developed within the eye ball. E, Fig. 21, shows the cut end of the inferior oblique muscle, F shows the lens fully developed, also the fibers. G, Fig. 21, shows the vitreous body and H shows the retina practically as is found in the adult eve.
 
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Fig. 22. Vertical section through eye of pig, 110 mm. long. Magnified
480 times.
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A, Fig. 22, shows the pupilary membrane stretching across the pupilary space, and in it may be seen Httle white areas. These are the branches of the hyaloid artery which furnishes the nutrition to the lens during its development, and it will be remembered that this artery atrophies before birth and that the pupilary membrane disappears, ostensibly being absorbed. At B, Fig. 22, is shown the iris growing out from the ciliary bodies. C and D shows the cornea and in it is shown the lacuna (small lakes), which are minute openings between the layers of the lamina propria (proper layer), and E shows the lid with its developing structures. F, Fig. 22, shows the conjunctival sack and G shows the ocular conjunctiva and just back of it the anterior portion of Tenon's space. H, Fig. 22, shows the levator palpebra superioris (the lifter of the upper lid). I shows the lids held together by the cement substance and J shows the vitreous body (glass-like body).
 
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CHAPTER II. ANATOMY.
 
Having hitrriedl}' described the development of the eye ball, we will Jiow go over the adult eye, giving the gross and leaving the minute anatomy until we have advanced farther with the subject. The adult eye ball is 24.5 mm. across, 24. mm. from front to back, 23.5 from top to bottom, weighs a fraction less than one-quarter ounce and is composed of the segments of two spheres ; the anterior portion, or the cornea, A, Fig. 23 (meaning hor^ilike), being the segment of a much smaller sphere than the posterior or scleral portion, the cornea comprising one-sixth of the outer surface, while the sclerotic (hard or tough), shown at B, makes up the other five-sixths. The cornea is transparent and thus forms the window through which the light is admitted to the eye ball and this transparency allows us to see the iris (rainbow), E, the structure lying directly behind the cornea. The iris is a circular structure pierced at the center by the opening known as the pupil. It contains two muscles, the one surrounding the pupil, which is a narrow band of circular fibers known as the sphincter pupillae muscle (meaning the bijider muscle), K. This muscle closes the pupil, to protect the delicate tissues at the back of the eyeball from bright or intense light, then the dilator pupillae muscle, the fibers of which extend from the base of the iris to the sphincter pupillae. This muscle enlarges the pupil when more light is required to form a denser picture on the retina. The lens, F, lies just back of the pupil but can only be seen after it has lost its transparency. Continuing backward from the base of the iris, will be seen the ciliary body, I, and between this structure and the sclerotic is found the ciliary muscle, H. In front of the ciliary muscle and at the base of the iris, is seen the pectinate ligament (comblike ligament), Q and J. This is made up of many small bundles of connective tissue, running from the periphery of the cornea to the base of the iris, across the angle formed by the junction of the cornea and the iris. This angle is known as the filtration angle, for the aqueous fluid, which fills the anterior and posterior chambers, leaves the eyeball, at this point. It passes into the spaces of fontana (fountain spaces), the spaces of fontana simply being the space between the bundles of fibers forming the pectinate ligament, and from these spaces the aqueous fluid, or nutrient lymph, as it is sometimes called, passes through the tissues to the canal of Schlemm, which is seen in Fig. 23 in the cornea just outside of the spaces of fontana. The canal of Schlemm is a circular channel within the corneal tissue, extending clear around the periphery of the cornea and the fluids pass from the canal of Schlemm to the anterior ciliary veins. Extending backward from the ciliary bodies and continuous with them, are the ciliary processes. These end near the ora serratta (saw tooth mouth), X, of the retina. Running from the ora serratta forward to the lens, imbedded in the outer layer of the hyaloid membrane and bound down firmly to the inner surface of the ciliary processes and bodies is the suspensory ligament or Zonule (belt) of Zinn, as Dr. Zinn first described it, G. The ligament proper is made up of very elastic fibers, which, as before stated, are imbedded in the outer layer of the hyaloid membrane. The hyaloid membra^ie surrounds the vitreous body and these fibers, the writer believes, to be elongated fibers of Mueller, which became attached to the lens during foetal life when the fornix (arch) of the primary and secondary optic vesicles were in apposition (touching) to the equator of the lens and as the
 
 
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Fig. 23. Cross section of the Eye, showing its construction.
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globe enlarged they elongated. See Figs. 7 to 20. This ligament leaves the ciliary bodies and passes across the space between them and the le,ns, a part of the fibers passing a little anterior of the equator and the rest a little posterior to the equator of the lens and are attached to the capsule of the lens. The triangular space formed by this separation of the suspensory ligament fibers is known as the Canal of Petit, shown at R. The lens, F, is a transparent body and occupies the space just back of the iris and between the circle of inward projecting ciliary bodies. It is round, and flattened from before backward, its anterior and posterior surfaces being convex, the posterior surface having the shorter radius of curvature. It lies in a depression m the anterior surface of the vitreous body. This depression is known as the fossae Patellaris (dishlike depression) and is supported by this and the suspensory ligament. The lens is surrounded by a dense transparent membrane known as the capsule. The space in front of the ciliary bodies, suspensory ligament and lens, and back of the iris, is known as the posterior chamber, T, and the space in front of the iris and lens at the pupilary space and behind the cornea, is known as the anterior chamber, S.
The sclerotic coat (tough coat), B, continues backward from the cornea by continuity (continuation of tissue by blending one into another) of tissue over the posterior five-sixths of the eyeball to the optic nerve, where it divides, the inner portion forming the lamina cribrosa (sieve layer), M, whilst the outer portion passes into the sheath of the optic nerve Y. It is pierced by the ciliary arteries P; by nerves which enter the eyeball in a circle surrounding the nerve; by the vena vortacosa, four or five of which leave the eyeball just back of the equator; and by the anterior ciliary arteries and veins which enter the eyeball at the attachments of the extrinsic muscles, just back of the cornea. The region where the cornea ends and the sclerotic begins is known as the limbus (seam), W, and the angle or depression formed by the difference in the radius of curvature of the two spheres, represented in the formation of the eyeball in the corneal and scleral portion Z, is known as the sclero corneal sulsus (furrow). This angle makes the eyeball stronger and more firm at this point and it is just inside,
Opposite lliis angle that the cihary muscle, II, is attached anteriorly, whilst posteriorly the longitudinal fibers are attached to the outer surface of the choroid, in the region of the ciliary processes and bodies, as this muscle is interposed between the sclerotic and choroid in this region. The ciliar)' muscle, H, is made up of two sets of muscular fibers, the longitudinal nuining antero-posteriorly which are placed farthest out, next to the sclerotic, and the circular fibers which lie farthest inward, just outside of the ciliary bodies. These last named fibers take a circular course ajid form a band of circular fibers extending entirely around the ciliary ring.
 
Just inside of the ciliary muscle and sclerotic is found a very vascular pigmented layer, C, knowii as the choroid (meaning membrane). This is loosely attached to the sclerotic by the exchange of bundles of tissue called trabeculae and this space so formed is known as the supra choroidal space. The choroid is the middle tunic, or coat, of the three grand tunics of the eyeball. It is extremely vascular and it is analogous to the pia mater of the brain. The choroid, ciliary processes, ciliary bodies and the iris constitute what is known as the uveal coat (grape skin coat) , and the three combined line all the scleral portion and compose the iris or curtain in front of the lens. Posteriorly the choroid is pierced by the optic nerve and this opening is known as the choroidal fissure (choroidal opening). As before stated, the posterior ciliary arteries ajid nerves pass through the sclerotic to reach the choroid. Here the short, posterior ciliary arteries, P, from twelve to twenty in number, divide, one branch running toward the optic nerve ; the others run anteriorly and begin to subdivide as they run forward supplying the choroid, and some branch to the sclerotic. Two of the interjial branches may be seen near the optic nerve in Fig. 23, the final destination of the anterior branches being the ciliary bodies, where they form capillary loops and turn backward as venous capillaries.
 
These capillaries keep joining with others and forming constantly larger veins, till finally there are great whorls formed in the region of the equator, where great numbers join to form the vena vortacosa which leave the eyeball just back of the equator to empty into the ophthalmic vein. Close inspection of this layer in Fig. 23 will reveal minute white spots al) through its expanse and these white spots are cross sections of the arteries and their branches as well as the veins of the whorls from which the vena vortacosa are formed within the tissue. There are two long posterior ciliary arteries which enter the eyeball with the short set of arteries; one enters just i.nside, the other just outside of the optic nerve. These pass forward in the choroid without giving off any branches, until they reach the ciliary region. Here they each divide into branches which take a circular course and form a circle of anastomosis at the base of the iris and form what is known as the circulus major (the largest circle), 2, of the iris. The anterior ciliary arteries also join in this network, forming an anastomosis with them ; then from this outer or larger circle branches pass into the iris and run toward the free margin or pupil, and when these reach the region of the sphincter pupillae muscle, another circle of anastomosis is formed and this is called the circulus m.lnor (smallest circle) of the iris ; from this smaller circle are given off capillaries, which form a circle of loops right at the free margin of the iris. These turn back as capillary loops, run one into another and become larger and larger and finally form veins known as the anterior ciliary veins ajid these veins also receive the aqueous humor from the canal of Schlemm, and therefore drain the anterior chamber. This was proven by injecting coloring matter into the anterior chamber, then after a few moments killing the animal and finding this colored matter in the anterior ciliary veins. The anterior ciliary veins leave the eye ball at the muscular attachments and pass away from the eye ball in the muscles finally reaching the ophthalmic vein from them.
The ciliary nerves, about twenty in number, which arise from the ciliary ganglion (knot), enter the eyeball in a circle just outside of the optic nerve. They run forv/ard in the supra choroidal space, giving off branches. Supplaying this structure, as well as the sclerotic, they run forward and form the ciliary plexus, which lies in the ciliary muscle. From this plexus branches run to the iris and cornea, supplying motor impulses to the sphincter pupillae muscle, dilator pupillae muscle, as well as trophic and sensory functions to the iris proper ; the branches passing to the cornea are trophic and sensory only.
Just outside of the optic nerve, where it pierces the eyeball, is ^und a circle of anastomosis, giving a pretty free blood supply to the sheath at this point and sending branches into the substance of the nerve, to supply nutrition to the sustentacular, or binding tissue, which forms trabeculae (beams) between the nerve bundles. This circle. O, is known as the circulus of Zinn, as he was the first to describe it.
 
Passing to the inner surface of the wall of the eyeball. we find the third of three grand tunics known as the retina (net), D. This lines the inner wall from the head of the optic nerve, also called the optic disc, or papillae, to the era serratta. It is made up of seven layers of nervous tissue, two layers of connective tissue and one single layer of columnar pigmented cell:^. The nine innermost layers are held together by the sustentacular or binding tissue, which is known as the fibers of Muller. The outer or pigmented columnar layer is intimately attached to the choroid, while the other nine layers are loosely attached to this layer, yet firmly attached to the choroid at the ora serratta, while the arrangement of the uorvc fiber kiycr and the passing of the axis cylinder processes through the choroidal fissure and their continuation into the optic nerve bind the retina down firmly at this point. The retina is the nervous tunic and the most sensitive in the eyeball and is the one v^hich makes possible the sense of sight. Its most sensitive area Hes just outside of the optic nerve and is known as the macula lutea, V (the yellow spot), so named from the fact that if examined after death, it will be seen to have a yellowish hue. Then again the central spot within the macula is known as fovea centralis (or central pit). The retina thins down ajid leaves a cone-shaped pit, there being only two layers at this central spot. The retina receives its blood supply from the arteria centralis retina (central ar^^^y)y 3- Ihis enters the eyeball in the substance of the optic nerve, having become incorporated in the nerve during the folding of the optic stalk and vesicles durijig foetal life. See Figs. 10 and 11. When it passes through the choroidal fissure it divides, one branch passing upward, the other downward. These are known as the superior and inferior branches. Each subdivide, making four branches ; one runnijig upward and toward the nose, another upward and toward the temple, another downward and inward toward the nose, and another outward and downward toward the temple and from the direction taken they are named. The one running upward toward the ^lose is known as the superior nasal branch, whilst the one running downward toward the nose is known as the inferior nasal ; the one running upward toward the temple is known as the superior temporal, the one running downward toward the temple is known as the inferior temporal branch. The farther subdivisions become so small and are so inconstant in their arrangement, that they have never beeji named. These vessels are imbedded in the retina, ramifying in the four innermost layers. They are readily seen with an ophthalmoscope from the fact that the retinal tissue surrounding them is transparent. These vessels keep dividing lill (hey become capillaries and turji back as venous capillaries. These capillaries keep joining and rejoining until the vena centralis retina is formed and this passes out by the side of the arteria centralis retina. These veins are normally about one-third larger than the arteries and as they carry vejious blood, which is loaded with waste products, they are of a darker red color when viewed with an ophthalmoscope.
 
As before stated the sclerotic coat posteriorly divides into three parts, the outer portion continuing into the sheath of the optic nerve, Y, the middle portion passes to the pial sheath, while the innermost portion breaks up into bundles and bridges across the space just back of the choroidal fissure, passing through the optic nerve and as these fibers come from all points and pass across in all directions, there is formed a sieve-like kyer which is known as the lamina cribrosa (sieve layer). This reinforces the globe at this point, which otherwise would not stand the strain exerted by the normal tension within the eyeball. The optic iierve fibers pass through the meshes in this sieve layer and the optic nerve proper commences just back of this, where the insulation in the form of the myelin (marrow) sheaths begin. The opening through the lamina cribrosa, through which the arteria centralis retina and veins pass, is known as the porus opticus. At the head of the optic nerve, at the inner wall of the eyeball, there is found a shallow, funnel-shaped pit, L, known as the physiological cup (normal cup). This pit is formed owing to the fact that when the axis cylinder processes reach the choroidal fissure and turn backward over the edge of the choroid, they make a gradual symmetrical turn, instead of running out and making a sharp right ajigled turn, so the innermost fibers join at the center, after having bent to a certain extent, thus leaving this normal depression. This depression of course is filled by the vitreous body.
 
The space surrounded by the retina, ciliary processes, ciliary bodies, suspensory ligament and lens, is filled by the vitreous body, U. This is made up of shapeless cells, more to be compared to an open meshed sponge than anything else, and fluid and the whole body is of the consistency of the white of an egg. It is surrounded by the hyaloid membrane, which lies on the injier limiting membrane of the retina. At the ora serratta, this hyaloid membrane divides. The outermost layer is firmly attached to the inner surface of the ciliary processes and bodies and passes from the ciliary bodies to the lens, and imbedded in it are the fibers of the suspensory ligament. The innermost layer continues over the front of the vitreous body and lines the fossae patillaris (dish-like depression), in which the lens rests. The vitreous body and its surrounding membrane are perfectly transparent. Running forward from the head of the optic nerve to the posterior of the lens, is a lymph space, known as the hyaloid canal, or the canal of Stilling; this was the channel through which the hyaloid artery passed to supply nutrition to the developing vitreous and lens, during foetal life. See Fig. i6. This artery atrophies before birth, and leaves this canal. The cornea, aqueous humor, lens and vitreous, form the refractive media of the eye, from the fact that they are transparejit and are of different densities and different curvatures, so arranged that light entering a normal eye is brought to a focus at the retina.
 
The eyeball has numerous lymph spaces and channels. The space between the sclerotic and choroid is known as the supra choroidal space. The greater portion of the contents of the eyeball are fluids, which are practically the same as lymph found in other parts of the body; they are furnished by the osmosis (passing out), of the fluids of the blood through the walls of the capillaries in the ciliary bodies. A portion passes into the canal of Petit and back into the vitreous body, while the rest passes into the posterior chamber, part directly from the anterior portion of the ciliary bodies and part from the canal of Petit. The supra choroidal space is filled with fluids ajid is drained by the lymph spaces accompanying the vena vortacosa. Tn healthy eyes all these fluids are constantly being supplied and rapidly passing out, so they do not become stagnant.
 
The orbits are four sided and pyramidal in form. The base is formed by the brim of the orbit, A, Fig. 24. The apex is at the sphenoidal fissure or opening, shown at B. The opening at the brim of the orbit, transversely, is one and one half inches, while vertically it is but one and one-fourth inches. Its depth, from the brim to the sphenoidal foramen, is one and three-fourths inches. The roof arches somewhat and the floor is slightly depressed, while the outer and inner walls are straight. The walls of the orbit are formed by seven bones. The roof is mainly formed by the orbital plate of the frontal bone, shown at C, and a very small portion at the posterior of the orbit by the lesser wing of the sphenoid, shown at D. The inner wall, from before backward, is formed by the nasal process of the superior maxillary, shown at E, lachrymal F, ethmoid H, orbital process of the superior maxillary G and the orbital portion of the sphenoid I. The floor is formed by the orbital plate of the superior maxillary J, orbital process of the plate K and a small portion of malar L. The outer wall is formed by the greater wing of the sphenoid M, and the orbital process of malar N.
The openings in the walls in the orbital cavity are as follows : On the interior wall, from before backward, the lachrymal canal, leading to the nasal cavity, through which the lachrymal duct passes ; the anterior and posterior ethmoidal foramen (opening), through which the nasal branch of the ophthalmic nerve and artery leave the orbit ; at the apex the sphenoidal fissure, through which the third, fourth, sixth and ophthalmic branches of the fifth nerve enter the orbit and the ophthalmic vein leaves it; above and to the inner side of the sphenoidal fissure is found the optic foramen O. It is through this opening that the second or optic nerve and the ophthalmic artery enter the orbit. At the lower, outer side, is found the spheno maxillary fissure P. It is through this
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Fig. 24. Tlie Human Skull.
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Opening that the upper branch of the superior maxillary or middle division of the fifth or trifacial nerve enters the orbit. It lies in a groove in the floor of the orbit at Q, and leaves the orbit with the infra orbital artery through the infra orbital foramen R.
 
Above the orbit, at its brim, is found a small opening, known as the supra orbital foramen, shown at S, through which the supra orbital nerve and artery leave the orbit. Sometimes this fails to fill in with bone at the brim and then only forms a notch, as shown at T. The inner walls are practically straight, from before backward, while the outer walls run obliquely backward and inward. Thus it will be seen that the axial poles of the two orbits diverge something like thirty degrees. The two eyeballs occupy the anterior central portion of the orbits. The rest of the orbit is filled with the orbital fat and the structures necessary for the performance of ocular functions and protection to the eyeball.
 
Covering the front, or base of the orbit and in front of the eyeball, are found the two lids, the upper and the lower, known as the palpebral and shown at G and H, Fig. 25. The opening between the two lids, through which the eyeball is seen is known as the palpebral fissure and where the two lids join, at the outer and inner sides of the eyeball, is called the outer and inner canthus, as shown at A and B. Near the inner canthus, the two lids approach one another, then separate again slightly, before coming together, and this little circular portion of the palpebral fissure is known as the lakus (meaning small lake, and is so called because the tears flow into it before leaving the palpebral fissure). Lying within the lakus is a small, red body, formed of mucous tissue and of some few very fine hairs, also the remains of the schneiderian gland, which is. found in those lower animals which have a third eyelid or nictitating membrane. This body is called the caruncle (small growth of flesh), shown at C, and just outside of the caruncle is found a fold of the conjunctiva (which membrane lines the lids and covers all the portion of the eyeball which is exposed when the lids are parted, except the corneal portion). This fold is the remains of the mem1)rana nictatans and is called the plica semilunaris (half moon fold), and is shown at F. All along the free margin of the lids, there is a row of hairs, which extend forward, with a slight turning upward at the outer ends on the upper lid and downward on the lower lid. These are the cilia (hairs) or eyelashes.
As before stated, when the lids approach, near the inner canthus, they arch away from each other, to form the lakus
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Fig. 25.
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and on the free margin of the lids at this angle, is found a small, slightly raised pdint, known as the lachrymal papillae (tear pimples), shown at I, from the fact that in the center of each one is found a little opening, called the lachrymal puncta (minute opening), so named from the fact that the tears pass out of the palpebral fissure through these two openings. At the anterior central portion of the eyeball is seen a round, dark area, shown at D, with a central, smaller, round and darker area, shown at E. Tlie outer, lighter portion, is the iris, and the smaller, darker portion is the opening through its center, known as the pupil. These are seen through the transparent cornea, M, and all the opaque, or white portion of the eyeball, seen from ni front, is the sclerotic, L, which is seen through the transparent conjunctiva. When the lids are separated, there is seen above the palpebral fissure, a fold of skin, J, which is caused by a bundle of fibers from the muscle which raises the upper lid passing outward and being attached to the skin, which draws the lower part of the skin, covering the lid, upward and al
 
 
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Fig. 26). Showin/j Tendo Oculi. +++++++++++++++++++++++++++++++++++++
lowing the skin covering the upper part of the lid to drop down, forming the fold, and in this way nature has provided against this loose skin dropping over the edge of the lid and obscuring vision, when the Hd is raised and the skin slackened.
Above the orbit, and covering a ridge, is a growth of hairs called the supra cilia (the hairs above) or eyebrows, K. This ridge is known as the supra ciliary ridge and is caused by a ridge of bone and a muscle underlying the skin. If the skin were dissected away, immediately beneath it would be found the superficial facia covering the deeper structure of the lids and stretching across the orbit. This is a thin, fibrous sheet, which is found immediately beneath the skin and areolar tissue in all portions of the body. At the outer and inner sides of the palpebral fissure, running from the canthi to the orbital walls, is seen the external and internal angular or palpebral ligaments, also called the orbicular ligaments (shown at A and B, Fig. 26), and just above the orbit would be seen the corrugator supra ciliary muscle (supra ciliary wrinkler) shown at C. It arises from the frontal bone near the median line and along the supra ciliary ridge,
 
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Fig. 27. Showing Orbicularis Muscle. +++++++++++++++++++++++++++++++++++++
and is attached to the upper and outer fibers of the orbicularis muscle. It is the contraction of this muscle which causes the vertical wrinkles in the skin at the lower central portion of the forehead. Its nerve supply comes from the facial nerve, yet there seems to be a reflex action between this muscle and those of accommodation, for we see this corrugation or wrinkling most frequently in those who are hyperopic.
If we dissect away the superficial facia, immediately beneath it will be found the orbicularis palpebrarum muscle (circular muscle of the lids) shown at D, Fig. 27. It arises from the bony walls of the orbit at the brim. The bundles of fibers pass inward and take a circular course and surround the palpebral fissure C, being continuous around the two canthi, A and B. This muscle is supplied by the facial nerve, and its action is to close the palpebral fissure and bring the free margins of the lids into apposition (touching), thus hiding the eyeball.
If the dissection is continued deeper, the deep facia would be exposed and in the region of the eye it is quite dense and fibrous and is called the ligament of Lockwood. It is shown
 
 
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Fig. 28. Showing Ligament of Lockwood.
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at A, Fig. 28. In it are embedded the tarsal (lid) cartilages, and above will be found the levator palpebrae superioris muscle (the lifter of the upper lid), shown at B. This muscle arises from the ligament of Zinn, which surrounds the optic foramen; it runs forward and upward and its tendon spreads out fan-shaped and is attached to the upper edge of the tarsal cartilage; a few fibers pass out and are attached to the skin. Its nerve supply is from the third, or motor oculi.
 
At the upper, inner side of the orbit, is seen the trochlea (pulley), shown at C, and passing through it and turning outward and downward, to be attached to the eyeball, is seen the superior oblique muscle D. It arises also from the ligament of Zinn, passes forward, upward and inward through the orhit, then becomes tendonous and passes through the trochlea, then runs outward, downward and backward, and is attached to the eyeball underucath and outside of the superior rectus muscle, just back of the equator. This muscle receives its nerve supply from the fourth or patheticus nerve. At the upper, outer side of the orbit is seen the lachrymal gland (tear gland), shown
 
 
 
 
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Fig. 29. Showing Arteries of the Lids.
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at E. This is a compound racemose gland (resembling a bundle of grapes), and its ducts empty into the conjunctival sac, at the fornix conjunctiva (arch of the conjunctiva), at the upper, outer angle. This gland secretes the tears which are poured into the conjunctival sac, when the eye is irritated, to wash away any foreign substance which may be the cause of the irritation. This gland is especially supplied with sensory nerves from the branch of the ophthalmic nerve, which is named after the gland. At the outer and inner cantlii are again seen the angular ligaments F, and beneath the internal angular ligament, is found the tensor tarsi muscle, which is supplied by the facial nerve. If the structures of the lids were dissected away, leaving only the arteries, their arrangement would be about as seen in Fig. 29. A is the angular artery, the terminal branch of the facial, and it is through this branch that collateral circulation to the brain is established, if the internal carotid is
 
 
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Fig. 30. Showing Veins of the Lids.
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occluded, for it forms an anastomosis with the frontal artery G, this being the terminal branch of the ophthalmic artery. B is the infra orbital artery which comes to the surface from the orbit, through the infra orbital foramen. D is the supra orbital which comes from the orbit to the face through the supra orbital foramen. H is the lachrymal branch of the ophthalmic artery, after piercing the lid. I shows a branch of the anterior temporal artery as it comes to the region of the eye. This branch is of importance, from the fact that in acute inflammations of the orbit, or its contents, leeching is resorted to on the temple, and it is the blood from this artery that is taken. E shows a branch from the transverse facial artery. Running across the lids, just above and below the opening, are seen two arterial trunks, F and J. They are divided into four arteries, the superior internal palpebral, the superior external palpebral, the inferior internal palpebral and the inferior external palpebral. It will be seen that the lids are well supplied with blood and that there is a free anastomosis of these vessels in and around the eyelids.
 
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Fig. 31. Showing Nerves of the Lids,
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Should all the structures of the lids be dissected away, leavuig only the veins. Fig. 30 would be a fair representation. The names of these veins are the same as those of the arteries. A is the angular ; B the infra orbital ; C shows the veins draining the palpebral margins, which are supplied by the four palpebral arteries; D shows the frontal, which forms the anastomosis and is the branch through which all parts of the orbit are drained, if there is occlusion of the ophthalmic vein, near the cavernous sinus, at the back of the orbit. E points out the infra orbital and F the anterior Iciiiporal. Thus it is seen that the drainage from the lids is abundant and this explains why it is that inflammatory conditions in this region are so easily controlled with hot or cold compresses.
 
If all other structures of the lids were dissected away, leaving the nerves only, Fig. 31 would give a fair idea of their arrangement. At A is seen the supra orbital nerve, after having emerged through the supra orbital foramen. At B, just outside of it, is seen the lachrymal nerve, after having pierced the lid, and at C are seen four branches coming from the facial nerve to supply the orbicularis palpebrarum. These are the only motor nerves shown in Fig. 31. The rest are all sensory nerves and are branches from the first and second divisions of the trifacial or fifth nerve. At D is seen the infra orbital nerve after emerging from the infra orbital foramen. It is the upper branch of the middle division of the trifacial nerve. At E are seen two branches emerging, the upper one passes above the trochlea and is known as the supra trochlear, while the lower passes below the trochlea and is called the infra trochlear nerve. The aggregation of small branches near the free margins of the upper and lower lids at F, is known as the plexus of Mises. It is thus seen that the lids are not wanting in sensory nerves.
 
If the lower portion of the nose were cut away and the deeper structures exposed between the palpebral fissure and the nosC; we would find the lachrymal (tear) conducting apparatus, A, Fig. 32, which shows the canaliculi (minute canals) above and below the lakus (small lake), B. These empty into the lachrymal sac (tear sack) C, which becomes smaller as it extends downward toward the nasal cavity and is known as the lachrymal or nasal duct, D. This empties into the nasal cavity below the inferior turbinate, E, into the space known as the inferior meatus, F. At G is shown the middle turbinate and H shows the nasal cavity proper. At I will be seen the tendo oculi or palpebral ligament cut short. The lachrymal sack occupies a triangular 8i)aec behind this structure, and in front of the tensor tarsi or Horners' muscle, and when these two structures are made taut, as is the case when the eye is closed, this arrangement causes a pulling forward and outward of the anterior portion of the lachrymal sac by the palpebral ligament, while at the same time the tensor tarsi muscle pulls the posterior portion outward and backward, thus distending the sac.
 
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Fig. 32. Showing Canaliculi and Lachrymal Sac and canal emptying into the nasal canal.
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Below the lachrymal sac there are valves in the lachrymal duct leading to the nasal cavity. These open downward and close the duct when there is suction from above, as is the case when the sac is distending, and the closing of the lids (which has distended the sac) has turned the lachrymal papilla, I, Fig. 25, so that their tips, where the lachrymal puncta are located, are pressed into the lakus, B, Fig. 32, and C, Fig. 25. As the lachrymal duct is closed there is produced a suction at these openings so that any of the lachrymal fluid (tear fluid) which may be in the lakus is drawn into the canaliculi and onward into the lachrymal sac. When the eye is opened and the lachrymal sac collapses the valves in the lachrymal ducts open and the fluid is given free passage into the nose.
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Fig. 33. vShowing Conjunctival Surface of the Lids.
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So it is seen that we have here a truly mechanical pumping apparatus to carry the tears from the eye. At J is seen the corrugator supracilia muscle.
Should we separate the lids from their attachments and leave only the attachments between them and the nose and swing them around forward, to clear the orbit, and look at the posterior or conjunctival surface of the lids, we would behold about the picture as seen in Fig. 33.
At A is seen the lachrymal gland and at B the openings through the conjunctiva where its ducts empty into the conjunctival sac at the fornix. C shows the conjunctival tissue, dissected from the back of the Hds, exposing the tarsal cartilages in which are imbedded the meibomian glands, shown at D, and their ducts opening onto the free margin of the lids, E. These glands secrete a sebaceous (oily) material which helps to lubricate the lids as they glide over the eyeball and also prevents the lids from sticking together when we sleep. Another function is that as the margins of the
 
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Fig. 34, Showing the Anterior Attachment to the Eye BaU of the Recti Muscles.
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lids are kept oiled all the time, the tears do not flow over them so readily and as the two lids come into apposition at the outer angle first and then gradually close the palpebral fissure from without inward toward the nose, the lachrymal fluid flows inward toward the lakus instead of over the margin of the lid and on to the cheek, as it would do if it were not for this sebaceous material being so freely distributed along the free margin of the lid. This oily substance also mixes with the tears and helps to prevent friction between the eye ball and lids, as well as keeping the cornea oiled so it does not dry so quickly as it otherwise v/ould.
F shows the location of the canaliculi and G the lachrymal sac ; H shows the tensor tarsi, or Homers' muscle, cut away ; I shows the corrugator supracilli ; J shows the levator labii superioris et aliqua nasi muscle (the lifter of the upper lip and the wing of the nose). This muscle arises just below the inner side of the orbit.
 
K shows the frontal sinus and L shows the maxillary sinus. These two sinuses sometimes become diseased and affect the eye on account of their nearness to it.
 
Should the lids be severed throughout their extent except at the inner side and swung out across the nose and all the tissue of the anterior part of the orbit dissected away, except the globe and recti muscles, as shown in Fig. 34, we could see the anterior portions and the attachments of the four straight recti muscles. A, B, C and D, the tendon E and pulley F, of the superior oblique and almost the whole of the inferior oblique muscle G as it arises from the floor of the orbit well forward and runs outward and slightly backward passing below the inferior rectus and is attached to the lower posterior quadrant of the eyeball. H shows the ocular conjunctiva, cut in a circle just outside of the cornea.
Should we make a horizontal cross section through the orbit and its contents, dissecting away all structures except the ligaments, fascias, etc., we would find the arrangement about as shown in Fig. 35. At A is shown the lid with the orbicularis palpebrarum muscle B, and the tarsal cartilage C, with the conjunctiva D, lining the conjunctival sac E, in which lies the plica semilunaris Q. At either side, in front, running from the Hd to the brim of the orbital bones, is seen the orbito tarsal ligamiCnt or tendo oculi F, and just back of it, at the internal side, is found the tensor tarsi muscle or Horner's muscle H. Just next to the wall of the orbit and placed between the tendo oculi and the tensor tarsi muscle is found the lachrymal sac I. At either side of the globe, running forward from the internal recti muscle K and the external recti muscle L, is seen the check ligaments of these muscles G. These are bands of fascia from the muscle sheaths, which run forward and blend with the deep fascia or ligament of Lockwood, which stretches across the front or base of the orbit within the lids, above and below the palpebral fissure. These check ligaments prevent extreme action of the muscles, which otherwise might do harmto the optic nerve, by rotating the eyeball too greatly.
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Fig. 35. Cross Section of Orbit and Contents.
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Just outside of the posterior portion of the eyeball is seen the space of Tenon N, which is a lymph space, and outside of it Tenon's sheath or capsule. Tenon's space is crossed by loose bundles of connective tissue, running from the sclera to Tenon's capsule and vice versa. These are known as trabeculae (fibrous bands). These are very loose and of sufficient length to allow free movements of the eyeball in the socket formed by Tenon's capsule. When the rectimuscles come near to the eyeball, the sheaths of the muscles blend with the capsule of Tenon, as shown at J, and it must be borne in mind that this connection greatly modifies the action of the recti muscles. Posteriorly is seen the optic nerve O, surrounded by the intra vaginal space P, and surrounding this space is found the sheath of the optic nerve, which is continuous with the sclerotic, and outside of the
 
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Fig. 36. Vertical Cross Section of Orbit.
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Optic nerve sheath is found the supra vaginal space S, which is continuous with the space of Tenon. This is surrounded by Tenon's capsule, filling in the spaces between the eyeball and the posterior or apex of the orbit, and between the muscles and other structures, is found the orbital fat T. This acts as a cushion for the eyeball as well as filling the spaces between the structures of the orbit.
Fig. 36 shows a vertical cross section of the orbit ; above and in front is seen the upper lid A and below in front is the lower lid B, the slit between them, the palpebral fissure C. Back of the lids, and in front of the cornea, is the conjunctival sac and above and below is seen the fornices (folds) D, where the conjunctiva leaves the lid (palpebral conjunctiva) and folds on itself, forming the fornix and then covering the anterior of the eyeball (ocular conjunctiva), ceasing at the edge of the cornea. At E, Fig. 36, is found the check ligaments of the levator palpebral «"n
 
 
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Fig. 37. Showing the Muscles of the Orbit.
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perioris J, and the iriferior rectus L, and at F is seen a band of tissue running from the upper side of the superior rectus muscle K to the lower side of the levator palpebrae. This band of tissue forms the check ligament of the superior rectus riiuscle. At H is seen the deep fascia or ligament of Lockwood. At G is seen the inferior oblique muscle with its sheath and the intimate relation of its sheath with the sheath of the inferior rectus I, and the capsule of Tenon. This is of importance from the fact of the modification of the action of the inferior obHque which it causes. At M is seen the orbital fat.
Should the roof of the orbit be cut away and all the structures of the orbit dissected away except the muscles, eyeball and the lachrymal gland, we would see about such a picture as shown by Fig. 37.
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Fig. 38. Showing Vessels of Orbit.
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The levator palpebrae superioris A, which occupies the uppermost portion of the orbit, is cut and thrown forward and exposes the superior rectus B, which lies just below it. At the inner side and above is shown the superior oblique C, running through the trochlea or pulley D, then its tendon E running obliquely outward and backward to its attachment to the globe F, beneath the superior rectus. Just beneath and outside of tlie superior obHque, is seen the internal rectus muscle K. At A IS seen the external rectus muscle and between it and the eyeball is seen the attachment of the inferior oblique muscle H. At the floor of the orbit, just back of the eyeball, is shown a small portion of the inferior rectus muscle J. All these muscles, except the inferior oblique, arise from the ligament of Zinn, which surrounds the optic foramen at the apex of the orbit. In the upper anterior portion of the orbit is shown the lachrymal gland I,
Should the roof of the orbit be cut away and all the structures of the orbit dissected away, except the arteries, veins, eyeball and lachrymal gland, we would see a picture about as portrayed in Fig. 38. Coming from the internal carotid artery, comes off the ophthalmic artery A, which enters the orbit through the optic foramen with the optic nerve. It first gives off the lachrymal branch D, which takes a course outward and upward to the position of the gland I, which it supplies, and after giving off branches to the gland, it pierces the lid and supplies the superficial structures of the lid, at the upper outer side of the orbit. The next branches given off are the several short posterior and long posterior ciliary arteries B, which run forward and pass into the eyeball in a circle around the optic nerve and run forward in the choroid. Shortly after these branches are given off, the arteria centralis retina is given off. This artery passes into the optic nerve ten or twelve millimeters back of the eyeball and passes through the choroidal fissure and gives the blood supply to the retina. There are also muscular branches given off which pass into the muscles and run forward in them to their attachments to the eyeball. These arteries pierce the sclerotic and enter the eyeball and are then known as the anterior ciliary arteries C. Then the supra orbital branch is given off, which runs upward and forward and passes out of the orbit through the supra orbital foramen and supplies the structures just above the orbit. The posterior H, and anterior E, ctliemoid branches, are then given off. These pass through the posterior and anterior ethmoidal foramen, which are found in the upper posterior portion of the internal bony wall of the orbit. They first pass into the cranial cavity, then run downward through the cribriform plate of the ethmoid bone to supply the internal and anterior portion of the nose. Anteriorly the ophthalmic artery gives off the frontal artery. These two then pierce the lids and one or the other forms an anastomosis with the angular artery, which is the terminal branch of the facial artery. This is of importance, from the fact that if the internal carotid artery or the posterior portion of the ophthalmic artery should be occluded (stopped up), collateral circulation would be established by this route. Accompanying all the larger arteries are found the veins, which carry the return flow of blood, and these veins are known by the sam.e name as the artery which they accompany. However, there are no veins leaving the eyeball with the posterior ciliary arteries, but the drainage from the choroid is by the Vena Vorticosse, J. These leave the eyeball just back of the equator and there are usually about five in number. All these veins join to form the ophthalmic vein L, which passes backward through the sphenoidal fissure and empties into the cavernous sinus. As shown at B, Fig. 38, the ophthalmic artery gives off several small branches which enter the eyeball in a circle around the optic nerve. There are some twelve to twenty of these, which are known as the short posterior ciliary arteries and two known as the long posterior ciliary arteries. Should we enucleate the eyeball and dissect away all the tissues down to the choroid and leave only the long and short ciliary arteries. Fig. 39 would give us a fair representation of their distribution. The short posterior ciliary arteries. A, from twelve to twenty in number, enter the eyeball by piercing the sclerotic in a circle just outside of the optic nerve. Immediately after entering the sclerotic, they divide, the main portion running forward (See P, Fig. 23) and enter the choroid, breaking up into smaller vessels and lay in three strata or layers, the layer of large blood vessels, the layer of small blood vessels, which is immediately beneath it, and the chorio capillaris or capillary layer, which is the innermost layer and is just beneath the retina.
 
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Fig. 39. Showing Ciliary Arteries.
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The larger vessels run forward in the choroid and ciliary processes to the ciliary bodies, which are just inside of the ciliary muscles B, where they end in capillary loops and turn back as venous capillaries, while the branches given off in their course form the layer of smaller blood vessels and these again break up into the chorio capillaris. The branches that turn toward the optic nerve, just after the short posterior ciliary arteries enter the sclerotic (See Fig. 23) form ii circle of anastomosis around the optic nerve, known as circulus of Zinn, as shown at O, Fig. 23. This circle furnishes a copious blood supply to the head of the optic nerve as well as furnishing a path for the establishment of collateral circulation, when there is trouble with the branches which supply the nerve sheath with which they also connect or anastomose.
There are two long posterior ciliary arteries, C, which enter the eyeball a little farther out than the short posterior ciliary arteries, one to the outer side of the nerve and one to the inner side. These run forward clear to the ciliary region, before they branch, and then when they do branch they join with, or anastomose with the anterior ciliary arteries, which enter the eyeball at the attachments of the recti muscles D, and these then form what is known as the circulus major (larger) of the Iris E, Fig. 39, and 2, Fig. 23. From this circle is given off the vessels for the iris, which run radially in toward the pupil G, and when these come near to the free margin of the iris another circle of anastomosis is formed, which is known as the circulus minor F, Fig. 39, and E, Fig. 23. Inside of this, toward the pupil, are given off arterial capillaries which turn back as veins, which are drained by the anterior ciliary veins, which leave the eyeball at the muscular attachments D. At H is seen the vena vorticosa (whirlpool) and at I is seen the optic nerve.
Should the eyeball be enucleated and the sclerotic and the tissues dissected oft', leaving only the veins of the posterior four-fifths of the eyeball, we would find practically the arrangement as seen in Fig. 40. The smaller veins pass back from the ciliary bodies at A from underneath the ciliary muscle F. These veins constantly join or anastomose with others and form four or five whirls, B, finally join to form the four or five vena vorticosae (whirlpool veins) C, which leave the eyeball just back of the equator and empty into the ophthalmic vein. See J. Fig. 38.
As previously mentioned the ophthalmic artery gives off one branch, which enters the optic nerve at its under surface and about ten to tv^elve millimeters back of the eyeball> which is known as the arteria centralis retina (central
 
 
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Fig. 40. Veins of the EyebaU
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artery of the retina), from the fact that it enters the eye ball at the optic disc and spreads out to supply the retina (See 3, Fig. 23), and if we should take an eyeball and make a coronal cut down through it at the equator, then hold it up and look at the inner surface of the globe, we would see the picture as portrayed in Fig. 41. At the disc A are seen the arteries emerging from the head of optic nerve or disc and the veins leaving. The artery first breaks up into two branches, one running upward, the other downward. These are known as the upper, B, and lower, C, branches. These in turn each divide into two branches. Each of these four branches runs obliquely outward from the disc, the upper one running inward toward the nose is called the superior nasal, D, and the one running upward and outward and toward the temple is called the superior temporal, E, while the one below, running inward toward the nose is known as the inferior nasal, F, and the one running downward and outward toward the temple is called the inferior temporal, G.
 
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Fig. 41. Arteries of the Retina
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The farther divisions of these arteries are unnamed. However, there are usually one or two small arteries, which run from the disc toward the maculae, which when present arc called the macular arteries, H. These arteries and veins lie in the retina, I, and the arteria centralis retina is what is known as a terminal artery, or in other words, it forms no anastomosis with any other set of arteries, consequently when it breaks up into capillaries, these turn back as veins. These keep joining together and get larger and larger until there are large veins formed, which are named the same as the arteries which they accompany. As there is usually a vein accompanying each artery, these join at the disc and form the vena centralis retina which leaves the eyeball within the optic nerve and lies within it for some ten or twelve millimeters. It then leaves the nerve and empties into the ophthalmic vein (See Fig. 23). The fact of the arteria centralis retina being a terminal artery in the retina having no collateral loops or anastomosis, as is the case in almost all other portions of the body, makes this of especial clinical significance, for if it becomes occluded, the nourishment is cut off from the retina and sight is lost and the retina atrophies in an exceedingly short period.
Just to the temporal side of the disc is seen the macula (spot) and at its center the fovea centralis (central spot) J. It is so named from the fact that it is the thinnest spot in the whole retina and turns yellow after death. It is not seen as a yellow spot during life, with an ophthalmoscope, as some inexperienced ones think, but as a dark area devoid of visible blood vessels and the yellow appearance which we see in examining the posterior inner surface of the eyeball after death is a post mortem (after death) change. K shows the choroid and L the scleral coat of the eyeball.
Should we cut away the roof of the orbit and dissect away all the tissues except the nerves, eyeball, recti muscles, levator superioris and the lachrymal gland, Fig. 42 would be a fair representation of what we would observe. At A we see the sixth cranial or abduceus nerve, which innervat-es the external rectus muscle J, and at B is seen the third cranial nerve or the motor oculi, which furnishes nerve impulse to the levator palpebrae superioris K, the superior rectus L, the internal rectus M, the inferior rectus N, and the inferior oblique O, besides giving branches to the ciliary or lenticular ganglion Q. At C is shown the fourth cranial or patheticus (cry) nerve which supplies the superior oblique muscle I. At D is shown the fifth cranial, trigeminus or trifacial nerve, and E the gasserian ganglion on the fifth nerve, and at F the upper or ophthalmic branch of the fifth nerve which supplies sensation to the orbit, eyeball and its structures as well as the lids, and G the superior maxillary nerve or the middle branch of the trifacial or fifth nerve, and H the lower branch or the inferior maxillary nerve. However, we are only particularly interested in the first, upper or ophthalmic branch, and just slightly interested in the second, or superior maxillary branch, for the ophthalmic nerve gives off first the nasal branch, R, which
 
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Fig. 42. The Nerves of the Orbit from Above.
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runs upward and inward through the orbit, giving a branch or root S to the lenticular ganglion Q, then passes out of the orbit to re-enter the cranial cavity through the ethmoidal foramen, then it leaves the cranial cavity again through the cribiform plate of the ethmoid bone and supplies sensation to the anterior portion and the tip of the nose, and it is this branch which accounts for the reflexes between the nose and the eye. Then the ophthalmic gives off the lachrymal branch, T, which runs upward and outward to the lachrymal gland, I J, and after supplying the gland it pierces the lid and supplies sensation to the upper outer part of the lid (See B, Fig. 31). The ophthalmic then gives one or two branches or roots to the lenticular ganglion direct and continues upward and forward. The main portion of the nerve leaves the orbit through the supra orbital foramen and is known as the supraorbital nerve (See A, Fig. 31). However, just before leaving the orbit it gives oflf a branch which divides, and one branch pierces the lid above the pulley or troclea, V, of the superior oblique muscle.
 
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Fig. 43. The Nerves of the Orbit from the side.
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This branch is known as the supra trochlear (See E, Fig. 31) ; the other one pierces the lid just below the trochlea and is known as the infra trochlear branch (See E, Fig. 31).
Should we make a vertical section of the walls of the orbit and dissect away the tissues, leaving only the eyeball, nerves and lachrymal gland, we would have the appearance as shown in Fig. 43. At A is shown the sixth or abduceus nerve, which is cut and thrown up at I, and at B is seen the third cranial or motor oculi nerve, and at J its branches or roots to the lenticular ganglion, K, and at D is shown the fifth cranial nerve, and at E the gasserian ganglion. At F is shown the first division, which is known as the ophthalmic nerve, and at L is shown its branches to the lenticular ganglion. At G is shown the second division or superior maxillary nerve, but in the study of the eye we are only interested in two of its branches ; first the one shown at M, known as the orbital nerve, which goes to the lower outer side of the eyeball, forming an anastomosis with the lachrymal nerve, T, and the terminal branch runs forward and passes out onto the face through the infra orbital foramen (See D, Fig. 31) and supplies the sensation to the lower lid and region just below the eye. This branch is known as the infra orbital nerve.
The lenticular ganglion, K, is of vast importance to the eyeball. It is a small pinkish body about the size of a pinhead and is situated some seven to ten millimeters back of the eyeball. On the outer side of the optic nerve, between it and the ophthalmic artery, it receives filaments, or roots, J, from the motor oculi nerve,, which are motor from the nasal nerve, L, as well as from the ophthalmic nerves which are sensory. It also receives filaments or roots from the sympathetic nervous system, which comes from the carotid plexus. Thus it is seen, there are motor, sensory and sympathetic filaments received by it. Then from this ganglion is given oflf the posterior ciliary nerves, N. These are mixed nerves and carry motor, sensory and sympathetic fibers. These nerves, from twelve to twenty in number, enter the eyeball posteriorly with the posterior ciliary arteries (See A, Fig. 39, and A, Fig. 44). These pierce the sclerotic just outside of the optic nerve in a circle and pass forward mostly in the supra choroidal space, and if we should enucleate an eyeball and dissect away the sclerotic and all other structures except the nerves, we would have a picture as shown in Fig. 44. The posterior ciliary nerves, ?», run forward in the supra choroidal space and give numerous branches to the choroid, C, in their course. They then break up into small branches, D, and these form a plexus in the ciliary muscle, E, and from thif plexus is given off branches to the ciliary muscle which are motor to the ciliary bodies which are sympathetic and sensory, then other branches to the iris, F, which are sensory, motor and sympathetic, the motor for the spincter pupillae (See K, Fig. 23), sympathetic, for the dilator pupillae muscle. Other branches go from the ciliary
 
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Fig. 44. Showing Ciliary Nerves. +++++++++++++++++++++++++++++++++++++
plexus to the cornea which are entirely sensory. Thus it will be seen that the nerve supply to the eye is abundant and of all three varieties, motor, sensory and sympathetic.
Having covered the gross anatomy of the eye pretty thoroughly, we will now pass to the more minute anatomy or Histology and in so doing it is well for the reader to be familiar with the gross anatomy, in order to be familiar with the relation of parts.
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CHAPTER III.
HISTOLOGY.
We will first take up the lids or palpebrae (from palpare — to stroke). These are two crescentic folds, which grow from above down and from below upward and cover the front of the eyeball. See Figs. 9 to 17, showing their development from the margin of the orbit. Their function is purely the protection of the eyeball and they contain many glands, all of which secrete substances which play their parts in the physiological functions of the lids. The lids also contain two semilunar plates with their convex border turned away from the palpebral slit. These are very dense, fibrous plates, known as the tarsal cartillages U, Fig. 45. However, they have nothing in common with cartillage, except their density, they being made up wholly of white fibrous tissue. However, they were named by the ancient anatomists prior to the time of our ability, by chemical analysis, to determine accurately the constituents of all tissues and bodies.
The outer or anterior surface is covered with epithelium while the inner or posterior surface is covered with mucous membrane, the epithelium changing its nature at the free margin of the lid.
Fig. 45 shows a vertical cross section of the upper lid. At A is shown the epithelium ; at B is shown the hair follicles of the small white hairs, which are scattered over the anterior surface of the lids. At C is shown the sweat glands ; at D the subepithelial tissue, or areolar tissue, which differs somewhat from that found in other parts of the body, from the fact that fat is not readily deposited in it, as is the case elsewhere in the body. Lying immediately below the areolar tissue is found the orbicularis palpebrarum muscle. The cross sections of the bundles are seen at E (also see Fig. 27) and at F are shown the hair folHcles of the ciha or lashes. At G are seen the modified sweat glands of Moll and at H are .shown the sebaceous glands connected with the lash or cilia in the lids. These glands are known as Zeisse's glands. At I is seen the muscle of Riolanis. This is the involuntary muscle for closing the eye ; it also re-enforces the orbiculars and brings the margins of the lids into close and firm apposition.
 
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Fig. 45. Vertical Cross Section of the Upper Eyelid.
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J points to the region where the epithelium changes its nature to that of a mucous membrane, such as lines all cavities or internal openings which communicate with the external world, and in this case is known as the conjunctiva. At K is shown one of the ducts of a Meibomian Gland, and L shows the secreting portion of the gland, which is imbedded in the tarsal cartillage. M shows the palpebral conjunctiva, and at N is seen a cross-section of one of the superior palpebral arteries (see G and J, Fig. 29). There is a free anastomosis between these arteries and those of the inner or conjunctival surface formed by numerous arteries piercing the tarsal cartillage. At O are seen the post tarsal papillae, which are folds, and the depressions be
 
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Fig. 46. Eyelid Showing Portion of a Hair FoUicle.
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tween them are called Henly's glands. At P are shown the glands of Waldeyer. At Q is shown the involuntary muscle of Mueller, and it is this muscular bundle which opens the eye involuntarily. At R are seen Kraus' glands, just above the fornix (arch) of the conjunctiva. S shows the fibers of the levator palpebrae superioris (see B, Fig. 28), and at T are shown the fibers from this muscle, which pass outward between the fibers of the orbicularis palpebrarum, and are attached to the skin as shown at A in Fig. 52.
This fasciculus is a wise provision of nature, for when the lid is raised it keeps the skin taut between its attachment and the free margin of the lid, and draws it up with the lid, while the skin above drops down over it, making a fold in the skin at about the middle of the lid, and in this way takes care of the loose skin when the lid is raised, otherwise it would drop down over the edge of the lid and interfere with vision.
 
 
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Fig. 47. Showing Zeisse's Glands, Modified Sweat Glands of Moll and Meibomian Glands.
 
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Fig. 46 shows the innermost portion of a hair follicle; G the papillae in the follicle, from which the hair grows and receives its nourishment mainly, and H the cup in the end of the hair shaft.
Fig. 47 also shows a hair follicle. B, Figs. 46 and 47, shows the sebaceous gland, known as Zeisses glands. In this location, these glands are compound sacular glands, the sacks filled with secreting cells, which secrete an oily material called sebum, which is poured into the hair follicle and travels along the lashes and keeps them oiled, so they are always soft and pliable. C, Fig. 46 and 47, shows the modified sweat glands of Moll, which are tubular glands, lined with secreting cells, which in other parts of the body lie doubled up in knots in the areolar tissue with straight tubes running to the surface. These modified sweat glands lie in the muscle of Riolanis, just back of the lashes. The modification of these glands on the margin of the lids is due to the fact that instead of opening onto the surface as sweat glands do elsewhere on the body, these empty into the hair follicle and this watery secretion becomes mixed with the sebum from Zeisse's glands and thereby renders it more viscid or watery. This serves the purpose of keeping the lashes constantly covered with this thin, viscid, oily substance, which facilitates their capacity for catching dust, thereby increasing the usefulness of the lashes in protecting the cornea against dust. F, Fig. 46 and 47, shows the muscle of Riolanis, which is a small muscular bundle, which surrounds the palpebral fissure and arises from the tendo oculi (see A and B, Fig. 26), however it is really a part of the orbicularis palpebrarum and is the involuntary muscle to close the eye when the cornea becomes dry. When acting in conjunction with the orbicularis in closing the eye, it causes a folding of the free margin of the lid and reinforces the orbicularis and brings the lid margins more closely together. D, Fig. 46 and 47, shows a duct of one of the meibomian glands and at E, Fig. 46 and 47, are the gland cells, which secrete the sebaceous material which is poured out on the free margin of the lid. The meibomian glands are modified sebaceous glands, being tubular with many blind pouches or sacks connecting, filled with secreting cells. There arc from twenty to thirty of these glands in each lid. They are imbedded in the conjunctival surface of the tarsal cartillage and arc readily seen in the human Hd (when inverted) as white Hnes, and their openings are readily seen on the free margin of the lid. See Figs. 33 and 34.
 
 
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Fig. 48. Showing the Tarsal PapiUae.
 
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This secretion renders four important services to the eye : First, this oily substance prevents the lids from sticking together when we sleep ; second, it keeps the margins of the lids oiled and prevents the tears from flowing over their edges, when the eyelids are being closed, for it will be remembered that the lids come into apposition at the outer canthus first, then the slit is gradually closed from without inward and any tears which have accunuilated in the palpebral fissure flow along in front of the closing edges; thus they arc directed into the lakus (see C, Fig. 25) ; third, it keeps the cornea oiled, which prevents the cornea from dessication or drying so readily ; fourth, its mixing with the tears and keeping the conjunctival sac so freely lubricated, prevents friction of the structures as they glide over each other in the opening and closing of the eye lids.
 
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Fig. 49. Showing Henle's Glands.
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A, Fig. 48, shows the post tarsal papillae, which in reality are only folds or Rouga of the conjunctiva which have the appearance of being small elevations when seen in cross section and where the mucous surfaces are brought into close proximity, as is the case in the furrows or depressions, the cells change their nature, and we find these furrows lined with columnar epithelial cells as shown at A in Fig. 49, and they are called Henle's glands. These glands, or folds, become more marked as age advances. These surfaces contain many goblet cells and secrete more or less mucus.
At A, Fig. 50, are seen the glands of Waldeyer. These glands are in the nature of sweat glands and they with Kraus' glands (A, Fig. 51) secrete the tears under ordinary circumstances. At B, Fig. 51, are shown cross sections of the lachrymal gland, which is a compound tubulo racemose gland resembling serous or fluid secreting glands in other parts of the body.
 
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Fig. 50.
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This is a rather larger gland than any we have seen in the lid before. It is located in the upper lid, at the upper outer side of the orbit (see E, Fig. 28) ; it is almond-shaped and the size of a small almond kernel. The secretions reach the conjunctival sac by some ten or twelve ducts (C, Fig. 51), which empty into the fornix (arch) of the conjunctiva. D, Fig. 51. The lachrymal gland, only pours forth its secretions when the eye is irritated, and this washes or floods the conjunctival sac quite freely.
as when the eye Is irritated by a foreign body or when we cry, and the secretion of tears is so copious, that our lachrymal apparatus cannot carry away all the fluids, and we find the tears flowing over the lower lid onto the cheek at such times.
The conjunctiva (joined together) is the mucous membrane which lines the conjunctival sac (the joined sac), which is really two crescentic culdesacs, one between each
 
 
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Fig. 51. +++++++++++++++++++++++++++++++++++++
eyelid and the eyeball, and they are really separated by the palpebral fissure, when the lids are open. However, it is a complete oval sac when the lids are closed. This mucous membrane commences at the free margin of the lid, at B, Fig. 52, by the transformation of the epithelium into mucous epithelium, the arrangement of the cells is the same as in the epithelium in other parts of the body, the outermost cells being squamous (scaly), the middle cells being irregularly round or polyhedral (many sided) cells, while the innermost are columnar (long) cells. These lie on a loose membrane which is well supplied with blood vessels, and the tissue being loose and transparent, it gives a free flow of Lymph. That part of the conjunctiva lining the posterior surface of the lids is known as the palpebral conjunctiva (C, Fig. 52). When it reaches well back under the lids, it folds on itself and becomes adherent to the sclerotic (H, Fig. 52).
 
 
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Fig 52.
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This fold is called the fornix conjunctiva (D, Fig. 52). The portion of the conjunctiva which covers the eyeball (E, Fig. 52) is called the ocular or bulbar conjunctiva. The ocular conjunctiva is transparent and through it we can see the sclera, which is opaque and white. It is freely movable over the sclerotic (K, Fig. 52) and by manipulation we can see the blood vessels of the conjunctiva (H, Fig. 52) change their position, while the blood vessels of the sclera, which are deeper set (J, Fig. 52), remain stationary. When the conjunctiva reaches the outer margin of the cornea (G, Fig. 52) the basement tissue ends, but the epithelium continues over the front of the cornea (F, Fig. 52) and forms the anterior or stratified epithelial layer of the cornea, and is called the conjunctival portion of the cornea. The blood vessels of the conjunctiva end at the corneal margin in a circle of capillary loops (I, Fig. 52, and F, Fig. 54), very superficially placed.
 
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Fig. 53.
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The cornea (horn-like) A, Fig. 23, and M, Fig. 25, forms about the anterior sixth of the eyeball. It is a highly transparent structure, allowing the light from the external world to enter the eyeball, and is the first of the refractive media through which this Hght passes on its way to the retina. It is made up of five layers, as shown in cross section in Fig. 53 ; A, the anterior stratified epithehal layer ; B, Bowman's membrane, or the anterior homogeneous membrane; C, the lamina propria (proper layer) ; D, Decimet's membrane or the posterior homogeneous layer, and E, the endothelial layer ; the latter lining the cornea on its surface bounding the anterior chamber. See S, Fig. 23.
 
The anterior stratified epithelium, as before stated, is continuous with the epithelial layer of the conjunctiva. As its name implies, its cells differ at different depths. The outermost, F, is made of squamous (scaly) cells, G is formed by hexagonal (many-sided) cells, and the innermost layer, H, is formed by Columnar (long, square) epithelial cells and this is the layer where all new cells are formed by the division and growth of these columnar cells, and as new cells are formed the older ones are pushed outward toward the surface and become hexagonal, and as this process continues, the cells are pushed farther and farther out. They lose their nuclei and become mere flat scales and finally lose their adhesive qualities and are disquamated (thrown off) and wash away with the tears. These cells are held together both from the cement substance lying between them and by the little projections from the surface of the cells themselves. When these projections are found on a cell, they are called prickle cells, and this is the nature of these cells in the lower or inner layers.
 
Passing from without inward, the next layer, B, is Bowman's membrane, or the anterior homogeneous lamina.
As the name, homogeneous lamina, implies, this layer when seen with the microscope reveals no structural frame work, but appears as a solid gelatinous layer. However, if this tissue be macerated (soaked) in an alkaline solution and the cement substance dissolved out, it will be found to be formed of connective tissue bundles. This layer ends at the periphery of the cornea. The next layer, C, is the Lamina Propria (proper layer) or substance of the cornea. It is formed of some sixty strata of connective tissue bundles. These cannot be stripped ofif in layers, but are made out by the microscope from the fact that the connective tissue bundles run in different directions ; that is, for instance, in one strata all the bundles run vertically across the cornea, the next layer may run horizontally, while the third strata may have its bundles laying at 45° or 135°.
 
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Fig. 54. Ciliary region, magnified 1,000 times.
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However, there is such a free exchange of bundles from one layer to another, in which they become lost, that the whole sixty are practically as one ; so that this arrangement forms a very firm, unyielding structure. At the sclerocorneal juncture or limbus, U, Fig. 54, the lamina propria continues backward, forming the sclerotic by continuity of tissue, the difference being simply that the nature of the tissue is changed at the Hmbiis, for in the cornea it has no blood vessels, while the sclera is fairly well supplied with blood vessels. In the cornea the tissue is quite dense and transparent, and in the sclerotic it is more loosely arranged and is opaque. In the cornea it is highly supplied with sensory nerves, while in the sclerotic it has only a moderate nerve supply, and, farther, if we should examine the cornea chemically, we would 'find it contained chondron ( found in cartilage), while the sclerotic would chow no chondron, but in its place we would find gelatin. It will then be seen that while one blends into the other, yet the two tissues are very much different.
 
The cornea is said to fit into the sclera as a watch glass fits into the bezel of a watch. This impression is given from the fact that the corneal tissue passes farther backward at its center, while the sclera runs forward farther at its outer and inner surfaces. When we view this with the microscope in stained sections, it appears as shown at U, Fig. 54. This is from the fact that the cornea being more dense than ihc sclerotic, it retains less of the stain in its preparation, so that we can make out the limits of the two tissues fairly well in this way.
 
Lying within the lamina propria is a network of openings or lymph channels, the lacunae, I, Fig. 53 (small lakes) and the minute canals (caniliculae) which run out in all directions from the lacunae and join the caniliculae from surrounding lacunae. Lying in these lacunae, I, Fig. 53, yet not entirely filling them, are found the fixed or corneal corpuscles. These cells in turn have very minute protoplasmic processes which run through the caniliculae and join or anastomose with the processes from the cells in the neighboring lacunae. These processes do not entirely fill the caniliculae in which they lie; thus it will be seen that we have a network of lymph channels through which the l>lood plasma, or nutrient lymph can have free passage to all parts of the cornea to supply it with nutrition. This lymph is given off from the capillary loops, forming :i circle around the margin of the cornea, which will be described later.
 
Passing inward, the next layer is Decimet's Membrane, or the internal homogenous lamina, D, Fig. 53. This is a very thin, highly transparent layer and has a tendency to curl up when stripped off of the cornea. When viewed with the microscope, it is impossible to make out any ground work, it seeming to be wholly made up of a hornlike membrane, but as with Bowman's membrane, if treated properly, to remove the gelatinous substance which forms the matrix or joins the component tissues together, it will be found to be formed of connective tissue. Many functions have been ascribed to this membrane, but the chief one is its great resistance to disease, such as corneal ulcers, etc. Some writers claim this membrane breaks up into connective tissue bundles, bridges across the filtration angle and forms the pectinate (comb) ligament, K, Fig. 54.
 
This is composed of hundreds of connective tissue bundles which run from the periphery of the cornea to the base of the iris, K, Fig. 54, and A, Fig. 53. This angle formed by the iris and cornea, V, Fig. 54, is known as the filtration angle from the fact that the aqueous fluid passes out of the anterior chamber between the bundles of tissue, forming the pectinate ligament, to the spaces of Fontana (fountain spaces), which comprises the openings in the pectinate ligament. The posterior, or fifth layer, is known as the endothelial layer, E, Fig. 53. This is formed of a single layer of cubical (square) endothelial cells, which are placed like paving blocks and are similar to cells which are found in other parts of the body, lining closed cavities, or cavities which have no opening communicating with the external world. These cells have the faculty to withstand the dissolving qualities of the aqueous fluid, or nutrient lymph, which fills the anterior chamber. 
 
Some anatomists divide the cornea into three portions; the conjunctival portion, consisting of the anterior stratified epitheHum, and Bowman's Membrane; the scleral portion, consisting of the lamina propria ; and the choroidal portion, consisting of Decimet's Membrane and the endothelial layer. This is from the fact that these layers are supposed to be derived from these structures.
 
The sclerotic (tough) coat, I, Fig. 54, forms the posterior five-sixths of the outer coat of the eyeball, except a small opening at the posterior pole, where the optic nerve pierces it. This opening is known as the choroidal fissure. See Figs. 23, 56 and 57. The sclerotic, as before stated, is continuous with the cornea by continuity of tissue. Just outside of the sclerotic is the space of Tenon, X, Fig. 54, and N, Fig. 35. This is a space between the capsule of Tenon and the sclerotic. The capsule of Tenon forms a fibrous socket for the eyeball, and this space of Tenon is a lymph space and is crossed by many connective tissue bundles passing from the capsule to the sclerotic. These are known as Trabeculae. Internal to the sclerotic, between it and the choroid, is another lymph space known as the suprachoroidal space, W, Fig. 54. This is also crossed by an abundance of trabeculae passing from the sclerotic to the choroidal coat. In fact, the trabeculae are so numerous that it is almost impossible to separate the two structures. The sclerotic, as its name implies, is very tough and opaque. The innermost portion contains quite a little pigment. It has four layers, from without inward ; they are the endothelial layer lining the space of Tenon, which is a single layer of pavement cells. Next comes the lamina propria (proper layer) ; the next layer is called the lamina fusca (Brown layer). The lamina propria and the lamina fusca are not sharply defined by any line of demarcation, but, as before stated, the innermost strata contains some pigment. It is therefore brown in color, the pigment not being sufficient to cause it to appear black. This pigment is deposited in branched cells. The innermost, or fourth layer, is the internal endothelial layer, lining the supra choroidal space, and is of the usual pavement variety. The lamina propria and lamina fusca are formed of tough fibrous tissue, the strands of which run in all directions with a general anterior posterior arrangement. Lying in the substance of the sclerotic are found lacunae, the same as in the cornea which contains the fixed or scleral corpuscles, analogous to the corneal corpuscles. In fact, the sclerotic is very similar to the cornea in the arrangement of the connective tissue, except that it is not learly so compact.
 
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Fig. 55. Magnified 2,500 times.
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The sclera, as before stated, is well supplied with blood vessels. These run forward and end in capillary loops at the limbus and form a complete circle extending clear around the periphery of the cornea. They, with the circle of capillaries formed by the conjunctival vessels, F, Fig. 54, and I, Fig. 52, near the outer surface, give off the nutrient lymph which flows through the lacunae (small lakes) and caniliculi (minute canals) and permeates the cornea. This lymph furnishes the nutrition for the cornea. To give the reader an idea of what is meant by capillary loops, we have taken a microphotograph of an injected section from the sole of the foot. Fig. 55, A. This is the Rete Mukosum (capillary layer, or malpighian layer),. of the skin, showing the fine capillaries running up and forming loops and passing back as venous capillaries. B shows some elevations, which form little ridges, which can be seen on the ball of the thumb so readily. C shows the branch of an artery, which breaks up into these small capillaries.
 
At J, Fig. 54, is seen the canal of Schlemm. This is a canal lying near the inner surface of the sclerotic, just at the limbus. It is circular in course, running clear around the margin of the cornea. It may be single, or may be composed of several small canals. They branch from and return to the main opening, so that it forms one continuous sinus, it is lined with endothelial cells, and is drained by the anterior ciliary veins. The aqueous humor passes from the spaces of fontana to the canal of Schlemm and eventually is carried back into the circulation by the anterior ciliary veins.
 
As before stated the sclerotic forms the posterior fivesixths of the outer coat of the eyeball. To it are attached the six recti (straight) muscles. See Figs. 35, 36 and 37. It is pierced by the anterior ciliary arteries and veins at these points of attachment. (See D, Fig. 39.) These pass through about eight to ten millimeters back of the limbus; then just back of the equator it is pierced by the vena vorticosa (whorl veins) four to six in number. (See H, Fig. 39, and B. and C, Fig. 40) Then posteriorly it is pierced by the cihary arteries and nerves, there being twelve to twenty of each. These pass through just outside of the optic nerve A, Fig. 39, and at A, Fig. 56, is seen one of these vessels passing through this structure. When the sclerotic reaches the optic nerve, it divides into three portions. The innermost, B, Fig. 56 and 57, breaks up into individual bundles. These pass across the choroidal fissure and form the lamina cribrosa (sieve layer), C, Figs. 56 and 57. These bundles pass across in all directions and reinforce the eyeball at this otherwise weak point, leaving meshes or openings through which the optic nerve fibers pass out of the eyeball. It is also pierced by the arteria centralis retinae (central artery of the retina) L, Fig. 57. The opening through the lamina cribrosa, through which the arteria centralis retinae and vein pass, is known as the porus opticus.
 
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Fig. 56.
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The middle portion passes to and blends with the pia mater of the optic nerve D, Figs. 56 and 57. The outer portion passes into the sheath of the optic nerve, F, Fig. 56 and 57. At E is shown the intervaginal space of the optic nerve, which is continuous with the sub-dural space of the brain at the optic foramen and contains cerebro spinal fluid.
 
 
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Fig. 57.
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H, Figs. 56 and 57, shows the choroid, and J, Figs. 56 and 57, shows the retina detached from the choroid. K shows the physiological cup ; M shows the optic nerve, and in Fig. 57 the nerve bundles are extremely well shown with the myelin sheaths surrounding them. These sheaths end normally just behind the lamina cribrosa, C.
The choroid is continuous from the optic nerve to the free margin of .the iris, or to the pupillary opening. It lies inside of the schlerotic and is the second grand tunic or coat of the eye. From the ora serratta of the retina (saw tooth mouth of the retina) to the choroidal fissure it Hes in touch with the sclerotic, only being separated from it by the supra choroidal space and intimately attached to the sclerotic by the interchange of trabeculae passing across the supra choroidal space from one to the other. It is a pigmented and highly vascular tissue, as its name implies, and supplies the greater part of the nutrition and secretions of the eyeball.
 
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Fig. 58. Showing Section of Choroid.
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It is composed of five layers of fibrous tissue with branched pigment cells in the meshes between the connective tissue fibers. First from without inward we have the endothelial layer lining the supra choroidal space, A, Fig. 58. Below that is the lamina supra choroidea (upper layer of the choroid), B, Fig. 58, also called the lamina fusca of the choroid, on account of its pigmentation and brown color.
The next layer is the layer of large blood vessels, C, Fig. 58. The next layer is known as the layer of small blood vessels, D, Fig. 58. The layer of large and small blood vessels are composed of the posterior ciliary arteries as they pass forward in the structure, giving off branches all along their course ; also the veins which go to form the vena vorticosa lie in these two layers. The next layer is called the choriocapillario (capillary layer of the choroid), E, Fig. 58. These capillaries are separated from the retina by the bacillary layer (basement layer), also called Bracks' Membrane or lamina vitrea, F, Fig. 58. The capillary layer of the choroid furnishes much of the nutrition to the outer layers of the retina, it reaching them by osmosis (passing through) Brucks' Membrane. This layer is free from pigment and is rich in cement substance, so much so that it is a homogeneous membrane or layer.
 
 
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Fig. 59.
 
 
Showing Ora Seratta, Ciliary Processes and Bodies and the Iris from Posterior Aspect.
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Fig.
 
 
Showing Same as Fig. 59 in Cross Section.
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The choroid is highly pigmented to prevent the light penetrating the wall of the eyeball, thus making an absolutely dark chamber of it. The choroid is extremely susceptible to disease on account of its extreme vascularity.
 
As before stated the choroid is continuous from the head of the optic nerve forward to the free margin of the iris. However, it is divided into the choroid ciHary process, ciHary bodies and iris. The choroid, I, Fig. 60, extends from the optic nerve to the ora serratta or anterior margin of the retina, A, Figs. 59 and 60. It then becomes somewhat ridged on its inner surface, B, Figs. 59 and 60. These ridges have an anterior posterior direction, and these ridges, about seventy in number, are known as the ciHary processes.
 
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Fig. 61. The CapiUaries of the Ciliary Processes.
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These end in blunt endings which project into the cavity of the eyeball towards the lens, H, Fig. 60, and are known as the ciliary bodies, C, Figs, 59 and 60. From the outer angle of the bases of the ciliary bodies, J, Fig. 60, the choroid or uvea leaves the outer wall of the eyeball and takes a transverse direction. This transverse portion is called the iris (rainbow), D, Figs. 59 and 60. At the center of the (Doll so called from the diminutive image of oneself as seen in the pupillary area when looking into anyone's eye), transverse portion there is an opening known as the pupil, G, Figs. 59 and 60. The free margin of the iris, F, Figs. 59 and 60, Hes free and rests on the anterior surface of the lens, H, Fig. 60. The short, posterior ciliary arteries run forward through the choroid in the layer of large blood vessels, C, Fig. 58, and B, Fig. 39, being bunched in straight vessels in
 
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Fig. 62. The Blood Vessels of the Iris.
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the ciliary processes and end in capillary tufts, Fig. 61, in the ciliary bodies, turning back as venous capillaries, A, Fig. 40. From the capillaries are given out the fluids of the blood which passes into the canal of Petit, N, Fig. 60, and the posterior chamber, M, Fig. 60. This fluid is known as the aqueous humor.
 
The two long posterior ciliary arteries run forward in the choroid in the layer of large blood vessels, C, Fig. 58, and C, Fig. 39. They join the anterior ciliary arteries, D, Fig. 39, and form one arterial trunk, which lies right at the base of the iris, A, Fig. 62, K, Fig. 60, and E, Fig. 39. This arterial trunk formed by the anastomosis (joining) of the long posterior and anterior ciliary arteries, is known as the circulus major (larger circle) of the iris, A, Fig. 62. From the circulus major is given off branches which run radially inward toward the free margin of the iris, B, Fig. 62, These radially coursing arteries in the iris may be likened to the spokes in a wheel. When they come near to the free margin of the iris, they anastomose (join) and form another circle known as the circulus minor (smaller circle) of the iris, C, Fig. 62. From the circulus minor is given off capillaries which run inward toward the free margin of the iris. They double back as venous capillaries, D, Fig. 62. The drainage from the iris is by the anterior ciliary veins which leave the eyeball at the attachments of the extrinsic muscles, while the drainage from ciliary bodies and the rest of the uveal tract is drained by the vena vortacosa (vortex veins). See Fig. 40,
 
Anterior to the ora serratta the outermost or pigment layer of the retina continues forward in two layers of columnar epithelial pigmented cells and lines the inner wall of the eyeball over the pars ciliaris, retina, ciliary processes, and bodies, O, Fig. 60, also continuing over the posterior surface of the iris, clear to the free margin at F, Fig. 60. This is known as the retinal portion of these structures and the amount of pigment contained in this layer over the posterior of the iris largely determines the color of the eye, for if there is no pigment in this layer, or the stroma of the iris, we would have the pink or albino eye. If there is a small amount of pigment in the retinal portion, then we would have a light blue eye ; a little more pigment and it will produce the dark blue eye, and so on as more pigment is deposited, the eye is gray, brown or black. However, in the brown and black eyes there is much pigmentation of the stroma of the iris.
 
Fig. 63 shows a cross section of an iris in which the retinal portion, A, is well pigmented, while the stroma, B, has but a small amount of pigment. This would have a tendency to produce a light grayish color when the iris is viewed from the front. Fig. 64 shows a cross section of an iris, which is highly pigmented, both in the retinal layer, A, as well as the stroma, B. This would produce a dark brown or black iris if viewed from the front.
The iris is a very delicate structure formed of a very thin network of connective tissue, with a large amount of cells filling in the spaces between the connective tissue fibers. In dark eyes these cells become more or less pigmented, C, Fig. 64. The iris contains two muscles, the Sphincter (binder) Pupillae, A, Fig. 65, and the Dilator (enlarger) Pupillae, B. It has four layers from within outward ; they are the pigment, or retinal layer, F, muscular, B, the stroma proper in which lie the blood vessels, E, and the endothelial or corneal layer, D. The front of the iris has deep depressions or crypts ; these run radially, or from the base toward the free margin. These depressions, or crypts, lie between the blood vessels, see Fig. 62, and in medium or light colored eyes this causes the stellate (star like) or radially spoke-like appearance of the anterior surface of the iris as seen in those eyes. Fig, 66 represents a quadrant of the front surface of an iris; A, the pigment layer at the free margin ; B, the circulus minor and capillaries ; C, the circulus major; D, a trabeculae or ridge in which runs a blood vessel; E, a depression or crypt; and F, the pectinate ligament. The spoke or stellate appearance is caused by the vessels being so near the surface that the reflection is greater over them than from the spaces between them. However, in very dark eyes, the pigment is so densely deposited that it. hides the blood vessels; therefore, in dark eyes this spokelike appearance is absent.
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Fig. 63. Cross section from iris of a light colored eye.
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Fig. 64. A cross section of Iris from dark Eye.
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Fig. 65. Showing cross section of Iris, its muscles and layers.
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Lying just behind the iris is found the crystalline lens (pea or lentil), and as the name implies, it is a transparent body shown at A, Fig. 6y. This lies in the Fossae Patilaris (dish like depression) in the anterior surface of the vitreous body and is held in place by the suspensory ligament, B. The lens has two portions, however, not divisible or sharply outlined; the central or nuclear portion and the outer or cortical portion. The central or nuclear portion is more dense than the cortical portion. The nuclear por
 
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Fig. 66. A quadrant of the front surface of the Iris.
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tion is composed of the elongated or spindle cells which first fill the lens vesicle by the elongation of the cells composing the posterior wall of the lens vesicle; see Fig. 8. These cells, as before stated, fill the whole cavity as shown in Fig. 68, D. The nuclei of these cells are pushed forward, as shown at K. After these first formed spindle cells have filled the lens vesicle, then at the transitional (transforming) zone, the original cells of the lens vesicle continue to elongate and grow around the ends of the cells which have formed the nucleus of the lens as shown at J, and in this section the cells, which will form the cortical portion, are just beginning to grow and elongate. These cells then form the outer or cortical portion of the lens and the ends of the fibers butt together, as shown at A, Fig. 69. These fibers, or spindle cells, have a diamond shape and these again are formed in layers bound together by transparent cement substance. These layers are then laid one on another, as the layers of an onion are found, and these layers in turn are bound together by the cement substance.
 
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Fig. 67. Cross section of the liuman eye.
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Over the anterior surface of the crystalline lens is formed a single layer of columnar cells, which are the cells composing the original lens vesicle wall. This layer extends back to the equator of the lens and then they become transformed into the spindle cells, which compose the lens substance. The area of transformation is known as the transitional Zone; see G, Fig. 60. Surrounding the whole lens is a thin transparent membrane known as the capsule of the lens, C, Fig. 67, and to this capsule is attached the suspensory ligament, the anterior fibers just in front of the equator and the posterior fibers just behind the equator.
 
 
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Fig. 68. Human embryo eye, 2 months. Magnified 1,080 times.
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The suspensory ligament of the lens or Zonule or Zinn. C and D, Figs. 70 to 74, is imbedded in the outer layer of the hyaloid membrane. This membrane divides into two layers at the ora seratta of the retina (saw tooth mouth), F, Figs. 70 to 74. The inner layer continues over the front of the vitreous body, while the outer layer in which the fibers of the suspensory ligament, I, Figs. 70 to 74, are imbedded, is firmly bound down to the inner surface of the pars ciliaris retina, ciliary processes and bodies G and H, Figs. 70 to 74. From the ciliary processes, H, the fibers and membrane leave the outer wall of the eyeball and turn transversely across toward the equator of the lens, P. The outer layer of the hyaloid membrane, I, Figs. 70 to 73, becomes very thin and fluid passes through it very readily. It passes across with the fibers of the suspensory
 
 
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Fig. 69. Human embryo eye, 5 months. Magnified 7,000 times.
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ligament, which are attached in front of the equator of the lens, and the triangular space bounded by it in front and the hyaloid membrane behind with its base at the equator of the lens. The apex at the ciliary bodies is called the canal of Petit, E, Figs. 70 to 73.
The fibers of the suspensory ligament, C and D, Figs. 70 to 74, arise from the retina at the ora seratta, F, and are continuous clear to their attachments to the lens, A and B. These fibers are believed to be specialized elongated fibers of Mueller, which are of a very elastic nature. These fibers become attached to the lens capsule during the development of the eye and as the eye enlarges become elongated. When they leave the ciliary bodies they divide and a part of them, C, Figs. 70 and 73, pass to their attachment to the lens capsule in front of the equator and others, D, Figs. 70 to 74, pass to their attachment to the lens capsule back of the equator, while a few pass across in the canal of Petit, E, Figs. 70 to 73. These are attached to the lens at its equator. By glancing at Fig. 70 and noting the attachment of the suspensory ligament, C and D, it will readily be understood that tension on the suspensory ligament, C and D, of the lens, P, Fig. 70, would have a tendency to flatten it in its anterior posterior diameter and enlarge its transverse diameter, thus increasing and decreasing its convexity, as this tension was exerted or relaxed.
 
 
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Fig. 70. Showing the suspensory Hgament.'
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Fig. 71. Showing ora seratta and ciliary processes.
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Fig 72. Enlarged view of ciliary muscle.
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Fig. 73. Showing ciliary processes and bodies.
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Lying between the ciliary processes, bodies and choroid, G, H and I, Figs. 70 to 73, and the sclerotic, K, Figs, ^^2 and y^, is found the ciliary muscle, L and M, Figs. 70 to 73 (hair-like muscle), composed of plain muscular fibers. It is composed of two portions, the longitudinal or the outer portion, L, Figs. 70 to yi, and the circular portion, M, Figs. 70 to 73. The fibers of the first or outer portion, L, run anterior posterior, arising at the limbus (seam), N,
 
Figs. 70 and "j^i^ a portion of them in front of and a portion posterior to the canal of Schlemm, O, Figs. 70 and 73. The circular portion, M, has the same origin, but takes a circular course and lies just ouiside of the ciliary bodies, H, Figs. 70 to y2). The longitudinal fibers are attached to the outer surface of the choroid, J, Figs. 71 to y2i- The)
 
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Fig. 74. Suspensory ligaments and lens drawn in.
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spread out fan shaped, some being attached as far forward as the posterior ends of the ciliary bodies, H, Figs. 70 to 74. Others extend backward and arc attached as far backward as the ora scratta, F, Figs. 70 to 74; thus it is seen they have a very extensive attachment to the choroid. The function of the ciliary muscle is to put the choroid, J, on the stretch. This is possible owing to the supra choroidal space, Figs. 70 to 73, separating the choroid and sclerotic, and the circular fibers, M, press the ciliary bodies, H, nearer to the equator of the lens, B, Fig. 74. As the suspensory ligament, C and D, is bound down to the choroid ciliary processes and bodies, and bridges across the space between the ciliary bodies, H, and the lens, P, the action of the muscle when it contracts is to slacken the suspensory ligament, allowing the lens P to become more convex by virtue of its elasticity or resiliency.
 
 
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Fig. 75. Vitreous darkened to show hyaloid canal.
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However, the main strain in accommodation seems to fall upon the circular portion M in Figs. 70 to y^, from the fact that in myopic eyes where the far point is within thirteen inches of the eye, where accommodation is never necessary, but few if any of these circular fibers are found upon examining the ciliary muscle after death, whereas in the hypermetropic eyes where accommodation is necessary for all vision, these fibers will be found to be very plentiful. In fact they have been known to make up as much as seventyfive per cent of the bulk of the muscle. The ciliary muscle receives its nerve supply from the posterior ciliary nerves, which arise from the lenticular or ciliary ganglion (to knit or weave), which receives its motor roots from the third cranial or motor oculi nerve. See Figs. 42, 43 and 44. The terminal portions of the posterior ciliary nerves break up into small anastomosing branches and form the ciliary plexus, which lies in the ciliary muscle.
 
The vitreous body, B, Fig. 75, composes the greater portion of the eyeball, filling all the cavity posterior to the lens. It is composed of shapeless transparent cells, very loosely arranged, so that it resembles a sponge and is filled with fluid resembling the aqueous humor, and is about of the density as the white of an egg, running through the vitreous body. Antero posteriorly from the posterior of the lens to the head of the optic nerve is found a lymph canal. A, Fig. 75, which was the space occupied by the hyaloid artery, which is present during the development of the lens during foetal life. See B, Fig. 16. This canal is known as the hyaloid canal or the canal of Stilling. The lens is imbedded in the anterior surface of the vitreous body, lying in a depression called the Fossae Patellaris (saucer-like depression), C, Fig. 75. The whole body is surrounded by the hyaloid membrane (glass-like membrane), which is transparent and homogenous (structureless), D, Fig. 75. This membrane divides at the ora seratta, F, Figs. 70 to 74, the inner layer covering the anterior of the vitreous and lining the fossae patellaris, C, Fig. 75, while the outer layer is intimately attached to the ciliary processes and bodies and leaves the ciliary bodies and extends to the lens. In this outer layer is imbedded the fibers of the suspensory ligament, I, Figs. 70 to 73. Posterior to the ora seratta the hyaloid membrane is very intimately attached to the retina, F, Fig. 75. This attachment is so firm that when the vitreous body is disturbed the nine innermost layers of the retina are usually detached.
The retina (net) lines the inner wall of the eye ball, it extends, properly speaking, from the head of the optic nerve, M. Fig. 'j^), to the Ora Serratta (Saw Tooth Mouth) X ; however, it is continuous clear to the free margin of
 
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Fig. 76. Cross section of the human eye, showing its construction.
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the Iris E, by means of a double layer of pigmented epithelial cells which cover the inner surface of the pars ciliaris retina (the part between the ciliary bodies and the retina), G, Fig. ^^, ciliary processes G, Fig. 74, and ciliary bodies H. Figs. 73 and 74, as well as the inner or posterior surface of the iris, E. Fig. 76. This anterior or pigmented portion is called the Uvea (Grape Skin) ; it is formed by the continuation forward of the outer or pigment layer of the retina and the anterior portion of the secondary optic vesicle which does not take part in the formation of the nine innermost layers of the retina or more properly speaking, the receiving and transmitting portion of this structure.
 
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Fig. 77.
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The retina is a very thin, delicate structure, being onehalf millimeter thick at its thickest portion near the optic nerve, and gradually becoming thinner toward the ora serratta, where it is but one-tenth millimeter thick. It is firmly attached to the choroid at the ora serratta, and is firmly bound down at the head of the optic nerve by virtue of the optic fibers passing from it through the choroidal fissure (the opening of the choroid), L. Fig. "jd. There is a less secure attachment at the macula lutea (yellow spot), J. Fig. 78. In all other portions of the retina the nine innermost layers are very loosely attached to the outer or pigment layer; this attachment is accomplished simply by the interlacing of the rods and cones with the processes which project inward from the cells forming the pigment or outer layer. It is held in place mainly by the interocular pressure.
 
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Fig. 78. Cross section of the Eye, showing its construction.
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The retina receives its blood supply from the arteria centralis retina (central artery of the retina) which reaches it through the choroidal fissure after having traversed the optic nerve for some ten millimeters back of the eye ball, L. Fig. yy. This artery is an end artery, or in other words, it is not joined by any other set of arteries, but it sends its branches to all parts of the retina, A. Fig. 78, terminating ill arterial capillaries and turning back as venous capillaries ; these keep joining and rejoining and form the vena centralis retina (the central vein of the retina), which leaves the eye ball through the choroidal fissure by the side of the entrance of the artery. See darker vessels in Fig. 78.
 
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Fig. 79.
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By staining cross sections of the retina it is shown to be divisable into ten layers. Seven of these are nervous tissue, two of neuroglia or nervous connective tissue and one of pigmented epithelium. The nine innermost layers are transparent and are bound together by the fibers of Mueller, which is the nervous connective tissue of the retina. The outermost or pigmented layer is more intimately attached to the choroid than it is to the other layers.
The layers from within outward are: First, the inner limiting membrane, A. Fig. 79. Second, the layer of nerve fibers, B. Third, the layer of ganglionic cells or ganglionic (knot like) layers, C. Fourth, the mner molecular or plexiform layer, D. Fifth, the inner nuclear or granular layer, E. Sixth, the outer molecular or plexiform layer, F. Seventh, the outer nuclear or granular layer, G. Eighth, the outer limiting layer, H. Ninth, the layer of rods and cones, I. Tenth, the pigment layer, J. K. shows the hyaloid membrane which lies just inside of the retina and L shows the choroid ^which is the structure just outside of the retina. In this section the choroid is somewhat torn and separated.
The pigment layer, as before stated, is composed of a smgle layer of columnar epithelial cells which are long hexagonal cells separated from each other by a well defined, clear, cement substance. They have long protoplasmic processes which project inward and interlace with the rods and cones. In these cells are deposited pigment granules which remain in the base or outer portions of the cells when the eye is closed or in darkness. See G, Fig. 80. F is the lamina vitrea or Bruck's membrane of the choroid. However, when the retina is exposed to the light these pigment granules flow into the processes which lie amongst the rods and cones (C, Fig. 81), thus protecting these delicate structures from destruction by too intense light as well as forming a screen right amongst the rods and cones, to receive the image which is formed by the refracting surfaces of the eye. See Fig. 81. A is the choroid, B the bases of the pigment cells and C the processes lying amongst the rods and cones.
The layer of rods and cones (I Fig. 79), especially the cones, are the real sensory cells of the retina, as it is their function to produce the impulse which is transmitted to the brain and there produces the sense of sight. Each rod and each cone is at the end of a process which comes from a cell in the outer nuclear layer (G, Fig. 79). These pass through openings in the outer limiting membrane (H, Fig. 79).
 
 
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Fig. 80. Showing Section of Choroid.
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The cones, as their name implies, are of a conical shape, shorter than the rods ; they have a large oval inner portion with a finer tapering point extending outward into, and interlacing with the processes extending inward from the pigment layer. The oval or enlarged inner portion is striated longitudinally, while the outer or tapering portion is formed apparently of discs. The rods are long cylindrical cells striated longitudinally, and are divided into two segments at about their middle. Their function is not clearly established. There are estimated to be about three niilHon cones in the human retina, and the rods exceed this many times. The cones predominate in the macula or most acute area of sight, while the rods predominate in all other portions of the retina, thus proving the cones to be the real sensory elements. 
 
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Fig. 81.
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The next layer from without inward is the outer limiting layer. (EP, Fig. 79.) This is formed by the overlapping of the flattened ends or feet of the outer extremities of the fibers of Mueller or nervous connective tissue, which will be explained later; this layer is punctured by millions of openings through which pass the processes on the distal ends of which the rods and cones are found. The next layer inside of the outer limiting membrane is the outer nuclear or granular layer. (G, Fig. 79.) It is almost wholly composed of bipolar cells; that is, they have two processes, one runs outward through the outer limiting membrane and ends in a rod or cone, whilst the other runs inward and ends in a brush like end or tuft in the outer molecular layer. The single layer of cells seen just inside of the inner limiting membrane in Fig. 79, are supposed to be the cells connected with the cones whilst the cells connected with the rods lie in the middle and inner portion of this layer. There are several varieties of nerve cells found in this layer, the functions of which are undetermined, and will be omitted in this description. The next innermost layer is the outer molecular or plexiform layer. (F Fig. 79.) This layer is composed of the end arborisations of the bipolar cells in the outer nuclear layer, which run inward, and the distal end tufts on the processes from the bipolar cells in the inner nuclear layer, which run outward, as well as some other nerve cells which have processes which extend to a greater or less extent in this layer. They are known as amacrine (long fiber) cells; their function is undetermined, but they seem to be association elements to join different portions of the same layer.
 
The next innermost layer is the inner nuclear or granular layer, E, Fig. 79. This layer is mainly formed of bipolar cells; they send one process outward into the outer molecular layer which ends in a brush-like end or tuft interlacing with the tufts on the inner ends of the inner processes from the bipolars of the outer nuclear layer and send another process inward into the inner molecular layer which ends in an end tuft or arborisation. There are other nerve cells in this layer also, the function of which has not been determined. The next innermost layer is the inner molecular or plexiform layer, D, Fig. 79. This, like the outer plexiform layer, is almost wholly composed of the end tufts of the processes from the bipolar cells ; however these come from the bipolars in the inner nuclear layer which run inward and the processes which run outward from the ganglionic cells in the ganglionic layer and, as explained about the other cells found in the outer molecular layer, those found in the inner molecular layer have not been thoroughly studied and their functions ascertained farther than that they associate different areas of the same layer. The next innermost layer is the ganglionic (knotlike) cell layer, C. These cells might well be called relay cells, for they are very large ; they send from two to three processes outward into the inner molecular layer from each cell, which form tufts and interlace with the tufts on the inner ends of the processes from the bipolar cells in the inner nuclear layer. It is from these ganglionic cells that the axis cylinder processes grow which form the next innermost layer, which is called the nerve fiber layer, B. These axis cylinder processes are continuous from the ganglion cells of the retina into the nerve fiber layer. They pass out of the eyeball through the choroidal fissure and form the optic nerve, which will be described later, and are continuous from the ganglion cells in the retina to the nuclei at the base of the brain. The next innermost layer is the inner limiting membrane, A. It is formed by the expanded or foot-like inner ends of the fibers of Mueller. The fibers of Mueller are the sustentacular (sustaining or binding) tissue of the retina and are the same as the neuroglia cells found in the brain and spinal cord. They are long, branching, connective tissue cells which extend from the inner to the outer limiting membranes and the overlapping of their expanded, or foot-like, ends form both the inner and outer limiting membranes. Their function is to bind the nine innermost layers of the retina together. The retina becomes quite thin at the macula and the cells which otherwise would occupy the space are piled up around it. The processes from these displaced cells, as well as the fibers of Mueller, run obliquely outward and toward its center.
 
The optic nerve, M, Figs, "j^ and 'JJ, leaves the eyeball at the choroidal fissure (opening through the choroid) and is made up of the axis cylinder processes, which arise from the ganglionic layer of the retina C, Fig. 79, and lie between this layer and the inner limiting membrane, A, forming the nerve fiber layer of the retina B. These nerve fibers, or axis cylinder processes, pass through the openings in the lamina cribrosa (sieve layer), C, Fig. 77, just back of the choroidal fissure. The fibers are bare, or, in other words, devoid of the myeline (marrow) sheaths or white substance of Sw^an, until after they pass through the lamina cribrosa (sieve layer). This covering is then added and this addition adds greatly to the bulk or size of the nerve at the choroidal fissure and at points posterior to the lamina cribrosa. All the fibers which arise from the ganglionic cells in the retina transmit visual impulses toward the brain. However, in the optic nerve are found many fibers which grow from the brain to the retina. These are sensory fibers of association and carry sensory impulses which cause the closure of the pupil when the retina is exposed to bright light, as well as causing the dilation of the pupil when the eye is in darkness and govern co-ordinate movements of the two eyes.
 
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Fig. Cross section of optic nerve showing neuroglia stained dark and nerve fibers light.
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Fig. 83. Cross section of optic nerve showing nerve fibers stained dark and the neuroglia stained light.
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The arteria centralis retina (central artery of the retina), L, Fig. ^"j, B, Fig. 78, and H, Fig. 82, and the vena centralis retinae (central vein of the retina), I, Fig. 82, and dark vessels in Fig. 78, enter and leave the eyeball with the optic nerve, after entering its substance some ten or twelve millimeters back of the eyeball.
 
The optic nerve is surrounded by three coverings ; the outermost being the optic nerve sheath, A, Figs. 82 and 83, and F, Fig. yy. This covering is formed by the continuation backward around the nerve of the outermost portion of the sclerotic, Y, Fig. jd, and F, Fig. yj. This sheath is continuous backward to the optic foramen (open mg), where it is continuous with the dura mater (hard or firm mother) of the brain. The optic nerve sheath is quite firm and is composed of connective tissue bundles. Beneath the optic nerve sheath is found a space surrounding the nerve which is known as the intervaginal space, E, Fig. jy, and B, Figs. 82 and 83. This space is continuous through the optic foramen with the sub-dural and sub-arachnoidal spaces of the brain, and this intervaginal space is filled with the cerebro spinal fluid.
 
 
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Fig. 84. Showing cross section of the head of a bird,
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Lying within the intervaginal space is found the arachnodial sheath (spider web sheath), G. Fig. 82. This is a very thin, web-like membrane, joined quite intimately to the outer and inner sheaths of the optic nerve, by trabeculse (beams), which cross the intervaginal space.
 
The innermost covering or sheath is known as the pia mater (thin mother) or pial sheath, D, Fig. "jj, and C, Figs. 82 and 83. This is formed of glial tissue (nervous connective tissue) and from it is given off the septa or trabeculae (beams) which surrounds the bundles of nerve fibers and forms the frame work of the optic nerve and holds it together, E, Figs. 82 and 83, and darker longitudinal striations in M, Fig. J'j, The pial sheath and trabeculse is highly supplied with minute arteries and veins which furnish it with nutrition.
 
The optic nerve is composed of about eight hundred bundles of medulated (covered with myelin) nerve fibers, D, Figs. 82 and 83, and light longitudinal striations in M, Fig. 77, each bundle being composed of from six to seven hundred axis cylinder processes or nerve fibers, each of which are insulated or covered by the myelin (marrow) sheaths.
 
The optic nerves, B, Fig. 84, leave the eyeballs. A, Fig. 84, just internal to the posterior poles of the eyeballs, and run obliquely backward and inward through the orbit and pass into the cranial cavity through the optic foramen, then join together and form the optic commissure (uniting band), C, Fig. 84. In the commissure a part of the nerve fibers decussate (cross over) and pass backward in the optic tract of the opposite side, while a portion pass into the optic tract of the same side.
 
The optic tracts extend from the optic commissure to the base of the brain, where a part of the optic fibers enter the external and internal geniculate (knee-like) bodies, others, the optic thalmus (bed), and the rest go to the anterior corpora quadrigemina (meaning the four bodies).
These latter fibers are supposed to be the sensory association fibers, which communicate with the different centers of the brain and their function is for co-ordinate movements of the two eyes as well as reflex movements and sensibilities, while the optic fibers which enter the other basilar (lower) nuclei (nut) come in contact with the protoplasmic processes of the ganglion (enlarged or swollen) cells in these bodies. From these ganglion cells extend the axis cylinder processes, which run upward and backward through the optic radiations to reach the centers of sight which are situated along the calcarian fissure in the cuniate lobe of the brain, which is located in the posterior or occipital region. It is by the interpretation of the impulses created by the cones in the retina and transmitted through the conducting elements in the retina, optic nerve, optic commissure, optic tracts, external and internal geniculate bodies, optic thalmus, and optic radiations to these centers, that sight is accomplished by man.

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