Human Embryology and Morphology 15

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Keith, A. Human Embryology And Morphology (1921) Longmans, Green & Co.:New York.

Human Embryology and Morphology: 1 Early Ovum and Embryo | 2 Connection between Foetus and Uterus | 3 Primitive Streak Notochord and Somites | 4 Age Changes | 5 Spinal Column and Back | 6 Body Segmentation | 7 Spinal Cord | 8 Mid- and Hind-Brains | 9 Fore-Brain | 10 Fore-Brain Cerebral Vesicles | 11 Cranium | 12 Face | 13 Teeth and Mastication | 14 Nasal and Olfactory | 15 Sense OF Sight | 16 Hearing | 17 Pharynx and Neck | 18 Tongue, Thyroid and Pharynx | 19 Organs of Digestion | 20 Circulatory System | 21 Circulatory System (continued) | 22 Respiratory System | 23 Urogenital System | 24 Urogenital System (Continued) | 25 Body Wall and Pelvic Floor | 26 Limb Buds | 27 Limbs | 28 Skin and Appendages | Figures

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Chapter XV. Development of the Structures Concerned in the Sense of Sight

The Nature of the Eye

It is in vain that we appeal to comparative anatomy for light on the various stages in the evolution of the eye ; the eye of vertebrates is already fully formed in the earliest form known. Our knowledge of its origin and nature rests on an embryological foundation ; during the 4th and 5th weeks of human development we see the eye compounded from three sources : (1) the retina and optic nerve arise as an outgrowth of the neural tube ; (2) the lens arises from the ectoderm or body covering ; (3) the tunics and mechanism of accommodation from the mesoderm. The union of these three tissues to form the most marvellous contrivance of the human body is a product of countless ages of evolution. A comparison with the olfactory organ, already mentioned in the last chapter, assists us in understanding the peculiar nature of the eye. The olfactory plates are neural in nature ; their sensory cells give rise to the fibres of the olfactory nerves. The plaques of olfactory epithelium are situated near the open anterior end (neuropore) of the neural tube ; one can easily understand how they might shift towards the neural tube, merge with it, and become enfolded with the part which forms the olfactory bulb. Were we to implant the olfactory epithelium in the olfactory bulb we should produce a structure comparable to the retina. During an early part of the 4th week the two retinal plates are represented by depressions on the sides of that part of the medullary folds which are enclosed to form the fore-brain (Fig. 203). The epithelium which lines the optic evagi nations, clearly parts of the original surface covering of the embryo, does not become ependymal cells but, like the olfactory plates, gives rise to those highly modified sensory cells — rods and cones. Besides the rods and cones the optic evagination gives rise to nerve and other cells, in this respect resembling a typical part of the neural tube. It is thus clear that the olfactory and optic nerves are of a totally different nature to the other cranial nerves. We must seek the origin of the retina as a superficial sense organ, which has become so modified in the course of evolution that its primitive simple nature is hard to detect.

The structures concerned in the sense of sight are :

  1. The Eyeball and the Optic Nerve ;
  2. The Eyelids and Lachrymal Apparatus ;
  3. The Orbit, and the Muscles, Nerves and Vessels contained in it ;
  4. The Nerve Centres and Tracts.

The Eyeball

The condition of the eye in the 4th week of foetal life is shown diagrammatically in Figs. 203, 204. The three elements which unite to form the eyeball are as yet separate. They are :

  1. Ectoderm, which forms (a) the epithelium of the cornea, (&) the lens, and probably (c) the capsule of the lens.
  2. Neuroderm, which forms {a) the optic nerves, (b) sensitive retina, (c) pars ciliaris retinae, (d) uvea, (e) pigmentary layer of retina, (/) the hyaloid membrane.
  3. Mesoderm, which forms {a) outer tunic (sclerotic and fibrous cornea) ; (6) middle tunic (choroid, ciliary-choroid and iris) ; (c) the vitreous humour

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Fig. 203. Diagrammatic Section across Fore-brain of a Human Embryo in early part of 4th week to show the Optic Evaginations. (After Professor Bryce.)

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Fig. 204. Diagram of the Elements which form the Eyeball.

Structures derived from the Ectoderm

[1] (a) The lens. — The lens is developed by a saccular invagination of the ectoderm situated over the optic vesicle at the beginning of the 5th week (Fig. 205). About a week later it becomes a closed sac by the severance of its connection with the ectoderm, its wall being formed by a single layer of epithelial cells. The cavity of the lenticular vesicle is gradually obliterated by the cells of the posterior wall becoming elongated (Fig. 206) until they reach the anterior wall (7th and 8th weeks). Each elongated cell is transformed into a lens fibre.

The cells of the anterior wall retain their primitive form (Fig. 206). New lens fibres are added by the cells at the margin (equator) becoming multiplied and elongated. The central fibres, which are formed first, are the shortest, the fibres of every additional layer produced become longer than those of the previous layer, hence the concentric arrangement of fibres. Further, the fibres of each layer are so graduated in length that, when produced, they meet along certain lines which radiate from the anterior and posterior poles of the lens. The lens is relatively large at birth, being two-thirds of its final size ; growth continues until puberty, and even then has not ceased, for Priestley Smith found that there is an appreciable addition to its weight with each decade of life. It wiU thus be seen that the lens is an area of modified epidermis, and in manner of development closely resembles the sense organs in the skin of fishes and amphibians. Like the epidermis, it shows a tendency in the aged to be transformed into keratin. The oldest cells .(the central or nuclear fibres) alter first ; hence the central position of the cataract which occurs so frequently in old people.

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Fig. 205. A. Depression of the Ectoderm to form the Lenticular Vesicle, early in the 6th week. (Hochstetter.) B. Separation of the Vesicle later in the 6th week. Both figures represent Coronal Sections of the Fore-Brain and Optic Vesicle in Human Embryo. (After Hochstetter.)

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Fig. 206. The Formation of the Lens Fibres from the Epithelium on the Posterior Wall of the Lenticular Vesicle and the ingrowth of mesoderm to form the substance of the Cornea and Vascular Capsule of Lens, 7th week. (After Lindahl.)

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Fig. 207. Diagrammatic Section of the Anterior Part of the Eyeball to show the state of the Anterior Chamber and Iris in the 5th month. (After Broman.)

(b) The cornea. — The epithelial covering of the cornea is continuous with the surrounding ectoderm. It becomes transparent. The mesoderm which grows in between the lens vesicle and ectoderm forms the connectivetissue basis of the cornea and, by a later invasion, the vascular capsule of the lens (Fig. 206).

(c) The capsule of the lens is a cuticular membrane formed by the lenticular cells. Outside the proper capsule a vascular tunic is formed from the mesoderm (Fig. 207).

2. Structures formed from the Optic Vesicles (neurodermal element). — Each vesicle is well developed soon after the commencement of the 4th week (see Figs. 203, 204) ; even before the medullary plates have quite met to enclose the cavity of the fore-brain the optic vesicles have commenced as evaginations of those plates. They form a great lateral diverticulum on each side of the fore-brain — a cavity which becomes the third ventricle in the adult. The condition of the right optic vesicle at the end of the 6th week is shown diagrammatically in Fig. 208. The stalk or neck remains constricted to become the optic nerve while the vesicle enlarges and becomes invaginated to form the optic cup.

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Fig. 208. Diagram showing the connection with the Fore-Brain and condition of the Optic Stalk and Vesicle at the end of the 6th week of development. After Wilhelm His (1831-1904)

Invagination of the optic vesicle

Almost as soon as it begins to grow out the optic vesicle becomes invaginated, one half being pushed within the other (Figs. 205, A, B). The lenticular bud lies within the indentation. The remarkable fact was discovered by Dr. Warren Lewis that the optic vesicle, if transplanted, can cause overlying ectoderm to produce a new lenticular bud. The invaginated vesicle is known as the optic cup. Fine fibres unite the neuroblastic cells which line the optic cup with the deep aspect of the lenticular vesicle (Cirincione). The invagination of the vesicle, which takes place in an oblique manner — as if pressure had been applied from below and behind — leads to the closure not only of the cavity of the vesicle, but also to that of the distal half of the stalk (optic nerve). The point at which the central artery enters the optic nerve marks the upper limit of the invagination of the optic stalk (Fig. 209). By the 5th week the optic vesicle no longer communicates with the cavity of the fore-brain, but the recessus opticus in the floor of the third ventricle, above the chiasma, remains to mark the point at which the original evagination took place (Fig. 208).

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Fig. 209. Certain parts of the Eye during the 7th week of development. After Wilhelm His (1831-1904)

The parts formed from the optic vesicles are :

(a) The optic nerve is formed from the stalk of the optic vesicle. The wall of the stalk is at first composed of a single layer of columnar epithelium ; in the second month these cells produce a sponge-work of fibres on the surface of the stalk.[2] During the 8th week, the optic fibres, developed as processes of the neuroblasts of the invaginated layer, begin to grow into the brain from the retina along the sponge-work of the optic stalk.[3] Thus are formed the greater number of the fibres in the optic nerve. The optic fibres also form the chiasma in the floor of the third ventricle and the optic tracts on the wall of the fore-brain (Fig. 224). It will thus be seen that the optic nerves and vesicles are of the same origin as the cerebral vesicle — both representing modified parts of the wall of the fore-brain.

(b) The pigmentary layer of the retina is formed from the ensheathing or outer layer of the optic cup (Fig. 210). At first the wall of the optic vesicle is composed of a single layer of epithelium ; the outer or pigmentary layer of the retina retains this embryonic form. Pigment appears as early as the 6th week, commencing at the marginal border.

(c) The uvea is the layer of pigmented epithelium which covers the posterior surface of the iris. It is formed out of both outer and inner layers of the optic cup, and represents the rim of the cup (Fig. 211).

{d) The pars ciharis retinae is formed out of that part of the inner or invaginated layer of the optic cup which lies in the shadow of the iris, and is therefore inaccessible to light rays. It also retains the primitive columnar or partly transitional form of the epithelium (Fig. 211). The ora serrata marks the junction of the pars ciliaris retinae and sensitive retina.

Ciliary Processes

At the commencement of the 4th month, the pars ciliaris retinae[4] becomes plicated or puckered into 60 or 70 small folds (Fig. 211) ; mesoderm of the middle tunic (choroid) grows into the puckers and forms the ciliary processes. It should be observed that the lens lies within the optic cup and the ciliary processes are formed round the equator or circumference of the lens. The retinal epithelium which covers the ciliary processes is secretory in nature. It forms the aqueous humour, thus recalling the ependyma, which covers the choroid plexuses of the ventricles of the brain. It is strange that from the same layer as gives origin to nerve cells there should also arise supporting (neuroglial) and secretory cells, and as we shall see anon, the unstriped muscle of the iris (Fig. 211).

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Fig. 210. Diagrammatic Section of the Optic Cup and Lens. The cavity Is represented as gaping, whereas from the 5th week onwards the outer and inner walls are in contact.

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Fig. 211. Section of the Iris, showing the folding of the marginal part of the Optic Cup to form the ciliary processes and the origin of the Sphincter Muscle of Iris from the anterior or outer layer of cup, in Human Foetus in 6th month. (Szily).

(e) The sensitive retina is formed out of the inner or invaginated layer of the optic cup (Fig. 212). At first the inner wall is composed of a single layer of epithelium. The ciliary part of the retina retains this form. What is called the outer aspect of the primitive retina is directed towards the pigmented layer, but is separated from that layer by what remains of the cavity of the optic vesicle (Fig. 210). That cavity, it will be remembered, is a prolongation of the neural canal or ventricular cavity of the brain. The inner or vitreous aspect of the retina, corresponding to the outer

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Fig. 212. Diagrammatic Section across Optic Cup to show the manner in which the Cells of the Inner Layer of the Optic Cup are differentiated to form the Retina. (After Fiirst.)

A, B, C, D, E, show stages in the development of the Retina from the simple layer of Cells.

1. The outer stratum of Sense Cells (rods and cones).

2. The middle stratum connecting (bipolar) Nerve Cells.

3. The inner stratum of Ganglionic Cells and Fibres.

The cavity of the Optic Vesicle, which is closed by the invagination of the retinal layer within the cup and obliterated by the outgrowth of the rods and cones, is represented by a wide black zone in the diagram.

aspect of the neural tube, is directed towards the lens. The manner in which the complicated strata of the retina arise from the single layer has been investigated by Professor Fiirst, and is represented diagrammatically in Fig. 212. Differentiation starts at the centre of the optic cup and spreads towards the periphery. The original layer, while dividing and producing broods of cells, still retains its position, the daughter cells being pushed towards the vitreous aspect of the retina, and by the middle of the 7th month of foetal life all the retinal elements are present, the fovea centralis being the last feature to appear. As far as mammals are concerned the fovea centralis is a characteristic of the higher primates.

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Fig. 213. The Optic Stalk and Cup, viewed on the lower and lateral aspect, showing the Closure o£ the Choroidal Fissure.

On each surface of the retina is developed a cuticular or limiting membrane. Some of the original epithelial cells are elongated between the limiting membranes and form the fibres of Miiller. On passing from the margin of the cup to its centre all stages will be seen between the single layer and the multi-stratified condition. Ultimately three strata can be recognized in the retina. Beneath the outer limiting membrane the original cells remain as the retinal sense epithelium ; processes from these cells break through the outer limiting membrane to form the rods and cones ; the middle stratum forms bipolar cells ; beneath the inner limiting membrane ganglionic cells are formed. The middle stratum by its processes links together the sense epithelium and the ganglionic cells, and thus stands in the same relationship to the sense epithelium and ganglionic cells as a posterior root ganglion does to the touch corpuscles of the skin and the cuneate and gracile nuclei of the medulla. In many ways the development of the retina recalls the development of the spinal cord. Both form part of the neural tube.

The Choroidal Fissure

Occasionally congenital fissures are seen in the lower segment of the iris (coloboma iridis) or choroid (coloboma choroidea) (Fig. 214). A white line, due to absence of pigment, may be seen in the corresponding segment of the retina when the interior of the eye is examined. These are due to imperfect closure of the choroidal fissure. The choroidal fissure is the result of the peculiar mode in which the optic vesicle is cupped or invaginated. The lens grows into it from the malar or lower lateral aspect and becomes lodged in the anterior part of the depression ; the posterior part becomes the choroidal fissure (Fig. 209).

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Fig. 214. Coloboma or Cleft of Iris. (After Seggel.)

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Fig. 215. Remains of Pupillary Membrane. (After Prof. Hippel.)

The margins of the fissure unite, fusion commencing near its middle and spreading distally to the margin of the cup and proximally until it reaches the point of entrance of the hyaloid artery (Fig. 209). By the 8th week all traces of the fissure should have disappeared. Its union recalls the closure of the fissures in the upper lip. Coloboma and harelip are lesions of a similar nature. With the closure of the choroidal fissure the optic cup is completed. Its brim or margin becomes the site of the pupil.

Binocular Vision.— At first the optic vesicles are directed laterally in the human embryo, and in mammals generally the eyes are so directed, each eye having its own field of vision. In the Primates the eyes swing forwards during the second month ; binocular vision is thus made possible. With binocular vision and the combination of images appear in the highest primates : (1) A fovea centralis and macula lutea (L. Johnston) ; (2) A partial crossing of the optic fibres at the chiasma ; (3) Certain alterations in the attachments of the oblique muscles of the eyeball.

The primitive cavity of the Optic Vesicle (Fig. 210) is of some clinical importance. It is obliterated by the invagination of the vesicle ; the rods and cones formed in the inner or invaginated layer grow out across the cavity into the outer or ensheathing pigmented layer of the retina (Fig. 212). From accident or disease the retina may be detached, thus causing blindness ; the separation takes place between the pigmented epithelium, which remains in situ, and the rods and cones, which fall inwards with the nerve layer. Fluid then collects in the site of the primitive cavity of the optic vesicle. The optic part of the medullary plate in amphibian embryos has been transj)lanted and produced a retina in its new site. Some experimenters found that the ectoderm over the optic graft gave rise to a lens.[5]

3. Parts of the Eyeball formed from the Mesoderm. — After the optic vesicle has been invaginated against the lens, a continuation of the same layer of mesoderm, which surrounds and forms the coverings of the brain, envelops the optic cup and spreads inwards between the ectoderm and the lens. As may be seen from Figs. 205, A, B, the lens at first lies in contact with the inner or retinal wall of the optic cup, no mesoderm intervening. When they move apart in the 3rd month a connecting network of fibres appears between them.

The structures formed from the mesoderm are : (1) The vascular tunic of the lens. — While the choroid fissure is still open, mesodermal tissue passes into the cup and in it is formed the hyaloid artery, which is enclosed, when the lips of the fissure fuse. Mesodermal cells also enter by the pupillary margin (Fig. 206), and in this way the actively growing lens becomes surrounded by a vascular tunic, in which the hyaloid artery terminates. Beneath this tunic lies the proper capsule of the lens, which is formed from the epithelium of that body.

(2) The vitreous humour is formed out of the mesoderm which passes into the optic cup behind the lens. KoUiker was of opinion that the mesodermal cells were absorbed and that the vitreous was wholly produced from the lenticulo-retinal fibrillar network mentioned m a previous paragraph. The closure of the choroidaL fissure cuts the vitreous humour off from the mesoderm which covers the outer layer of the optic cup and becomes transformed into the tunics of the eyeball. The vitreous humour — like Wharton's jelly of the umbilical cord — represents an early form of embryonic tissue. It consists of cells embedded in a jelly-like matrix.

(3) The hyaloid artery is the vessel which supplies the mesodermal tissues within the optic cup ; it terminates in the vascular capsule of the lens (Figs. 209, 216). In the 7th month foetus a trace of the artery can still be seen passing through the vitreous humour from the optic disc to the lens. With the gradual obliteration of the artery, the mesodermal capsule of the lens becomes thin and clears up. A foetus born in the seventh month is blind, because the vascular capsule of the lens has not quite disappeared. The anterior part of the capsule — -filling the pupil — is the membrana pupillaris. A trace of the membrane may occasionally be seen crossing the pupil (Fig. 215). The part of the hyaloid artery within the optic nerve persists as the central artery of the retina. The canal of the artery within the vitreous humour, from the optic disc to the lens, remains as the hyaloid canal — a lymph path. The hyaloid artery may persist and cause partial or complete blindness. It disappears some days after birth in cats and rabbits.

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Fig. 216. Diagrammatic Section of the Eye showing the parts formed from the Mesoblast or Mesoderm. (After His' Model of the Eye of a 3rd month human foetus.)

(4) The aqueous chamber is formed between the cornea and lens, its walls being entirely of mesodermal origin. In Fig. 216 the mesoderm which invades the space between the ectoderm and lenticular vesicle is represented as forming not only the basis of the cornea but also the anterior wall of the vascular tunic of the lens, these two parts being supposed to become separated by the formation of the aqueous chamber. Dr. Lindahl[6] finds, however, that these two parts are formed separately, the mesodermal basis of the cornea in the 6th week and the lenticular capsule later — at the 9th week, the aqueous chamber being the potential chamber between these two formations (Fig. 206). Fluid begins to collect in the pupillary area of this space in the 6th month and spreads, so that in the 7th month the chamber has extended to the corneo-scleral junction. Almost to the time of birth, the anterior chamber of the aqueous is very shallow (Fig. 217), the lens lying near the cornea. Even so late as the 6th month (see Fig. 207) the posterior part of the aqueous chamber — the part which lies between the iris in front and the lens behind — is not opened up. We must regard the aqueous system as strictly comparable to the cerebro-spinal and not as part of the lymph system.

(5) The choroid, ciliary processes and iris form the middle or vascular tunic of the eye, and are developed out of the mesoderm which covers the optic cup. They form a vascular and pigmented covering through which the optic cup is nourished, and correspond to the combined pia mater and arachnoid membranes of the brain. The ciliary muscle is formed in this tunic in the 4th month. The iris is late in its development. The uvea on its deep surface is formed from the brim of the posterior surface (Fig. 211). In the 6th month the sphincter, and then the dilator muscles, are produced — their origin being peculiar. The muscle fibres arise from the epithelial cells of the uveal part of the optic cup (Fig. 211). The iris is fully formed in the 7th month and can then react to light.

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Fig. 217. Section of the Eye and Orbit at birth.

(6) The sclerotic is derived from the outer mesodermal envelope of the optic cup and is strictly comparable to the primitive cranial capsule. It is continuous in front with the cornea ; behind, with the sheath of the optic nerve and dura mater. In some vertebrates, but not in mammals, plates of bone are developed in the anterior half of the sclerotic, recalling the deposition of dermal bones in the primitive capsule of the brain.

The tapetum lucidum is absent in the human and primate eye. It gives the metallic lustre seen on the retinal surface of the eye of the ox, and is formed by a layer of fine fibres which are developed on the retinal surface of the cboroid.

(7) The capsule of Tenon, the bursa or connective-tissue socket of the eyeball, is developed in the mesoderm surrounding the eyeball. A lymph space separates it from the sclerotic, which is but slightly marked until after birth. The choanoid muscle (retractor bulbi or orbital muscle) which surrounds the sclerotic part of the eyeball as a muscular hood in mammals and vertebrates generally and arises in common with the external rectus, has become greatly reduced in man and the higher primates. Remains of the retractor bulbi — a striated muscle — have been described by Prof. Whitnall in the human orbit.[7] The unstriped muscle of the orbit occurs in two jDlaces ; the orbital part (Miiller's muscle) bridges the sphenomaxillary fissure ; the palpebral part forms the non-striated musculature found in the insertions of the levator palpebrae (Groyer). The non-striated muscle is supplied by sympathetic nerves. Its function is obscure, but is probably designed to regulate the pressure and circulation of the venous blood of the orbit.

Growth of the Eyeball

The eyeball is relatively large at birth, its diameter (17-18 mm.) being three-fourths of the adult diameter (24 mm.). In rate and precocity of growth it is comparable to the brain. The maculalutea and fovea centralis are said to have reached their full size at birth. A child born at the end of the 7th month is sensitive to light and darkness ; appreciation of form comes towards the end of the 1st year, while colours are not recognized until the 2nd or 3rd years — or in some cases the colour sense is not developed. The colours at the opposite ends of the spectrum (red- violet) are the first to be recognized (Edridge Green).

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Fig. 218. The Origin of tiie Bones entering into Formation of the Orbit.

Formation of the Orbit (Fig. 218). — The orbit is formed (1) above by the capsule of the fore-brain in which the frontal bone is developed ; (2) externally and below by the maxillary process. In the maxillary process the malar bone and superior maxilla (except the ascending nasal process) are developed. (3) The inner wall is formed by the lateral nasal process, in which the nasals, lachrymals and lateral mass of the ethmoid, are formed. The optic nerve enters the orbit between the orbito- and presphenoids, both of which help to form the orbit. The orbital surface of the great wing is formed at a later period in a membranous basis (see Fawcett, p. 135). The orbital plate of the malar cuts the orbit off from the temporal fossa ; it is develojDed in higher primates only. The nasal duct is formed between the maxillary and nasal processes (Figs. 154 and 219). In lower primates and mammals generally the hamular process of the lachrymal appears on the margin of the orbit ; the pars facialis lachrymalis is sometimes seen in the human skull (Fig. 154, p. 160). Mention has been made of the division of the orbital region of the primitive skull (Fig. 133) into orbital and temporal parts during the evolution of the temporomandibular joint (see page 139). The division is effected by the upbuildiAg of a lateral wall to the mammalian orbit ; the lateral orbital wall must be regarded as part of the bony scaffolding for giving attachment to the muscles of mastication.

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Fig. 219. Malformed Face of a newly born Child in which the Double Formation of the Eyelid is seen. The Lateral Nasal and Maxillary processes have not fused. Two folds separate the Eye from the Nasal Cavity. The inner fold represents the Caruncula Laclirymalis and the outer the Plica Semilunaris.

The eyelids are formed in the earlier weeks of the 3rd month by folds of ectoderm which commence above and below the superficial part of the eyeball. Mesoderm grows into the folds and forms the tarsal plates. The upper eyelid is formed from the capsule of the fore-brain, the lower from the maxillary process. About the middle of the 3rd month the edges of the lids meet, adhere, and remain adherent until the end of the sixth month. In rabbits, mice, kittens and puppies the lids are still closed at birth. The upper eyelid is developed in two parts — outer and inner ; occasionally a notch remains on the margin, and marks the point at which the two parts unite (Fig. 219). The upper end of the plica semilunaris is attached in the embryo at the position of the notch. The ectoderm on the deep surface of the lids retains a columnar shape, and forms the palpebral conjunctiva. It is continuous with the ectodermal stratum of the cornea. From the ectoderm between the adherent edges of the lids, buds grow during the 4th and 5th months, and form the eyelashes, Meibomian and other glands, in the same manner as hairs and sweat glands are developed. The Meibomian glands represent modified sebaceous glands, but the hair or cilia from which they primarily arose have vanished. The curious epicanthic fold is shown in Fig. 220. It is represented in all races during foetal life.

The plica semilunaris (Fig. 221), a fold of conjunctiva in the inner canthus of the eye, is a vestige of the third eyelid (membrana nictitans) which is fully developed in birds and reptiles. In the snake's eye Mayou found that this membrane formed what is commonly called the anterior lamina of the cornea ; it is the epithelium of this membrane which desquamates and renders the animal temporarily blind. The plica semilunaris is relatively large in the human foetus, reaching its maximum development in the 5th month. It is well seen in the cat, partially crossing the cornea as the lids are shut. The lachrymal papillae in man rub in the grooves at the outer and inner margins of the fold.

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Fig. 220. Epicanthic or Mongolian fold. (After Meckel.)

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Fig. 221. Diagram of the Plica Semilunaris and Lachrymal Oanaliculi.

The Lachrymal Gland

The Lachrymal Gland[8] arises at the beginning of the 3rd month as a number of ectodermal buds which spring from the fornix of the conjunctiva beneath the upper lid, and grow into the tissue of the outer and upper segment of the orbit (Fig. 222). The outer buds form the orbital part of the gland ; the more internal buds form the palpebral part. Smaller lachrymal glands may occasionally be found at the outer angle of the eye, which is the position occupied by the lachrymal glands of birds and reptiles (Wiedersheim). The lachrymal oanaliculi and sac and nasal duct are formed out of solid epithelial cords enclosed between the maxillary and lateral nasal processes (see p. 201). The canaliculi are formed during the 3rd month as sprouts from the upper end of the solid rod of epithelium representing the nasal duct. While the bud of the upper canaliculus opens at the inner end of the upper lid (Fig. 222, A), the inferior canaliculus extends some way along the lower lid before it conies to the surface (Ask). It may form a secondary communication nearer the inner angle of the eye, thus giving rise to a congenital lachrymal fistula. With the formation of the lachrymal canaliculus, part of the lower eyelid is cut off and forms the caruncula (Fig. 222, A and B).

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Fig. 222. A. — Showing the Termination of the Lower Lachrymal Canaliculus some distance from the Mesial End of the Lower Eyelid, in a foetus 2 months old. The tubular outgrowths of the lachrymal gland are also shown. B. — The Mesial Extremity of the Lower Eyelid cut off to form the Caruncula. The lachrymal outgrowths are more complex in structure. From a foetus in 4th month of development. (After Ask.)

The Orbital Muscles

[9] We have already seen that the head is composed of nine segments, at least four of these being occipital ; also, that each segment gives rise to a muscle plate (Fig. 149). The muscle plate of the maxillary or premandibular — usually called the first — segment forms the muscles supplied by the third cranial nerve — which is the motor nerve of that segment. The mesencephalon (crura cerebri) contains the corresponding segment of the neural tube. The ciliary muscle and sphincter of the iris also belong to this segment, and are supplied by the Ilird nerve (Fig. 223). The muscle plate of the mandibular, usually named the second head segment, produces the superior oblique. The dorsal decussation of the IVth nerves is evidently the result of a mutual migration of their nuclei — following Kapper's neuro-biotactic law. The muscle plate of the hyoid or third cephalic segment gives rise to the external rectus ; the Vlth nerve is the nerve for the somatic musculature of the segment, the Vlith supplying the splanchnic muscles.

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Fig. 223. Diagram of the Motor Nerves of the Muscles of the Eye derived from the 1st, 2nd, and 3rd Cephalic Segments.

The sensory nerves of these three segments are fused together in the three divisions of the Vth nerve. The ciliary ganglion is the splanchnic (sympathetic) ganglion of the premandibular segment. The nerves for the retractor muscle, the non-striated muscle of the upper eyelid, and the dilator fibres of the iris, issue from the upper three dorsal segments of the spinal cord, and reach the eye by the cervical sympathetic chain and cavernous plexus. The nerve fibres for the orbicularis palpebrarum pass out with the facial, but they are said to arise from, or have connection with, cells in the first segment of the neural canal (oculomotor nucleus). The ophthalmic division of the fifth represents the sensory somatic nerve of the same segment to which the third nerve belongs ; hence the reflection of pain along this nerve (frontal headache) in disorders of accommodation, the muscle of accommodation being the ciliary, and its nerve, the oculomotor, both also derivatives of the first segment. Mention has been made of the origin of the retractor muscle with the external rectus from the 3rd segment. The levator palpebrae superioris is a late delamination from the superior rectus.

Development of the Nerve Centres concerned with Sight

Five parts of the brain are concerned with vision. They are :

  1. The optic tracts.
  2. The basal centres surrounding the termination of the aqueduct of Sylvius in the 3rd ventricle.
  3. The optic radiations.
  4. The occipital lobes — in part at least.
  5. The angular gyri.

(1) The optic tracts are made up of fibres developed from the ganglionic cells of the retina and also in part of efferent fibres developed from cells of the basal ganglia in which the optic tracts are seen to terminate. The fibres grow in by the optic stalk, those from the nasal fields of the retina decussating in the floor of the third ventricle between the origins of the optic vesicles, and thus form the chiasma. The optic fibres grow backwards on the surface of thalamencephalon (see Fig. 224) and on the optic thalamus to reach the nerve centres which afterwards form the pulvinar, lateral geniculate bodies and the superior corpora quadrigemina. In these centres the optic fibres end. It is said that 80 per cent, of the fibres from the central area of the retina terminate in the lateral geniculate bodies.

(2) The basal ganglia. — The corpora quadrigemina. — Almost in every structure the human embryonic condition resembles the adult condition of lower vertebrates. A good example is seen in the corpora quadrigemina

The human foetus at the end of the 2nd month (Fig. 224) shows the corpora quadrigemina represented by a prominent thickening in the roof of the cavity of the mid-brain, which forms subsequently the aqueduct of Sylvius. The thickening is divided into lateral halves by a median sulcus, each half being nearly as large as the cerebral vesicle of that period. In Fig. 225 is shown the condition in an adult lizard ; there is one body on each side — the optic lobes or corpora bigemina. As the human foetus grows older, each lateral lobe becomes divided into an upper and lower part by the formation of a transverse groove, the upper and lower pairs of the corpora quadrigema being thus formed. The upper pair are connected with sight. In the mole they are vestigial, but in compensation the inferior corpora are well developed as they are connected with the sense of hearing, which is very acute in that animal.

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Fig. 224. Diagram of the Foetal Brain at the end of the 2nd month, showing the position in which the Optic Tracts are developed.

The pulvinar and lateral geniculate body, in which the upper division of the optic tract ends, are developed in the wall of the 3rd ventricle (thalamencephalon). The mid-brain is the part primarily connected with sight ; in the floor of its cavity — the aqueduct of Sylvius — are situated the motor nuclei for the muscles of the eye ; on its roof — the terminal centres for the optic tract (see p. 95). As the vertebrate scale of animals is ascended, the termination of the optic tracts is found to be transferred more and more to the centres on the thalamencephalon. The projection of retinal stimuli to the occipital cortex from the nucleus of the pulvinar is shown in Fig. 113.

(3) The optic radiations[10] connect the basal optic centres just named with the mesial surface of the occipital lobes, and vice versa. The fibres join the posterior part of the internal capsule, and pass under and round the posterior horn of the lateral ventricle to end in the cortex of the calcarine fissure and neighbourhood. The cortex in which the optic radiations terminate is divided by a narrow white stratum — the line of Gennari — into a superficial and deep layer.

(4) Tiie occipital lobe and calcarine fissure. — A mesial view of the 5th month foetal brain is shown in Fig. 226. The occipital lobe is already well formed ; its inner aspect shows the calcarine and jDarieto-occipital fissures. A section across the occipital lobe is shown in Fig. 226 ; the posterior horn is large ; the calcarine fissure indents its inner wall, giving rise to the calcar avis or hippocampus minor, a feature which is seen in the brains of nearly all mammals (Elliot Smith).

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Fig. 225. Mesial Section of the Brain of a Lizard, showing the resemblance to the Human Foetal Brain (Fig. 224), especially in the development of the Corpora Bigemina.

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Fig. 226, A. — View of the Mesial Surface of the Brain in the 5th month. B. — Section of the Occipital Lobe at the position marked in A.

The calcarine is one of the first fissures to be formed on the brain ; it appears early in the fifth month. This and the hippocampal depression, which is connected with the sense of smell, are the two fissures most commonly present in the mammalian brain. The posterior part of the calcarine fissure is a later formation, a,nd is distinguished as the retrocalcarine (see Fig. 127, p. 131). The optic radiations end in the cortex of the retro-calcarine fissure.<ref>For a description of the cortex of the visual areas see Elliot Smith, Journ. Anat. and Physiol. 1907, vol. 41, p. 237. See also references on p. 116. ,/ref> In Fig. 227 the condition of the occipital lobe in the 5th week is shown. The lateral ventricle is as yet undifferentiated into horns, and only the rudiment of the occipital lobe is present. The occipital lobe is produced by a backward growth of the cerebral vesicle, the posterior horn being produced as a diverticulum of the cavity of the vesicle. By the 5th month the occipital lobe has reached far enough back to overlap the cerebellum. The striate or visuo-sensory area of the human brain is not larger than that of the anthropoid ape, but the association or visuo-psychic area is infinitely more extensive. " Thus, we can take it that the superiority of the human over the ape's brain as a psychical organ must be the result mainly of the higher development of the association or peri-striate areas " (Elliot Smith).

(5) The angular gyrus is connected with the calcarine region by association fibres. In it are seated the word-seeing and word-understanding centres. It is developed round the posterior end of the 1st temporal or parallel fissure (Fig. 123). It is part of the wall of the cerebral vesicle. The first temporal or parallel fissure appears during the sixth month and is one of the primary fissures. It is found in the brains of all primates except the lowest.

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Fig. 227. Mesial Section of the Brain at the 4th week, showing the rudiment of the Occipital Lobe. After Wilhelm His (1831-1904)


It will thus be seen that three parts of the neural tube are specialized in connection with sight.

  1. The optic vesicle, an outgrowth from the fore-brain (thalamencephalon).
  2. The occipital region of the cerebral vesicle, which receives fibres projected from the basal nuclei connected with the eyes.
  3. The walls of the 3rd ventricle (thalamencephalon) and mid-brain (mesencephalon), in which the terminal nuclei of the optic fibres are developed.

The tunics of the eye are extensions of the embryological coverings of the brain. The choroid coat and the vitreous humour spring from the same layer as forms the pia mater and arachnoid. The sclerotic is a prolongation of the primitive cerebral capsule, in which the skull bones are formed. The optic vesicle carries with it a prolongation of the arteries and veins of the fore-brain. Part of the oj)tic vesicle is transformed into a secretory epithelium over the ciliary processes in the same way as the wall of the neural tube becomes a covering for the choroidal villi of the brain.

  1. E. Kallius, Ergebnisse der Anat. 1904, vol. 14, p. 234 ; 1906, vol. 16, p. 746 ; 1907 vol. 17, p. 463 (Development of Eye) ; F, Keibel, Keibel and Mali's Manual of Human Embryology, 1912, vol, 2.
  2. Prof. Robinson, Journ. Anat. and Physiol. 1896, vol. 30, p. 319.
  3. Prof. Cameron, Journ. Anat. and Physiol. 1905, vol. 39, p. 135.
  4. M. von Lenhossek, Verhand. Anat. Gesellsch. 1911, p. 81 (Dev. of Ciliary Body).
  5. Most of these instructive experiments have been carried out by American investi gators. For a recent list of researches see Spemann, Zool. Jahrbuch, 1912, vol. 32, Heft 1. W. H. Lewis, Amer. Journ. Anat. 1907, vol. 7, p. 259.
  6. Anat. Hefte, 1915, vol. 52, p. 195.
  7. Journ. Anat. and Physiol. 1912, vol. 46, p. 36.
  8. Development of lachrymal gland, F, Ask, Anat. Hefte, 1910, vol. 40, p. 489, 1908, vol. 36, p. 189.
  9. For an account of the development of orbital vessels see F. Dedekind, Anat. Hefte, 1909, vol. 38, p. 1. See also Dr. Eliz. A. Fraser, Proc. Zool. Soc. 1915, p. 299.
  10. For fuller details of optic tracts see Prof. Elliot Smith, Cunningham's Text-Book of Anatomy.

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Human Embryology and Morphology: 1 Early Ovum and Embryo | 2 Connection between Foetus and Uterus | 3 Primitive Streak Notochord and Somites | 4 Age Changes | 5 Spinal Column and Back | 6 Body Segmentation | 7 Spinal Cord | 8 Mid- and Hind-Brains | 9 Fore-Brain | 10 Fore-Brain Cerebral Vesicles | 11 Cranium | 12 Face | 13 Teeth and Mastication | 14 Nasal and Olfactory | 15 Sense OF Sight | 16 Hearing | 17 Pharynx and Neck | 18 Tongue, Thyroid and Pharynx | 19 Organs of Digestion | 20 Circulatory System | 21 Circulatory System (continued) | 22 Respiratory System | 23 Urogenital System | 24 Urogenital System (Continued) | 25 Body Wall and Pelvic Floor | 26 Limb Buds | 27 Limbs | 28 Skin and Appendages | Figures