Human Embryology and Morphology 16

From Embryology

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 XVI. The Organ of Hearing

The Nature of the Labyrinth

It often happens, when we seek to interpret the developmental changes which give rise to an organ or system of the human body, that a reference to the condition seen in certain groups of fishes — especially those belonging to the shark kind, selachians — gives us great assistance. This is true as regards the organ of hearing. In a shark or ray every part of the internal ear — the labyrinth with its semicircular canals — is already evolved with the exception of one part— the canal of the cochlea ; it is represented by a mere rudiment. The labyrinth of the shark is not an organ of hearing, for it is generally admitted that fishes are insensitive to sound-waves, but for the balancing or orientation of the body. Most men who have investigated the nature of the labyrinth of fishes agree that it represents a specialization of one of a series of superficial sense organs set on the sides of fishes — the organs of the lateral line — these also being connected with the functions of balancing and movement. Hence we find that the labyrinth begins as a pocket-like invagination of the ectodermal covering in the head region. The essential element of the labyrinth is its ciliated epithelium ; movements of the cilia, produced in various ways, give rise to stimuli which pass by the Vlllth nerve to the hind-brain. The auditory or cochlear part of the labyrinth appeared when the land-forms of vertebrates were evolved. In vertebrates above fishes the rudiment of the cochlea begins to be differentiated and an apparatus for converting sound waves into mechanical waves in the labyrinth is evolved.[1] A vibrating drum was established in the site of the first of the pharyngeal or visceral clefts. We must also suppose that in the piscine type, which gave origin to the ancestry of the mammals, the mammalian form of mandible was already evolved, for it is from remains of the primitive cartilaginous skeleton of the lower jaw that the malleus and incus are differentiated in the human and mammalian embryo.


The Structures which form the Organ of Hearing

In Fig. 228 is shown diagrammatically the derivation of the five elements which unite together to make up the organ of hearing. The five elements are : form, to become the epithelial lining of the membranous labyrinth. Some of its lining cells become differentiated into ciliated sensory epithelium.


(1) The otocyst — an area or plaque of ectoderm covering the head of the embryo above the first visceral cleft which becomes invaginated in a saccular


(2) A ganglion of somewhat uncertain origin, one view being that it arises from the neural crest as is represented in Fig. 228, but there is a growing conviction that some at least of the ganglionic cells arise from the ectoderm of the otocyst. The nerve cells form the cochlear and vestibular ganglia. Each cell sends out two processes, one to become connected with the epithelium of the otocyst, the other to end in groups of nerve cells in the floor of the hind-brain, their collective fibres forming the Vlllth nerve. The development of the auditory nerve thus resembles that of the posterior or sensory root of a spinal nerve.

(point at which acoustic ganglia arise)

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Fig. 228. Diagrammatic Section through the Cephalic Region of an Embryo, showing the origin of the Auditory System.


(3) The otocyst (membranous labyrinth) becomes surrounded by a capsule of cartilage — the periotic capsule. This ossifies from several centres, and forms the bony labyrinth and petro-mastoid.

(4) The Eustachian tube, the tympanum and antrum of the mastoid arise in connection with the pharyngeal pocket between the mandibular and hyoid arches ; the corresponding external cleft depression forms the point of origin for the external auditory meatus ; while out of the tissue between the internal pocket and external cleft, representing in position a " cleft-membrane," is formed the membrana tympani.

(5) The hyomandibular cartilage (Fig. 173), which served primarily to bind the cartilages of the maxillary process, mandibular and hyoid arches to the base of the skull, becomes the stapes. The incus and malleus arise from the upper end of the mandibular bar of cartilage (Fig. 132).


In fishes the auditory apparatus is composed of the three elements named first. In amphibians, reptiles and birds a membrana tympani is developed, Avhich is connected with the inner ear by an unjointed derivative of the hyomandibular cartilage, the columella. In mammals a tympanic cavity, external auditory meatus, and auditory ossicles appear.

External Auditory Meatus

A section along the external meatus of a newly born child shows that it is divided by a constriction into outer and inner parts (Fig. 229, B). The outer part is derived directly from the first cleft depression ; the inner part arises during the 2nd and 3rd months by a solid ingrowth of epithelium which, commencing from the cleft depression or pit, grows inwards until it comes in contact with the handle of the malleus, when it expands to form the fundus of the meatus (Figs. 230, 236).

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Fig. 229, A. A Section of the External Auditory Meatus of the Adult. B. A Section of the External Auditory Meatus at Birth. (After Symington.)

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Fig. 230. Showing the growth of the external meatal plug and its relationship to the tympanic recess of the pharynx. (Prof. Frazer.)


During the 7th month the deeper part of the meatus and outer aspect of the drum are formed by a breaking down of the central, and therefore older, cells of this ingrowth. Cartilage surrounds the part of the meatus derived from the cleft ; the floor of the deeper part is formed at birth by a fibrous plate continuous with the tympanic ring. In the adult the tympanic ring has grown outwards in the fibrous tissue, as we have already seen (p. 178), to form the tympanic plate and the inner two-thirds of the meatal floor. The squamous part of the temporal, which is developed in its roof, also grows outwards, and forms a thick, horizontal plate in the inner two-thirds of the meatal roof (Fig. 229, A and B). Over the roof lies the third temporal convolution.


The meatus is supplied in front by the nerve of the mandibular arch (auriculo-temporal branch) and also by a branch from the nerve of the hyoid arch — the facial. Why the vagus should supply it with a branch (Arnold's nerve) is obscure. In fishes a branch of the vagus passes backwards beneath the skin on each side and supplies the sense organs of the lateral line. Many regard the auricular branch of the vagus as a vestige of such a branch.


In the newly born child the membrana tympani is so obliquely set that its outer surface is almost in contact with the meatal floor (Figs. 229, B, 236). With the development in length of the meatus, it becomes more vertical in position. The deeper part of the meatus may fail to form, or the whole cleft may become closed. In such a case there is commonly a corresponding absence of development of the middle and internal ear.

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Fig. 231. Showing the Tubercles which arise round the First Visceral Cleft to form the External Ear.


The External Ear

Six tubercles appear on the mandibular and hyoid arches round the 1st cleft depression during the 6th week and form the basis of the external ear (Figs. 231 and 232). Three of these tubercles grow from the mandibular arch and form the tragus, crus of the helix, and helix ; three from the hyoid to form the lobule, antitragus and antihelix. The posterior margin of the ear, or descending helix, with the lobule, arise as a mere thickening or elevation of the skin behind the tubercles on the hyoid arch. During the latter part of the 2nd month and first part of the 3rd, the pinna begins to assume its definite form. The tubercles of the helix and antihelix send out processes which cross the upper part of the cleft and obliterate it, while the neighbouring tubercles fuse to form the definite parts of the ear. The posterior margin and lobule rise up at the same time as a free fold. The auricular tubercles may not fuse completely and thus leave fistulae between them. Such fistulae are commonly seen between the tragus and root of the helix, or between the antihelix and the helix. The mandibular part of the auricle is supplied, as one would expect from its origin, by the third division of the 5th, while the sensory fibres for the hyoid part come from the 2nd cervical by the great auricular and small occipital nerves.


Darwin's Tubercle

The human ear appears to be derived from a form in which the margin was pointed at the posterior superior angle, such as is seen in many of the lower forms of apes and mammals generally. With the retrogression of the j)osterior border or descending helix and increased development of the antihelix in the human ear, the posterior margin became infolded ; hence the tip appears as a tubercle on the inturned posterior margin or welt of the human ear (Fig. 232). The small size and restricted mobility of the external ears of higher primates result from the free manner in which these animals can turn their heads in the direction of sounds.

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Fig. 232. Showing the part of the Adult Ear formed by each Tubercle.


Muscles of the External Ear are derived from the platysma sheet and are supplied by the nerve of that sheet — the 7th or facial. The part of the platysma sheet which surrounds the external meatus and acts on the ear appears to have been the first of the facial muscles to be evolved. The ear muscles are not so reduced in man as in some other primates, such as the orang.


The Eustachian Tube

The Eustachian tube has usually been regarded as a derivative of the first of the inner cleft recess — a diverticulum of the lining membrane of the primitive pharynx between the mandibular and hyoid arches (Fig. 198, A). Recently Professor Frazer[2] has made a thorough enquiry into its development, and has found that its origin is more complicated than was supposed. In Fig. 233, A, the left half of the floor of the pharynx of a human embryo, five weeks old, is represented. Between the 1st and 2nd and between the 2nd and 3rd arches the lining mucous membrane of the pharynx is seen to dip outwards and at first is actually in contact ; as yet there is no sign of Eustachian tube or of tympanum. In Fig. 233, B, the opposite half of the floor of the pharynx of a human embryo towards the end of the 2nd month of development is shown ; the basis of the Eustachian tube and tympanum is now apparent as a wide recess between the first and third arches, the hyoid arch being squeezed outwards on the outer wall of the recess. The oblique fold forming the roof and posterior wall of the Eustachian tube is formed by the forward growth of the substance of the 3rd arch, which, it will be remembered, also contributes to the formation of the soft palate. The Eustachian tube retains through life the ciliated epithelial lining of the primitive pharynx. Its inner two-thirds is bounded behind by a triangular plate of cartilage, which is attached at its inner or pharyngeal end to the internal pterygoid plate, by its outer to the tympanic ring, both of which are probably derived from the palato-quadrate bar (Fig. 173, p. 172). The cartilage is developed in the 4th month of foetal life. The tympanic plate grows inwards and forms the floor of the outer third of the tube (Fig. 235), while the periotic capsule (petro-mastoid) which is developed above and behind the 1st cleft, grows forwards and forms the roof of its outer third. The part of the petro-mastoid which grows over it is the tegmen tympani (Fig. 229) ; it also forms the roof of the tympanum and of the antrum of the mastoid. The tensor tympani and tensor palati are developed on the mandibular side of the first cleft and are supplied from the nerve of the mandibular process through the otic ganglion.


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Fig. 233. Figures illustrating the Development of the Eustachian Tube and Tympanum. (After Prof. Frazer.) A. The Floor of the Pharynx of a Human Embryo Ave weeks old. The visceral arches are cut across and the inner cleft recesses indicated. B. The Floor of the Pharynx of a Human Foetus seven weeks old, showing the origin of the Eustachian Tube by an Evagination of the Pharynx opposite the 2nd or Hyoid Visceral Arch. T, rudiments of tongue on floor of pharynx ; L, larynx.

The Tympanum

The tympanum can scarcely be said to exist until the 3rd month of foetal life. Until then, the Eustachian recess ends in jelly-like tissue containing the cartilaginous bases of the malleus and incus. It is directed outwards and backwards between the periotic capsule to its posterior and inner side, and the external cleft depression (meatus) and developing squamosal to its outer (Fig. 234). As the tympanic recess, in which are represented both 1st and 2nd pharjoigeal pockets, extends outwards and backwards, the gelatinous tissue is absorbed, so that, in the later months of development, the malleus and incus and developing stapes, with the chorda tympani, become surrounded by the entodermal lining of the recess and thus appear to lie within the cavity thus formed — the tympanum. The tympanic plate forms the floor of the tympanum, the membrana tympani and squamosal its outer wall, while the petro-mastoid forms its inner wall and roof (Fig. 235). The nerve of the 2nd arch — the facial — lies in its inner or mesial wall. That part of the tympanum which lies above the level of the membrana tympani is named the attic, and contains the head of the malleus and body of the incus (Fig. 229).

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Fig. 234. Showing the condition of the Auditory Organs in a 7th week Human Foetus. (After Siebenmann.)

In carnivora and some other mammals the floor of the tympanum, formed by the tympanic plate, is inflated into a bulla, the tympanic bulla. Its meaning is unknown, but when a bulla is developed the antrum of the mastoid is small or absent.


Auditory Ossicles

In the 3rd month the auditory ossicles become clearly differentiated in cartilage in the mesodermal tissue between the meatal recess on their outer side and the Eustachian recess on their inner. Concerning their development, the exact researches of Broman[3] of Hammar, and of Jenkinson[4] give us a very full account. The malleus represents the upper or articular end of Meckel's cartilage (Figs. 237, 132) ; the incus, developed beyond the articular end of Meckel's cartilage, represents the cranial articular base — the quadrate of lower vertebrates. The stapes (Fig. 237) is developed at the upper end of the hyoid arch, the sides of the stirrup being formed round the dorsal end of the artery of the hyoid arch. Even in the 4th month of development the cavity of the tympanum has only reached the handle of the malleus (Fig. 236). The upper part of the drum (pars flaccida) is not yet differentiated. The attic, antrum, head of the hammer, and body of the incus are still outside the cavity of the tympanum.


600px Fig. 235. Showing the Cavities derived from the Eustachian Recess of the Primitive Pharnyx.


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Fig. 236. Section of the External Auditory Meatus, Drum and Tympanum of a Human Foetus in the 4th month of development. The meatal plug fills the deep part of the meatus and only the handle of the hammer is in the tympanic cavity. (After Broman.)

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Fig. 237. The Auditory Ossicles of the Left Side, seen on their Inner Aspect, during the 3rd month of development. (After Broman.)


The Antrum of the Mastoid represents the extreme outer or posterior end of the chamber derived from the extension of the Eustachian recess (Fjgs. 234 and 235). It is formed during the 6th and 7th months by an expansion of the tympanic cavity upwards and backwards in the surrounding mucoid tissue. Its use is uncertain, but it has frequently to be exposed by the surgeon to remove the effects of chronic middle-ear disease. At birth its outer wall is formed by the thin post-auditory part of the squamosal (Figs. 238 and 239). The squamosal forming its outer wall is then only 2 mm. thick, but every year until the 20th, or later, this plate increases nearly 1 mm. in thickness, so that by the 20th year the antrum is buried by a plate of bone about 20 mm. thick. There is a great individual variation, however, in the thickness of its outer wall. The antrum lies above and behind the level of the external auditory meatus ; the postauditory spine and supra-meatal triangle formed by the post-auditory part of the squamosal lie over it and serve as surface guides to it. The antrum opens in front into the attic of the tympanum. The tegmen tympani (Fig. 239) forms its roof and the petro-mastoid its floor and inner wall. The canal for the Vllth nerve runs down the inner wall of its mouth (Fig. 240), and in its inner wall is situated the external semicircular canal. The petro-squamosal suture in its roof (Fig. 239) and the masto-squamous suture on its outer wall (Fig. 181, p. 179) become closed the second year, and thus the escape of pus from it is rendered more difficult. The rudiments of the mastoid cells are already present as evaginations or pits of the antral lining at birth (Arthur Cheatle).


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Fig. 238. The Temporal Bone at birth, showing the formation of the Antrum between the Squamosal and Petro-mastoid.

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Fig. 239. A Transverse Section showing how the Walls of the Antrum are formed. Fig. 240. — Showing the outer aspect of the Petro-mastoid at birth after the Squamosal is removed.

Petro-Sguamous Sinus

We have seen (page 133) that the primitive vein of the head, part of which persists as the cavernous sinus, escapes from the cranial cavity just in front of the auditory capsule. Before escaping from the skull it receives a tributary from the hind-brain — which afterwards occupies the petro-squamosal suture. This vein, frequently of considerable size, runs forwards from the lateral sinus, and commonly ends in a tributary of the middle meningeal vein. It receives as it runs along, venules from the antrum and attic and may be the means of carrying infection from the middle ear to the lateral sinus or to the meningeal veins (Cheatle). The petro-squamous sinus may open in man, as it does in mammals generally, at the post-glenoid foramen, situated at the outer end of the Glaserian .fissure, near the base of the zygoma. The vein thus emerging may represent the primitive vein of the head,

The Membrana Tympani

As may be seen from Figs. 230, 234, the membrana tympani is of very considerable thickness until the gelatinous tissue in the tympanum is absorbed. It has an inner covering of entoderm and an outer of ectoderm. In the mesodermal tissue between the coverings lie parts of the malleus, incus and chorda tympani. As the gelatinous tissue round the fundus of the Eustachian recess is absorbed during the later months of foetal life, the tympanic lining membrane expands, and thus the handle of the malleus and chorda tympani come to appear as if they lie on the membrane, although really within it (Fig. 236j. The mucous lining of the tympanum covers them. The membrane is supported by the tympanic ring, the age changes of which have already been dealt with (p. 178). The membrane contains tissue derived from both mandibular and hyoid arches, and hence receives nerves and vessels from both.

The Membranous Labyrinth

The various parts of the membranous labyrinth of the internal ear are represented in Fig. 241. It consists of (1) the utricle ; (2) three semicircular canals opening into the utricle ; (3) the saccule ; (4) a canal uniting the utricle and saccule — ^from, or near which, springs the ductus endolymphaticus. All of these parts constitute the vestibular or balancing part of the labyrinth. (5) The cochlear canal - — the part connected with hearing. The labyrinth, although a complicated structure, has a very simple beginning. The cells of a certain area of ectoderm, situated above and behind the first cleft and lying against the 4th and 5th neuromeres of the hind-brain (Figs. 80, 93, 231) become invaginated during the 4th week. In this manner, and at this early date, there is formed a simple closed pyriform sac, the otocyst, which lies above the first visceral cleft and is soon surrounded by the mesodermal tissue which forms the primitive capsule of the cephalic part of the neural canal. The sac contains a fluid, the endolymph, and also otoliths are found in it later. The otocyst lies at first close to the side of the hind-brain with the ganglionic mass belonging to the 7th and 8th cranial nerves to its inner and anterior side (Fig. 93). The epithelial cells lining it, all of which are originally columnar, soon become flattened, except at the maculae acoustica, where they retain the columnar form and develop hair-like processes. The hair-like processes are to serve as levers and become capable of being moved by various means to evoke nerve stimuli. Under the influence of gravity otoliths serve to move them ; so do the currents in the semicircular canal as the head is moved and, so too, do the movements of the stapes. The hair cells become connected with the hind-brain by the auditory nerve fibres of the cochlear and vestibular ganglia. The otocyst clearly represents a sense organ which was primarily situated in the skin and through its hair-like processes was sensitive to the position and movements of the body. Its auditory function arose at a later stage.

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Fig. 241. Diagram of the Membranous Labyrinth.


In the lower vertebrates, as in the earlier embryonic stages of the higher mammals, the otocyst is of a saccular form with a stalk above — the ductus endolymphaticus (Fig. 242). The simplest form of vertebrate otocyst is seen in the lamprey ; the superior and posterior semicircular canals are present, but, as in the mammalian embryo, the primitive cyst is undivided into utricle, saccule and cochlear canal. The semicircular canals grow out from the vesicle as fiat, hollow plates, but only the circumferences of the plates persist, the centres disappearing.

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Fig. 242. The Otocyst in an Embryo of five weeks ; it shows a demarcation into the various parts of the Membranous Labyrinth.


The development and differentiation of the human otocyst has been closely studied by Prof. Streeter.[5] In Fig. 243 three stages depicted by him are represented. At the 5th week there are three parts : (1) the ductus endolymphaticus, at one time regarded as the stalk which connected the cyst with the surface of the head, but now known to be an outgrowth formed after the stalk is obliterated ; (2) the vestibular pouch or part ; (3) the cochlear pouch or rudiment. At the 6th week a higher stage of differentiation is reached ; all the parts of the adult labyrinth are indicated — the ductus and saccus endolymphaticus (both of uncertain import), the semicircular canals, with their ampullae ; the utricle and saccule. All of these are derived from the vestibular part of the otocyst. The cochlear rudiment has extended into a bent canal, and its communication with the saccule is constricted to form the canalis reuniens. In the 10th week all the various parts are present, almost in their adult form. The utricle and saccule are now separated and only communicate by means of the ductus endolymphaticus. The cochlear canal has assumed its spiral form.

The primitive utricle or vestibular pouch, which represents the main part of the otocyst, subdivides into the saccule and utricle (Fig. 243, C C, G"). The division occurs at the entrance of the endolymphatic canal, which thus comes to open into both saccule and utricle. The endolymphatic duct is enclosed in the petro-mastoid, its extremity appearing at the hiatus vestibuli, where it ends beneath the dura mater in a dilatation. The cochlear canal (scala media), the real auditory part of the labyrinth, although late in point of evolution, is not late in its developmental appearance. There is merely a rudiment of the cochlea in fishes and other amphibians. In reptiles, birds and monotremes it is a straight canal — the Lagena. Only in mammals is it arranged spirally. In it the organ of Corti is developed.

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Fig. 243. Three stages in the development of the Human Membranous Labyrinth. A, at the end of the 5th week ; B, at the end of the 6th week ; C, at the end of the 10th week. (Streeter.)

Periotic Capsule

The mesoderm surrounding the membranous labyrinth and dorsal aorta (internal carotid) above the first visceral cleft becomes cartilaginous at the end of the 2nd month of foetal life, forming the periotic capsule (Figs. 228, 234, 139). There are two centres of chondrification, one for the vestibular part — surrounding the vestibular division of the labyrinth, and one for the cochlear part — surrounding the cochlea. The course of the facial nerve indicates approximately their line of union. The cartilage of the cochlear part fuses with the parachordal or basilar cartilage ; the vestibular part becomes continuous with the occipital plate (see p. 137).


Perilymph System

The tissue which immediately surrounds the membranous labyrinth does not undergo chondrification, but becomes converted into an open meshwork of cells, the intercellular spaces containing perilymph. The chief or vestibular cistern of the perilymphatic system is formed round the saccule and utricle. In its tympanic or outer wall (Fig. 240) there is an oval area in which the fenestra ovalis and foot plate of the stapes are formed. Streeter[6] found that the vestibular cistern is the first to form, commencing at the stapedial plate when the foetus is 50 ram. in length (11 weeks old) ; an extension grows out along one side of the cochlear canal to form the scala vestihuli. Another area of the inner tympanic wall remains unchondrified, subsequently subdivided to form the fenestra rotunda (Fig. 240) and the aqueductus cochleae. In the 11th week a second cistern — the scala tympani — begins to form at the fenestra rotunda, growing along the side of the cochlear canal, opposite to the scala vestibuli, thus bringing that canal to lie between two perilymphatic spaces. The vestibular and tympanic extensions meet and fuse at the lip of the cochlear canal, at the end of the 3rd month, thus forming the helicotrema.

Ossification of the Petro-mastoid

About the end of the 4th month, four ossific centres appear in the periotic capsule ; one, the pterotic, gives rise to the tegmen tympani which forms the roof of the antrum, tympanum, and Eustachian tube ; the petro-squanious suture marks its outer edge ; the hiatus Fallopii marks its junction with a second centre — the opisthotic. This centre forms the posterior or vestibular half of the petrous bone. The pro-otic forms the anterior or cochlear half ; the mastoid part, which appears on the surface of the skull, is developed from the epiotic centre. While the greater part of the petro-mastoid is formed in a cartilaginous basis, the dense layers which form the immediate bony capsule of the labyrinth, the modiolus and lamina spiralis of the cochlea, are laid down by the lining membrane of the perilymphatic space.


The Mastoid

The mastoid part of the petro-mastoid is flat at birth ; about the 2nd year the mastoid process appears as a slight knob, and it gradually grows downwards to form a cephalic lever for the sterno-mastoid, splenius and trachelo-mastoid muscles. The period of its most active growth is marked by the eruption of the permanent teeth. In most mammals the mastoid grows out as a flat, wing-shaped process continuous with the occipital crest, and thus increases the basal area of the skull on which the neck muscles are inserted (Fig. 148). The post-auditory process of the squamosal forms a considerable part of the mastoid process ; it reaches to the apex and forms the anterior border (Fig. 181, C). As the mastoid process grows the diploic sj)aces within it enlarge into air spaces. Those round the antrum come to open into it, but the more distal remain closed. These spaces occupy the whole of the mastoid part of the temporal, but they also extend forwards in the post-auditory process of the squamosal, and may spread backwards to the occipital. Three varieties of mastoids are recognized : (1) Dense processes in which the air cells are minute or absent (infantile type of Cheatle) ; (2) a type containing numerous large spaces (pneumatic) ; (3) an intermediate type with large cells round the antrum, and a few small ones near the surface. The third type is the commonest.

The Floccular or Subarcuate Fossa

At birth there is a fossa situated on the posterior aspect of the petro-mastoid. It is filled with a process of the dura mater in the human embryo, but in all except the highest primates it contains the paraflocculus (Fig. 90), a part of the cerebellum which is quite vestigial in man. The posterior semicircular canal surrounds the fossa. This is the condition in most mammals throughout life, but soon after birth the fossa becomes closed in man, merely a remnant being seen above and internal to the hiatus vestibuli in the bone of the adult.


Organ of Corti

In Fig. 244 is given a diagrammatic section across the cochlear canal to show the manner in which its ectodermal lining is modified to form the organ of Corti[7] — the machinery concerned in producing auditory stimuli. The canal has become three-sided — one side lying against the scala vestibuli (vestibular wall), another against the scala tympani (tympanic wall), the third being peripheral or outer. The ectoderm on the vestibular wall atrophies and disappears — the fibrous base forming Reissner's membrane. The ectoderm is modified to form a secretory apparatus — the vascular body (striae vasculares). On the tympanic wall the ectoderm is modified to form, (a) hair or sensory cells, (6) supporting or pillar cells — comparable to neuroglial cells in the spinal cord, and fibres of Miiller on the retina ; (c) tectorial cells, producing a peculiar cuticular substance, which forms the tectorial membrane — in which the hair processes of the sensory cells are embedded. The auditory nerve fibres commence round the hair cells.

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Fig. 244. Diagrammatic section across the Cochlear Canal of a newly born child to show the differentiation of Ectodermal Epithelium to form the Organ of Corti. (After Keibel.)


While in the saccule, utricle and ampullae of the semicircular canals, the hair cells are planted on a fixed base, their hair-like processes being moved by otoliths acting under the influence of gravity, or by currents set up in the semicircular canal, the hair cells of the cochlea are planted on a movable base — tbe basilar membrane, wbich responds to every movement of the stapes, because of the displacement of perilymph in the adjoining scalae. The tectorial membrane bends the hair-like processes with every movement of the basilar membrane, because the tectorial membrane is attached to a fixed base on the spiral bony lamina while the hair cells rest upon a movable one.

The Acoustic Ganglia

The origin of the mass of nerve cells lying between the otocyst and hind-brain has already been mentioned (p. 224). It becomes divided into three parts : (1) the geniculate ganglion of the facial nerve, which is included in the petro-mastoid, but has no functional relationship to the labyrinth ; it gives rise to the great superficial petrosal nerve, chorda tympani and pars intermedia (root part of ganglion) in the same manner as a ganglion of the posterior root produces the sensory fibres of a spinal nerve (Dixon) (Fig. 93) ; (2) the vestibular part — applied to the vestibular part of the labyrinth ; (3) the cochlear part, which becomes applied to the cochlear canal (scala media). The differentiation of the vestibular and cochlear ganglionic masses proceeds at the same rate as the development of the membranous labyrinth.[8]

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Fig. 245. The differentiation of the Ganglion of the Labyrintli. (Streeter.) A. The otocyst and ganghon of a human embryo in the 4th week ; A^. In the 5tli week. The parts are those of the left side, and are viewed on their lateral aspect. B. From a foetus in the 7th week (16 mm. long) ; C. From a foetus in the 9th week (30 mm. long).

1. Branch from ampulla of superior canal. 3. Branch from utricle. 2. Branch from ampulla of lateral canal. 4. Branch from saccule. 5. Branch from ampulla of posterior canal. A. Cochlear ganglion. A1. Cochlear nerve.


In Fig. 245 four stages in the differentiation of the nerve equijjment of the ear are reproduced. The figures are those of Professor Streeter[9] and represent stages in the first, second and third months of develojDment. Towards the end of the first month the cochlear part becomes apparent {A) ; in the second month this part is undergoing rajjid growth [B) ; early in the third month (C) it has assumed a spiral form, and lies within the spiral lamina of the cochlea, and hence is often named the spiral ganglion. The cells of the spiral ganglion send out two sets of processes — to the organ of Corti (peripheral fibres), to ganglia situated in the hind-brain (root fibres). The cochlear fibres form the lateral root of the VIIIth nerve. The vestibular ganglionic mass becomes partially subdivided into a dorsal mass — connected with the areas of sensory cells in the utricle and the ampullae of the superior and external semicircular canals (Fig. 245, 1, 2, 3) ; the lower or ventral mass, which sends fibres to the saccule and posterior semicircular canal. The vestibular ganglion is lodged in the fundus of the internal auditory meatus. Its ingrowing or centripetal fibres form the mesial root of the Vlllth nerve. While the cochlear root enters the floor of the 4th ventricle superficial to the inferior peduncle of the cerebellum, the vestibular or mesial root passes deep to it. The lateral or cochlear root is connected with hearing, the mesial or vestibular with balancing.

Nerve Centres

(1) Cochlear or auditory. — By the end of the 5th week (Fig. 246) the ingrowing root fibres of the cochlear ganglion have reached a central mass of nerve cells (central cochlear mass) developed in the alar lamina of the hind-brain. The central cochlear ganglion gives rise to the acoustic tubercle (situated on the restiform body) and a lateral accessory nucleus on the outer aspect of the restiform body. By means of the striae acousticae and lateral fillet the cochlear central ganglia are united with the superior olive, inferior corpus quadrigeminum (mid-brain) and internal geniculate body (thalamencephalon) of the opposite side. Projection fibres connect the geniculate body with the cortex of the first temporal gyrus (see Fig. 113). Heschl's gyri (audito-sensory) of the first temporal convolution are already apparent at the beginning of the 7th month (see Fig. 247). The cortex of these gyri, with the neighbouring area of the first temporal, receives the fibres from the internal geniculate nucleus, and forms the audito-sensory areas. It is highly probable that the cortex of the greater part of the temporal lobe forms association areas, for the interpretation of sounds. The auditory centres are necessarily connected with the centres for sight, movement and speech, but the development of these connections is as yet imperfectly known.

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Fig. 246. Section across on half of the Wall of the Hind-Brain of an Embryo at the end of the first month. (His.) A. Peripheral cochlear ganglion; A^. Central cochlear ganglion ; B. Peripheral vestibular ganglion ; B^. Central vestibular ganglion ; C. Geniculate ganglion of facial ; C". Motor nucleus of facial nerve ; the motor nucleus of the Vlth cranial nerve is shown adjacent to that of the Vllth.

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Fig. 247. Lateral view of the Cerebrum of Foetus in the seventh month of development. (Retzius.) The audito-sensory area on Heschl's gyri is stippled.


(2) The ingrowing fibres of the vestibular ganglion pass beneath the inferior peduncle of the cerebellum to terminate in the nerve cells of the dorsal nucleus and Deiter's nucleus in the floor of the 4th ventricle (Fig. 246). These nerve cells and fibres are in no sense auditory, but concerned with the balancing of the body. Through the inferior peduncle of the cerebellum, the nuclei in which the vestibular root ends are connected with both the vermis and lateral cerebellar lobes. The cerebellum and acoustic ganglia arise from the same part of the hind-brain ; there is a close developmental relationship between the origin of the vestibular or balancing part of the ear and the cerebellum.

Internal Auditory Meatus

The internal auditory meatus is formed round the 8th nerve, its ganglia, and the 7th nerve. The falciform crest separates the fibres of the dorsal and ventral parts of the vestibular nerve. The meatus also contains a prolongation of the arachnoid and subarachnoid space. Fractures of the base of the skull frequently cross the petro-mastoid in the line of the internal auditory meatus, vestibule and membrana tympani. In such cases the cerebro-spinal fluid and perilymph may escape by the external auditory meatus.


Summary

A study of the development and evolution of the human ear leads to the following conclusions :

  1. That the otocyst was originally an external sense organ connected with the balancing of the body ; it became encysted above the first visceral cleft, and part of it became sensitive to sound waves.
  2. Parts of the dorsal laminae of the hind-brain were connected with it, and from those were developed the acoustic ganglia and nuclei, and probably also the cerebellum (see page 85).
  3. The first and part of the second clefts were modified in air-breathing forms, to become air passages for transmitting sounds.
  4. Parts of the skeletal bases of the first and second visceral arches became the auditory ossicles.



  1. G. L. Streeter, Journ. Experiment. Zoology, 1906, vol. 3, p. 543 ; 1907, vol. 4, p. 431 ; 1914, vol. 16, p. 149 (Results of Experiment on Developing Internal Ear) ; A. Keith Proc. Roy. Soc. Med. 1919, vol. xiii. p. 1 (Otological Section).
  2. Journ. Anat. 1914, vol. 48, p. 391.
  3. See Broman's excellent Normale und abnormale EntwicHung des Menschen, Wiesbaden, 1911.
  4. J. W. Jenkinson, Journ. Anat. and Physiol 1911, vol. 45, p. 305; Hugo Frey, Anat. Hefte, 1911, vol. 44, p. 363.
  5. American Journal of Anatomy, 1907, vol, 6, p, 139 ; 1907, vol. 7, p. 337 (Development of Ganglia of vii, viii),
  6. See Geo. L. Streeter, Contributions to Embryology, 1918, vol. 7, p. 5.
  7. For differentiation of Organ of Corti see 0. van der Stricht, Contrib. to Embryology, 1920, vol. 9, p. 109.
  8. Cameron and Jlilligan, Journ. Anat. and Physiol. 1910, vol. 44, p. 111.
  9. Amer. Journ. Anat. 1907, vol. 6, p. 139.



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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