Book - A Laboratory Manual and Text-book of Embryology 13

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Prentiss CW. and Arey LB. A laboratory manual and text-book of embryology. (1918) W.B. Saunders Company, Philadelphia and London.

Human Embryology 1918: The Germ Cells | Germ Layers | Chick Embryos | Fetal Membranes | Pig Embryos | Dissecting Pig Embryos | Entodermal Canal | Urogenital System | Vascular System | Histogenesis | Skeleton and Muscles | Central Nervous System | Peripheral Nervous System | Embryology History
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Chapter XIII. The Peripheral Nervous System

The nerves, ganglia, and sense organs constitute the peripheral nervous system. The peripheral nerves consist of bundles of myelinated and unmyelinated nerve fibers and aggregations of nerve cells, the ganglia. The fibers are of two types: afferent fibres which carry sensory impulses to the central nervous system, and afferent fibers, which carry effective impulses away from the nervous centers. The peripheral efferent fibers of both brain and spinal cord take their origin from neuroblasts of the basal plate. Typically they emerge ventro-laterally from the neural tube. Those arising from the spinal cord take origin in the mantle layer, converge, and form the ventral roots of the spinal nerves. The efferent fibers of the brain take origin from more definite nuclei and constitute the motor or effector portions of the cerebral nerves. The peripheral afferent fibers originate from nerve cells which lie outside the neural tube. Those sensory nerve cells related to the spinal cord and to the brain stem caudal to the Otic vesicle are derived from the ganglion crest, the origin of which has been described (Chapter X, p. 304).

A. Spinal Nerves

The spinal nerves are segmentally arranged and each consists of dorsal and ventral roots, spinal ganglion, and nerve trunks. In embryos of 4 mm. the ventral roots are already developing as outgrowths of neuroblasts in the mantle layer of the spinal cord (Fig. 357). The spinal ganglia are represented as enlargements along the ganglion crest and are connected by cellular bridges.

In 7 mm. embryos (five weeks old) the cells of the spinal ganglia begin to develop centrally directed processes which enter the marginal zone of the cord as the dorsal root fibers (Fig. 358). These fibers course in the dorsal funiculi and eventually form the greater part of them. Perpihcral processes of the ganglion cells join the ventral root fibers in the trunk of the nerve (Fig. 360) At ]0 mm. , (Fig. 359) the dorsal root fibers have elongated and the cellular bridges of the ganglion crest between the spina! ganglia have begun to disappear. In transverse sections at this stage (Fig. 325 and 350) the different parts of a spinal nerve may I be seen. The trunk of the nerve Just ventral to the union of the dorsal and ventral roots gives off laterally the dorsal, or /jc supply the dorsal muscles. The ventral ramus > ramus communicans to the sympathetic ganglion nj ventral (anterior) terminal rami. The efferent muscles of the lateral and ventral body wall, and integument of the same regions.

Fig. 357. Reconstruction of an embryo of 4 mm., showing the development of the cerebrospinal nerves (Streeter). X 17. C/., 2., etc., cervical spinal nerves.

At the points where the anterior and lateral terminal rami arise, connecting loops may extend from one spinal nerve to another. Thus in the cervical region superficial and deep nerve plexuses are formed. The deep cervical plexus forms the ansa hypoglossi and the phrenic nerve (Fig. 359).

Fig. 359. — Reconstruction of the

The Brachial and Lumbosacral Plexuses

The nerves supplying the arm and leg also unite to form plexuses. In embryos of 10 mm. (Fig. 359) the trunks of the last four cervical nerves and of the,first thoracic are united to form a flattened plate, the anlage of the brachial plexus. From this plate nervous cords extend into the intermuscular spaces and end in the premuscle masses. The developing skeleton of the shoulder splits the brachial plexus into dorsal and ventral lamirite. From the dorsal lamina arise the musculocutaneous, median, and 1 ulna nerv-fs; from the ventral lamina, the axillary and radial nerves.

In 10 mm. embryos the lumbar and sacral nerves which supply the leg unite 1 in a plate-like structure, the anlage of the lumbosacral plexus (Fig. 359). The J plate is divided by the skeletal elements of the pelvis and femur into two latera and two mL-rlian trunks. Of the cranial pair the lateral becomes the femor<d nerve; tht- median, the obturator nerve. The caudal pair constitute the sciatic nerve; the lateral trunk is the peroneal nerve, and the median trunk is the tibial.

Fig. 360. Transverse section Sympathetic ganglian I. embryo showing the spinal e mi components. Diagrammatlc

Save for the neurones from the special sense organs fnnsc, eye. and ear) whichi.V form a special sensory group, the neurones of the peripheral nerves, both spinal and cerebral, fall into four functional groups (Fig. 360).

(1) Somatic afferent, or general sensory, with fibers ending in the integumer of the body wall.

(2) Visceral afferent or sensory, with fibers ending in the walls of the visct

(3) Somatic efferent or motor, with fibers ending on voluntary muscle fibers

(4) Visceral efferent or motor: (a) with fibers ending about sj-mpathetio ganglion cells, which in turn control the smooth muscle fibers of the viscera a blood vessels (spinal nerves); or (b) with fibers ending directly on visceral muscl fibers (mixed cerebral nerves).

B. The Cerebral Nerves

The cerebral nerves of the human brain are twelve in number. They differ from the spinal nerves: (1) in that they are not segmentally arranged, and (2) in that they do not all contain the same types of nervous components. Classed according to the functions of their neurones they fall into three groups:

Sfecul Somatic Siksoky.

Somatic Motoi ob Efteuht.


II. Olfactory.

III. Oculomotor.

V. Trigeminal.

II. Optic.

IV. Trochlear.

VII. Facial.

VIU. Acoustic.

VI. Abducens.

XII. Hypoglossal.

X. Vagus complei, including XI. Spinal Accessory.

It will be seen (1) that the nerves of the first group are purely sensory, corresponding to the general somatic afferent neurones of the spinal nerves; (2) that the nerves of the somatic motor group are purely motor and correspond to the somatic efferent or motor neurones of the spinal nerves; (3) that the nerves of the third group are of mixed function and correspond to the visceral components of the spinal nerves.

The Special Somatic Sensory Nerves

I. The Olfactory Nerve, though purely sensory, has no ganglion. Its nerve cells lie at first in the olfactory epithelium of the nose and are of the bipolar type (fourth week). From these cells pjeriph++++eral processes develop and end directly at the surface of the olfactory epithelium {Fig. 361). Central processes grow toward the olfactory lobe and form the strands of the olfactory nerve. They end in the glomeruli of the olfactory bulb in contact with the dendrites of the mitral cells, or olfactory neurones of the second order. Some olfactory cells migrate from the epithelium, with which, however, they retain peripheral connections. Such bipolar cells, found along the entire course of the nerve, resemble ordinary dorsal ganglion cells. The olfactory nerve fibers are peculiar in that they remain unmyelinated. Nerve fibers from the epithelium of the vestigial vomero-nasal organ (of Jacobson) also end in the olfactory bulb. When the ethmoidal bone of the cranium is developed, its cartilage, as the cribriform plate, forms around the strands of the olfactory nerve.

Fig. 361. Diagram of the relations al the fibers in the olfactory nerve.

The ganglionated n. terminalis courses in close assocktion with the olfactory nerve. Its fibers end in the epithelium of the vnmero-nasal organ and of the nose. Although evidently a distinct nerve its significance is obscure.

2. The Optic Nerve is formed by fibers which take their origin from neuroblasts in the nervous layer of the retina. The retina is differentiated from the wall of the fore-brain and remains attached to it by the optic stalk (Fig. 343), hence the optic nerve is not a true peripheral nerve, but belongs to the central system of tracts. The neuroblasts from which the optic nerve fibers develop constitute the ganglion cell layer of the retina (Fig. 381). During the sixth and seventh weeks these cells give rise to central processes which form a furce fiber layer on the inner side of the retina. The optic fibers converge to the optic stalk and grow through its wall back to the brain. The cells of the optic stalk are converted into a neurogUa framework and the cavity is obliterated. In the floor of the fore-brain, at the boundary between telencephalon and diencephalon, the fibers from the median half of each retina at about the end of the second month cross to the opposite side, and this decussation constitutes the optic cHasma (from Greek letter Jf or "chi"). The crossed and uncrossed fibers constitute the-; optic trad which rounds the cerebral peduncles laterally and dorsally (Fig. 354), Eventually the optic fibers end in the lateral geniculate body, thalamus, and superior colliculus.

Efferent fibers, terminating in the inner reticular layer of the retina, are also present. In certain fishes where their function has been studied these fibers resemble visceral efferent components (Arey, Jour. Comp. Neurol., vol. 26, 1916).

8. The Auditory Nerve, or N. Acusticus, is formed by fibers which grow from the cells of the acoustic ganglion. l"he origin of these cells is unknown, though they appear in 4 mm. embryos just cranial to the otic vesicle {Fig. 358). The cells become bipolar, central processes uniting the ganglion to the luberculum acusticuti! of the myelencephalon and peripheral fibers connecting it with the wall of the otocyst. The acoustic ganglion is differentiated into the vestibular and spiral ganglia (Fig. 362). Its development has been studied by Streeter (Amer. Jour. Anat., vol. 6, 1907). The ganglion elongates and is subdivided into superior and inferior portions in 7 mm. embryos. The superior part supplies nerves to the utriculus and to the ampulla of the anterior and lateral semicircular canals. It forms part of the vestibular ganglion of the adult. Part of the inferior portion supplies nerves to the sacculus and to the ampulla of the posterior semicircular canal, and this portion, together with the pars superior, constitutes the veslibtitar ganglion. The greater part of the pars inferior is, however, differentiated into the fibers form the cochlear division of the acoustic nerve. This is distinctly separated from the central fibers of the vestibular ganglion which constitute the vestibular division of the acoustic nerve, the fibers of which are not auditory in function. The pars inferior of the vestibular ganglion becomes closely connected with the n. cochJearis, and thus in the adult it appears as though the sacculus and posterior ampulla were supplied by the cochlear nerve, Spiral ganglion, the peripheral fibers of which innervate the hair cells of the spiral organ (of Corti) in the cochlea. The spiral ganglion appears in 9 mm. embryos and conforms to the spiral turns of the cochlea, hence its name. Its central nerve

Fig. 362. The development of Uie acoust c ganglia and nerves The vestibular gangUon is finely stippled the sp lal ganglion coarsely stippled (Strecter)

n. The Somatic Motor Nerves

The nerves of ihis group, consisting of the three nerves to the eye muscles and the n. hypoglossus, are purely motor nerves, the fibers of which take origin | from the neuroblasts of the basal plate of the brain stem, near the midline. They i are regarded as the homologues of the ventral motor roots of the spinal cord, but I have lost their segmental arrangement and are otherwise modified. The nuclei I of origin of these nerves are shown in Fig. 364.

12. N. Hypoglossus. — This nerve is formed by the fusion of the ventral root I fibers of three to five precervical nerves. Its fibers take origin from neuroblasts ■ of the basal plale and emerge from the ventral wall of the myclencephaJon in several groups (Fig. 357). In embryos of live weeks (7 mm.) the fibers have converged venlrally to form the trunk of the nerve (Fig. 358). Later they grow cranially, lateral to the ganglion nodosum, and eventually end in the muscle fibers of the tongue fPig, 359), The nerve in its development unites with the first three cervical nerves to form the ansa hypoglossi. IXs nucleus of origin is shown in Fig. 364.

That thL- hypoglossal is a composile nerve homologous with the venlral roots of the ( spinal nerves is shown; (1) by the segmental origin of its fibers; (2) from the fact thai ii nucleus of origin is a cranial continuation of the ventral gray column, or nucleus of origin for that in the ventral sjMtial roots; (3) from the fafi rudimentary dorsal ganglia are dorsal root lo the hypoglossal, rudimenlary structure (Figs. 359 ai.>. cervical nerve may also degenerate at Neurol., vol. 20, I'oiO) has found two and . i.i. which dorsal roots extended to the root fascicles o( .alian embryos (pig, sheep, cat, etc.) al least (Froriep's ganglion) sends a. iriep's ganglion may be present as a absent and the ganglion of the first pig embryos Prentiss (Jour. Comp. y ganglia (including Froriep's) from, 1 ypoglossiil nerve (Fig, 121).

3. The Oculomotor Nerve originates from n^ ^ in t the mesencephalon (Fig. 339 B). The fibers em( * small iaes ventral surface of the mid-brain in the concavity du ceph lesure 359 and 364). The fascicles converge, form the tru ner\ id end ii premuscle masses of the eye. The nerve eventua es i the extrindt muscles of the eye save the superior oblique and external rectus. A branch is also supplied to the ciliary ganglion. In the chick embryo, bipolar cells migrate along the fibers of the oculomotor nerve to take part in the development of the ganglion. The ciliary ganglion of human embryos is derived entirely from the semilunar ganglion of the trigeminal nerve.

4. The Trochlear Nerve fibers take their origin from neuroblasts of the basal plate, located just caudal to the nucleus of origin of the oculomotor nerve. They are directed dorsally, curve around the cerebral aqueduct, and, crossing in its roof, emerge at the isthmus (Fig. 339 A). From their superficial origin each is directed ventrally as a slender nerve which connects with the anlage of the superior oblique muscle of the eye (Fig. 359).

6. The N. Abducens takes origin from a nucleus of cells in the basal plate of the myelencephalon, located directly beneath the fourth neuromere of the floor of the fourth ventricle (Figs. 359 and 364). The converging fibers emerge ventrally at a point caudal to the future pons, and, as a single trunk, course cranially, mesial to the semilunar ganglion, finally ending in the anlage of the external rectus muscle of the eye. Vestigial rootlets of the abducens and hypoglossal nerve tend to fill in the gap between these two nerves, according to Bremer and Elze.

m. The Visceral Mixed Nerves

The nerves of this group, the trigeminal, facial, glossopharyngeal, and vagus complex (vagus plus the spinal accessory), are mixed in function. The trigeminal nerve, beside its visceral nerve components, contains also numerous somatic sensory neurones which supply the integument of the head and face.

5. The Trigeminal Nerve is largely sensory. Its semilunar ganglion is the largest of the whole nervous system and is a derivative of the ganglion crest, but very early is distinct from the other cerebral ganglia (Fig. 358). It arises laterally at the extreme cranial end of the hind-brain. Central processes from its cells form the large sensory root of the nerve which enters the wall of the hindbrain at the level of the pontine flexure (Fig. 359). These fibers fork and course cranially and caudally in the alar plate of the myelencephalon. The caudal fibers constitute the descending spinal tract of the trigeminal nerve, which extends as far caudad as the spinal cord (Fig. 364). The peripheral processes separate into three large divisions, the ophthalmic, maxillary, and mandibular rami, and supply the integument of the head and face and the epithelium of the mouth and tongue.

The motor fibers of the trigeminal nerve arise chiefly from a dorsal motor nucleus which lies opposite the point at which the sensory libers enter the brain wall (Fig. 364). In the embryo these fibers emerge as a separate motor root, course along the mesial side of the semilunar ganglion, and, as a distinct trunk, 1 supply the premuscle masses which later form the muscles of mastication, rrom 1 the chief motor nucleus, a line of cells extending cranially into the mesencephalon . constitutes a second source of origin for motor fibers. In the adult, the motor] fibers form a part of the mandibular division of the nerve.

The facial, glossopharyngeal, and vagus nerves are essentially visceral in function. Their sensory fibers, chiefly of the visceral type, supply the sense organs

Fig. 3. Reconstruction of the cerebral n o( an embryo of 10.2 mm. {Stieeter). X 16.7. of the branchial arch .nrf yii respective nerves, caudaljy as the sm , . . „

the same origin and col e in the myelencephalon, supply the adjacent integument. i originate in the ganglia of theif J of the myelencephalon, course somatic sensory fibers,

In aquatic vertebrates, special somatic sensory fibers from the laler, e org, facial, glossopharyngeal, and vagus nerves, and their goi an cells torr I of t late, petrosal, and nodose ganglia. In human embryos ih jans of the il line i sented by ectodermal thickenings or placodes which o^- emporarilj r these j The nervous elements supplying these vestigial organs :ompletely ppeared.

7. The Facial Nerve is largely composed of efferent motor fibers which supply the facial muscles of expression. In 10 mm. embryos these fibers arise from a duster of neuroblasts in the basal plate of the myelencephalon located beneath the third rhombic groove or neuromere (Fig. 364). The fibers from these cells course laterally, and emerge just mesial to the acoustic ganglion. The motor trunk then courses caudally and is lost in the tissue of the hyoid visceral arch, tissue which later gives rise to the muscles of expression (Fig. 359) . The sensory fibers of the facial nerve arise from the cells of the geniculate ganglion, which are in turn derived from the ganglion crest (Streeter). This ganglion is present in 7 mm. embryos (Fig. 358), located cranial to the acoustic ganglion. The centrally directed processes of the geniculate ganglion enter the alar plate and form part of the solitary tract. The peripheral fibers in part course with motor fibers in the chorda lympani, join the mandibular branch of the trigeminal nerve, and end in the sense organs of the tongue. Other sensory fibers form later the great sttperficial petrosal nerve, which extends to the spheno-palatine ganglion.

Fig. 364. Reconstruction of the nucin of origin and termination oF the cerebral n of 10 mm. The somatic motor nuclei are colored red (Streeter).

The motor fibers of the facialis at first course straight laterad passing cranial to the nucleus of the abducens. The nuclei of the two nerves later gradually shift their positions, that of the facial ncr\-c moving caudad and laterad, while the nucleus of the abducens shifts cephalad. As a result, the motor root <rf the facial nerve in the adult bends around the nucleus of the abducens producing the genu or knee, of the former. The two together produce the rounded eminence in the floor of thi' fourth ventricle known as the f<u:ial colliculus.

9. The Glossopharyngeal Nerve takes its superficial origin just caudal to the otic vesicle (iMgs. 35S, 363 and 365). Its few motor fibers arise from neuroblasts in the basal plate beneath the fifth neuromeric groove. These neuroblasts form part of the nucleus ambiguus, a nucleus of origin which the glossophar\-ngeal shares with the vagus (Fig. 364). The motor fibers course laterally beneath the spinal tract of the trigeminal nerve and emerge to form the trunk of the nerve. These fibers later supply the muscles of the pharynx.

The sensory fibers of the glossopharyngeal nerve arise from two ganglia, a superior, or root ganglion, and a petrosal, or trunk ganglion (Figs. 359 and 365). These fibers constitute the greater part of the nerve and divide peripherally to form the tympanic and lingual rami to the second and third branchial arches. Centrally, these fibers enter the alar plate of the myelencephalon and Join the sensory fibers of the facial nerve coursing caudally in the solitary tract.

10, 11. The Vagus and Spinal Accessory. The vagus, like the hypoglossal, is composite, representing the union of several nerves, which, in aquatic animals, supply the branchial arches (Figs. 359 and 365). The more caudal fascicles of motor fibers take their origin in the lateral gray column of the cervical cord as far back as the fourth cervical segment. These fibers emerge laterally, and. as the spinal accessory trunk (in anatomy a distinct nerve), course cephalad along the line of the neural crest (Figs. 358, 359 and 365). Other motor fibers take their origin from the neuroblasts of the nucleus ambiguus of the myelencephalon (Fig. 364). Still others arise from a dorsal motor nucleus which lies median in position. The fibers from these two sources emerge laterally as separate fascicles and join the fibers of the spinal accessory in the trunk of the vagus nerve. The accessory fibers soon leave the trunk of the vagus and are distributed laterally and caudally to the visceral premuscle masses which later form the stcmo<leido-mastoid and trapezius muscles of the shoulder (Fig, 359). Other motor fibers of the vagus supply muscle fibers of the pharynx and larynx.

As the vagus is a composite nerve it has several root ganglia which arise as enlargements along the course of the ganglion crest (Figs. 359 and 365). The more cranial of these ganglia is the ganglion jugiUare. The others, termed accessory gtuif^lia. are vestigial structures and not segmentally arranged. In addition to the root ganglia of the vagus the ganglion nodosum forms a ganglion . of the trunk (Fig. 365). The trunk ganglia of both the vagus and glossopharyngeal nerves are believed to be derivatives of the ganglion crest, their cells migrating ventrally in early stages.

The central processes from the neuroblasts of the vagus ganglia enter the wall of the myelencephalon, turn caudalward, and, with the sensory fibers of the facial and glossopharyngeal nerves, complete the formation of the solitary tract. The peripheral processes of the ganglion cells form the greater part of the vagus trunks after the separation from it of the spinal accessory fibers.

Fig. 365. A reconstruction of Uk peripheral nerves in an embryo of 17.5 mm. (Streeter). X 16.7.

The Segmentation of the Vertebrate Head

The vertebrate head undoubtedly of fused segments. This was suggested to the earlier workers by the arrangement of the branchial arches (branchiomerism). and by the discovery, in the embryos of lower vertebrates, of so-called head carilies, homologous with mesodermal segments. (Note also the presence of neuromeres, p. 334.)

Assuming that the branchiomeres arc portions of the primary head segments — and there are recent observations which tend to disprove this— iheir segmentation is still not comparable to that of the trunk, for the branchial arches are formed by the segmentation of splaiiclmir mesoderm, tissue which in the trunk never segments. The branchial arches, therefore, represent a different son of metamerism.

Only the first three head cavities persist. These form the eye muscles, innervated by* the third, fourth, and sixth cranial nerves respectively. All the remaining muscles of the head are derivcif from the branchiomeres. From what has been said it is evident that one cannot compare the relation of the cranial nerves to the branchiomerie muscles with the relation of a spinal nerve to its myotomic muscles. For this reason the cranial nerves furnish unreliable evidence as to the primitive number of cephalic segments. Various investigators have set this number between eight and nineteen.

C The Sympathetic Nervous System

The sympathetic nervous system is composed of a series of ganglia and peripheral nei-ves, the fibers of which supply gland cells and the smooth muscle fibers of the viscera and blood vessels. It may function independently of the central nervous system and is hence known as the aulonomic system.

The sympathetic ganglion cells are derived from the cells of the ganglion crest. In fishes discrete cellular masses become detached from the spinal ganglia. At an early stage (6 to 7 mm.) in human development, on the contrary, certain cells of the ganglion crest migrate ventrally and give rise to a scries of ganglia, which, in the region of the trunk, are segmentally arranged (Figs. 139 and 360). According to Kuntz (Jour. Comp. Neurol., vol. 20, 1910), the primary source of these errant cells is the neural tube, from which they migrate along both dorsal and ventral nerve roots. At 9 mm. the ganglionated cord is formed and fibers connecting the sympathetic ganglia with the spinal nerves constitute the r communicanles (Streeter). The more peripheral ganglia (cardiac and cceliac) and the sympathetic ganglia of the head may be found in 16 mm. embryos (Fig. 366).

The cells which are to form the ganglia of the sympathetic chain migrate ventrally in advance of the ventral root fibers and take up a position lateral to the aorta. These ganglionic anlages are at first distinct, but soon unite with each other from segment to segment, forming a longitudinal cord of cells. After the formation of the primitive rami communicantes by the root fibers from the spinal nerves, centripetal processes from tJie sympathetic cells grow back and join the trunks of the spina) nerves. The visceral, spinal fibers later become myelinated j and constitute the white rami; the sympathetic, centripetal fibers remain unmyeIinated and form separately the gray rami. Nerve fibers appear in the paired longitudinal cords, which were at first purely cellular, in such a manner that segmental masses of cells {sympathetic ganglia) become linked by fibrous, commissural cords.

In the head region the sympathetic gangUa are not segmentally arranged, but are derived from cells of the cerebrospinal ganglia which migrate to a ventral position (Fig. 365). These cells likewise give rise to nerve fibers which constitute longitudinal commissures connecting the various ganglia of the head with the ganglionated cord of the trunk region. The small cranial sympathetic ganglia are probably all derived from the anlage of the semilunar ganglion (Fig. 366).

Fig. 366. The sympathetic system io a 16 mm. human embryo (Streeter In Lewis and Stfthr). X 7. The ganglkmated trunk is heavily shaded. The first and last cervical, thoracic, lumbar, sacraj and coccygeal spinal ganglia are numbered, a., Aorta; arc, accc<isoTy nerve; cor., carotid arter>'; cil., ciliary ganglion; coe., cocliac artery; Ht., heart; nod,, nodose ganglion; at., otic ganglion; prt., petrosal ganglion; s-m,, submaxHIary ganglioD; s.uks., superior mesenteric artery; »pk.-p., ^henopalatine ganglion; spl., splanchnic nerve; St., stomach.

The ciliary ganglion is related by a ramus communicans to the ophthalmic division of the trigeminal nerve and receives fibers from the oculomotor nerve. Its cells are probably derived entirely from the semilunar ganglion. The sphenopalatiMlM submaxillary, and otic ganglia probably take their origin from migrating cells of J the semilunar ganglion, but as they are connected with the geniculate ganglion the facial nerve, the latter may contribute to their formation. The sphenopal tine ganglion is connected directly with the semilunar ganglion by two 1 municatjng rami. The submaxillary ganglion is intimately related through thl mandibular division of the trigeminal nerve to the semilunar ganglion, while t otic ganglion is united to the latter by a plexus and is related to the glossopharyi gcal nerve through its tympanic branch.

The cerdcal ganglia lose their segmental arrangement and represent the fusion ' of from two to five ganglia of the cerWcal and upper thoracic region. The more distally located prevertebral ganglia (of the cardiac, cceliac. hjpogastric, and pelvic plexuses) are derived from cells of the neural crest which migrate to a greater distance ventrally (Fig. 366). The visceral ganglia (of the myenteric and submucous plexuses^ and the prevertebral cardiac plexus as well, are derived by Kunt2 chiefly from migratory cells from the hind-brain and from the vagus ganglia.

The sympathetic nerve cells give rise to axons and dendrites, and are thus t>'pically multipolar cells. Their axons possess a neurilemma sheath, but remain unmyelinated.

D. Chromaffin Bodies Suprarenal Gland

Certain celts of the sympathetic ganglia Jo not form nerve cells, but are transformed into peculiar gland cells which secretion formed by these cells causes the, chrome salts, hence they are ( from the ganglionated cord of as citromuffin bodies. Chrom mesenchimal tissue, form the 1 reaches a rclati\ely large size in hur 1

The Chromaffia Bodies of masses partly embedded in the dorsal si ce they may attain a diameter of 1 to 1.5 mm, several for each gan^on.

Similar chromaffin bodies niay ocoir in aU The largest of these structures found in the ab

duce an internal secretion. The to stain brown when treated with cells. Cells of this type deriv item give rise to structures knoi the cocliac plexus, together wil iprurenal gland, an organ whii '(Fig. 232).


the aortic chromaffin bodies (of Zuckerkandl). These occur on either side of the inferior mesenteric artwy, ventral to the aorta and mesial to the metanephros. At birth they attain a length of 9 to 12 mm. and are composed of cords of chromaffin cells intermingled with strands of connective tissue, the whole being surrounded by a connective tissue capsule. After birth the chromaffin bodies degenerate, but do not disappear entirely.

Glomus Caroticum

Associated with the intercarotld sympathetic plexus is a highly vascular chromaffin body known as the carotid gland. Its anlage has been first observed in 20 mm. embryos.

The Suprarenal Gland is developed from chromaffin tissue which becomes its medulla, and from mesodermal tissue which gives rise to its cortex. In an embryo of 6 mm. the anlage of the cortex is present, according to SouUt, and is derived from ingrowing buds of the coelomic epithelium. At 8 mm. the glands are definite organs and at 9 mm. their vascular structure is evident. The cellular elements of the cortex are at first larger than the chromaffin cells which give rise to the medulla. The ardages of the glands form projections in the dorsal wall of the ccelom between the mesonephros and mesentery (Figs. 221, 232 and 233).

The chromaffin cells of the medulla are derived from the cceliac plexus of the sympathetic system. In embryos of IS to 19 mm. (Fig. 368) masses of these cells begin to migrate from the median side of the suprarenal anlage to a central position, and later surround the central vein which is present in embryos of 23 mm. The primitive chromaffin cells are small and stain intensely. They continue their immigration until afUr birth. The differentiation of the cortex into its three characteristic layers is not completed until between the second and third years. The inner reiicular zone is formed first, next \h^ Jasciculate zone, and last the glomerular zone.

Fig. 367 .^Section through a chromatfin body in a 44 nun. human fetus (after Rohn). X 450. p, Mother chromaffin cells; sy, sympathetic cells; 6, blood vessel.

When the cells of the medulla begin to produce an internal secretion thqr I give the chrome reaction. By using extract of the aortic bodies, which are « tirely composed of chromaffin cells. Biedl and Wiese! have proved that its eSec^a like that of adrenalin, is to increase the blood pressure. The logical conclusion ub that the effect of adrenalin, an extract of the suprarenal glands, is due lo SOLf internal secretion produced by the chromaffin cells of the suprarenal medulla.

Fig. 368. — Transverse Biyce). sy. Sympathetic cells; gland.

Portions of the suprarenal aniage may be separated from the parent glnnd and (omi accessory suprartnals. As a rule, such accessor>- glaods are composed only of conical sub- i stance; they may migrate some dialance from their original position, accompanying the I genital glands. In fishes the cortex and medulla perast normally as separate organs.

E. Development of the Sense Organs

The sense cells of primitive animals (e. g.. worms) are ectodermal in origin and position. Only those of the vertebrate olfactory organ have retained this primitive relation. During phylogeny the cell-bodies of all other such priman sensory - neurones migrated inward to form the dorsal ganglion (Pariter), hei their peripheral processes either end freely m the epithelium or appropriate new cells to serve as sensory receptors (taste, hearing).

The nervous structures of the sense organs consist of the general sense organs of the integument, muscles, tendons, and viscera, and of the special sense organs which include the taste buds of the tongue, the olfactory epithelium, the retina, optic nerve and lens of the eye, and the epithelial lining of the ear labyrinth.

L General Sensory Organs

Free nerve terminaiians form the great majority of all the general sensory organs. When no sensory corpuscle is developed, the neurofibrils of the sensory nerve fibers separate and end among the cells of the epithelia.

Lamellated corpuscles first arise during the fifth month as masses of mesodermal cells clustered aroimd a nerve termination. These cells increase in number, flatten out, and give rise to the concentric lamellae of these peculiar structures. In the cat these corpuscles increase in number by budding.

The tactile corpuscles, according to Ranvier, are developed from mesenchymal cells and branching nerve fibrils during the first six months after birth.

n. Taste Buds

The anlages of the taste buds appear as thickenings of the lingual epithelium in 110 mm. (C H) fetuses (Graberg). The cells of the taste bud anlage lengthen and later extend to the surface of the epithelium. They are differentiated into the sensory taste cells, with modified cuticular tips, and into supporting cells. The taste buds are supplied by nerve fibers of the seventh, ninth, and tenth cerebral nerves; the fibers branch and end in contact with the periphery of the taste cells.

In the fetus of five to seven months taste buds are more widely distributed than in the adult. They are foimd in the walls of the vallate, fungiform, and foliate papillae of the tongue, on the under surface of the tongue, on both surfaces of the epiglottis, on the palatine tonsils and arches, and on the soft palate. After birth many of the taste buds degenerate, only those on the lateral walls of the vallate and foliate papillae, on a few fungiform papillae, and on the laryngeal surface of the epiglottis persisting.

m. The Olfactory Organ

The olfactory epithelium arises as paired thickenings or placodes of the cranial ectoderm (Fig. 369 A ) . The placodes are depressed to form the olfactory pitSf or fossce, about which the nose develops (Fig. 89).

In embryos of 4 to 5 mm, (Fig. 369) the placodes are sharply marked off from Ihe surrounding ectoderm as ventro-lateral thickenings near the top of the bead. They are flattened and begin to invaginate in embryos of 6 to 7 mm. In 8 embryos the invagination has produced a distinct pit, or fossa, surrounded everywhere save ventraily by a marginal swelling.

The later development of the olfactory organ is associated with that of the face. It will be remembered (cf. p. 145) that the first branchial arch forks into | the maxillary and mandibular processes. Dorsal to the oral cavity is the frontonasal process of the head, lateral to it the maxillary processes, and \'entral to it are the mandibular processes (Fig. 97). With the development of the nasal pits the fronto-nasal process is divided into paired laUral nasal processes and a single > median frontal process, from which are differentiated later the median nasal ffroc- ] esses, or processus globulares (Fig. 370). Thf nasal pits are at first grooves, each bounded mesially by the median frontal process and laterally by the lateral nasal process and the maxillary process (Fig. 370 .4). The fusion of the maxillary processes with the ventro-lateral ends of the median frontal process converts the nasal grooves into blind pits or fosse, shutting them off from the mouth cavity (Fig. 370). Thus in embryos of 10 to 12 mm. the nasal fossa has but one opening, the external naris, and is separated from the mouth cavity by an ectodermal plate (Fig. 369 D, E).

Fig. — Sections ih (X 13); C* Lai. Kosat proem Mrd. nasal prnrtss iloxillary procrss Epilketi.ll plaSe li the oi&ctory anlages of human embn'os. A, 4.9 nun. (X 20): B, fi-Sn m. {X 13); D and £. 10 mm. (.1, B and C from Kicbel and Hie.)

The ventro-lateral ends of the median frontal process enlarge and become the median nasal processes which fuse with the lateral nasal processes and reduce the size of the external nares (Fig. 370 B). Externally, the nares are now bounded ventrally by the fused nasal processes. The epithelial plates which sq)arate the nasal fosse from the primitive mouth cavity become thin membranous structures caudally, and, rupturing, produce two internal nasal openings, the primitive choana (Fig, 153). Cranially, the epithelial plate fc destroyed by ingrowing mesoderm of the maxillary process and median nasal process which replaces it, thereby forming the primitive palate (Fig. 369 D). The primitive palate forms the Up and the premaxillary palate. The nasal fosse now open externally through the external nares and internally into the roof of the mouth cavity through the primitive choana.

Fig. 370.— Two stages in the development of the jaws and nose. A , Ventral view of the end of the head of a 10.5 nun. human embryo (after Peter) i S, of an 11 .3 mm. embryo (after RabI).

Coincident with these changes the median frontal process has become relatively smaller and that portion of it between the external nares and the nasal fosse becomes the nasal septum (Fig. 370). As the fadal region grows and elongates, the primitive cboanx become longer and form slit-like openings in the roof of the mouth cavity. By the development and fusion of the palatine processes (described on p. 147) the dorsal portion of the mouth cavity is separated off and constitutes the nasal passages (cf. Figs. 371 and 373). The nasal passages of the two sides for a lime communicate thrcsugh the space between the hard palate and the nasal septum. Later, the ventral border of the septum fuses with the hard palate and completely separates the nasal passages (Fig. 372). The nasal passages of the adult thus consist of the primitive nasal fossse plus a portion of the primitive mouth cavity which has been appropriated secondarily by the development of the hard palate. The passages of the adult thus open caudally by secondary choana into the cavity of the pharynx.

Fig. 37 L— Transversa ^.eaiim ihro embryo. In the nasal seplum is il pas&agc<> anil jiabiine i)r<tcesses of a 20 mm. human on of the vomero-nasal organ (of Jacobson). X 30.

Part of the epithelium which lines the nasal fossfe is transformed into the sensory olfactory epithelium (Fig. 371). The remainder covers the concha: and lines the vomero-nasal organ (of jacobson), the ethmoidal cells, and the cranial sinuses.

The Vomero-nasal Organ {of Jacobson) is a rudimentary epithelial structure which first appears in 8.5 to 9 mm. embryos on the median wall of the nasal fossa (Fig, 369 C, E). The groove deepens and closes caudally to form a tubular structure in the cranial portion of the nasal septum (Fig. 371). During the sixth month it attains a length of 4 mm. Nerve fibers, arising from cells in its epithelium join the olfactory nerve, and it also receives fibers from the n. terminalis. In late fetal stages it often degenerates, but may persist in the adult (Merkel, Mangakis).

Fig. 372. — Transverse section through the nasal passages of a 65 mm. human fetus. X 14.

Fig. 373.— Right naul passage of a fetus at tenn (after Killian). /, MuUlo-tuihinal; II-VI, ethmoturbinals. The slight clevatioD at the left of / and // is the naso-turhinal.

Special cartilages are developed for its support (Fig. 371). The organ of Jacobson is not functional in man, but in many animals evidently constitutes a spedat olfactory organ.

The Conchas are structures which are poorly developed in man. Tliey ap-1 pear on the lateral and median walls of the primitive nasal fosss. The inferior I concha, or maxillo-lurbinal. is developed first in human embryos (Figs. 371 and | 372), It forms a ridge along the caudal two-thirds of the lateral wall and is I marked off by a ventral groove which becomes the inferior nasal meatus (Fig. 373). The naso-lurhinal is very rudimentary and appears as a slight elevation dorsal and cranial to the inferior concha (Fig. 373). Dorsal to the inferior concha arise five dbmo-turbinals. which grow progressively smaller caudaUy. According to Peter, the ethmo-turbinals ansi il wall of the nasal fossa, and, by

a process of unequal growth, are tranai„. .o the lateral wall (Fig. 372), Acccs- , sory concha; are also developed (Killian),

In adult anatomy, the inferior concha forms from / (Fig. 373), the middle concha Iroux I //, and the superior concha from /// and IV.

In addition to the ridges formed by the concha;, there are developed in the grooves J

t'thtno-turbinals the ethmoidal cells. between / anri //, Fig. 373) gives rise to Vne frontal Ji lary sinus grows out from the inferior recess of the sa nasal fossa becomes the sphenoidal sinus, which, as

The cells of the olfactory epithelium become ciliated, but only a small area, representin the primitive epithelial invagination, functions as an olfactory sense organ. The olfaetoryl cells of this area give rise to the libers which constitute the olfactory nerve (cf. p. 357).

' birth the frontal recess (localedi|

(J. During the third month ihe n.

e groove. The most caudal end of ihsl

■£, invades the sphenoiil'l

IV. The Development of the Eye

The anlage of the human eye appears in embryos of 2.5 mm. as a thickeningj and evagination of the neural plate of the fore-brain. At this stage the neural groove of the fure-brain has not closed optic vesicles are larger, but still may brain cavity (Fig. 374 A, B). ^ vesicle is attachefl to the ven

The thickening, flattening the optic vesicle gives rise to me tion also extends ventrally along the „ the chorioid fissure (Figs. 331, 375 and

At the same time that the optic vesicle ib ectoderm overlying the former thickens, as see plale, or optic placode. This plate invaginates opening of which closes in embryos of 6 to 7 i lens vesicle, which remains at first attached to

U, 330 and 382). At 4 mm. the ctcd by a wide opening with the I shown in Fig. 374 C, the optic , distinct optic stalk (cf. Fig, 343). n of the distal and ventral wall of 174 B-D). The area of invaginaand produces a groove known as embryo of 10 mm. (Fig. 376) the lens vesicle has separated from the ectoderm, which will form the epithelium of the coraea. The lens vesicle in earlier stages

—Stages in Uie early devdopment of the buin&n eye. A, B,at 4 mm. (X 27); C, a 23); D. at 6.25 mm. (X 18) (after Kdbel and Elze).


{Fig. 374 D) is closely applied to the inner wall of the optic cup, but now it has separated from it, leaving a space in which the vitreous body is developing. The inner retinal layer of the optic cup has become very thick and is appUed to the outer layer, so that the cavity of the primi- j^. tive optic vesicle is nearly obliterated (Fig. 376). Pigment granules have begun to appear in the outer cells which form the pigment layer oi the retimt. Mesenchymal tissue surrounds the FiG.37S.-The optic stalk, cup and lw» of a human optic cup and is beginning to embryo of 12.5 mm. The chotioid fissure has not yet ex, . , , , tended along the cqjtic stalk (from Fuchs, after Hocb++++make its way between the lens .tetter), x 90++++vesicle and the ectoderm. Here the anterior chamber of the eye develops later as a deft in the mesoderm. The distal mesenchymal tissue (next the ectoderm) forms the substantia propria of the Mcsentkyma Lens vesicle VUrtous body OpHc stalk Opfk ri

Epilkelium of cornea Pigmml laytr ofrtrm Nenous layer of et na

Fig. 376. — A transverse section through the optic cup stalk and lens of a 10 mm human embryo.

Epithelial layer t^ lens Pigment layer of the rt


Fig. 377. — Transverse section passing through the optic cup at the level of the chorioid fissure. The central artery of the retina is seen entering the fissure and sending a branch to the proximal surface of the lens; from a 12.5 mm. human embryo. X 105.

cornea and its posterior epithelium, while the proximal mesenchyma (next the lens) differentiates into the vascular capsule of the lens. The mesenchyme surrounding the optic cup is continuous with that which forms the cornea and later gives rise to the sclerotic layer, to the chorioid layer, and to the anterior layers of the dtiary body and iris.

Both the inner and outer layers of the optic cup are continued into the optic stalk, as seen in Fig. 376, This is due to the trough-like invagination of the ventral wall of the optic stalk, the chorioid fissure, when the optic vesicle is transformed into the optic cup (Fig. 375), Into the chorioid fissure grows the central artery of the retina, carrying with it into the posterior cavity of the eye a small amount of mesenchyme (Fig. 377). Branches from this vessel extend to the posterior surface of the lens and supply it with nutriment for its growth. At a later stage the chorioid fissure EtrUhdial Una closes, so that the distal rim of the optic cup forms a complete circle.

If the chorioid fissure fails to close, the optic cup remains open at one point and this results in the defective developmcnl of the iris, ciliary body, and chorioid layer. Such a defect is known as coloboma.

The old view that the development of the lens vesicle causes the formation of the optic cup by pushing in its distal wall has been disproved by W. H, Lewb, for if an aniage of the optic vesicle from an amphibian embryo be transplanted to some other part of the embryo, it can develop into an optic cup in the absence of a lens. Furthermore, it is the contact of optic vesicle with ectoderm that furnishes the stimulus for lens fonnalion, both normally and after transplantation to foreign regions, e. g., abdomen.

The lens vesicle, and its early development from the ectoderm, have been described. Its proximal wall is much thickened in 10 mm. embryos, and these cells form the lens fibers (Fig. 376) which will soon obliterate the cavity of the vesicle, as in embryos of 15 to 17 mm. (Fig. 378). The cells of the distal layer remain of a low columnar type and constitute the epithelial layer of the lens. When the lens fibers attain a length of 0.18 mm. they cease forming new fibers by cell division. New fibers thereafter arise from the cells of the epithelial layer at its line of union with the lens fibers. The nuclei are arranged in a layer convex toward the outer surface of the eye and later degenerate, the degeneration beginning centrally. Lens sutures are formed on the proximal and distal faces of the lens when the longer newly formed peripheral fibers overlap the ends of the shorter central fibers. By an intricate but orderly arrangement of fibers these sutures are later transformed into lens-stars of three, and finally of six or nine rays (Fig. 379). The structureless capsule of the lens is probably derived from the lens cells. The lens, at first somewhat triangular in cross section, becomes nearly spherical at three months (Fig. 379).

Pig. 378.^ — Section through the lens and corneal e derm of a 16 mm. pig embryo. X. 140.

The origin of the vitreous body has been in doubt, one view deriving it from the mesenchyma which enters the optic cup through the chorioid fissure and about the edge of lens, another view holding that it arises from cytoplasmic processes of cells in the retinal layer.

It is certain that the vitreous tissue is formed before mesenchyma is present in the cavity of the optic cup. Szily {Anat. Hefte, Bd. 35, 1908) regards this primitive vitreous body as a derivative of both retinal and lens cells, it forming a non-cellular network of cytoplasmic processes which are continuous with the cells of the lens and retina. With the ingrowth of the central artery of the retina, from which the artery of the lens passes to the proximal surface of the lens and branches on it, a certain amount of mesenchymal tissue invades the optic cup, and this tissue probably contributes to the development of the vitreous body (Fig. 377). The vitreous body may therefore be regarded as a derivative both of the ectoderm and of the mesoderm.

The mesenchyma accompanying the vessels to the proximal surface of the lens, and that on its distal surface, give rise to the vascular capsule of the lens (Fig. 377). On the distal surface of the lens this is supplied by branches of the anterior dliary arteries and is known as the pupillary membrane; the vessels disappear and the membrane degenerates just before birth. The artery of the

Fig. 380.— Section of the nervous layer of the retina from a diagiaimnaticaUy the cellular elements of the rel

mm. human fetus. At the left is shown according to Cajal. X 440.

lens also degenerates, its wall persisting as the transparent hyaloid canal. Fibrillae extending in the vitreous humor from the pars cilia ta of the retinal layer to the capsule of the lens persist as the zonula ciliaris or suspensory ligament of the lens.

Differentiation of the Optic Cup.— We have seen that of the two layers of the optic cup the outer becomes the pigment layer of the retina. Pigment granules appear in its cells in embryos of 7 mm. and the pigmentation of this layer is marked in 12 mm. embryos (Fig. 377).

The inner, thicker layer of the optic cup, the retinal layer, is subdivided into a distal zone, the pars caca, which is non-nervous, and into the pars optica, or the nervous retina proper. The line of demarcation between the pars optica and the pars oeca is a serrated circle, the ora serrata. The blind portion of the retinal layer, the pars caca, with the development of the ciliary bodies is differentiated into a pars ciliarts and pars iridis retina. The former, with a cx>rresponding zone of the pigment layer, covers the ciliary bodies. The pars iridis forms the proximal layer of the iris and blends intimately with the pigment layer in this region, its cells also becoming heavily pigmented (Fig, 379),

Fig. — Outer reticular layer

Inner nuclear layer

Inner reikular layer

The pars optica, or nervous portion of the retina, begins to difTerentiate proximaljy. the differentiation extending distally. An outer cellular layer and an \nncT fibrous layer may be distinguished in 12 mm. embryos (Fig. 377), These correspond to the cellular layer (ependymal and mantle zones) and marginal layer of the neural tube. In fetuses of 65 mm. (C R) the retina shows three layers, large ganglion cells having migrated in from the outer cellular layer of rods and cones (Fig. 380). In a fetus of the seventh month all the layers of the adult retina may be recognized (Fig. 381). As in the wall of the neural tube, there are differentiated in the retina supporting tissue and nervous tissue. The supporting elements, or fibers of Mailer, resemble ependymal cells and are radially arranged (Figs. 380 and 381). Their terminations form internal and external limiting membranes.

The neuroblasts of the retina differentiate into an outer layer of rod and cone cells, the visual cells of the retina, which are at first unipolar (Fig. 381). Internal to this layer are layers of bipolar and multipolar cells. The inner layer of multipolar cells constitutes the ganglion cell layer. Axons from these cells form the inner nerve fiber layer of optic fibers. These converge to the optic stalk, and, in embryos of 15 mm., grow back in its wall to the brain. The cells of the optic stalk are converted into neuroglia supporting tissue and the cavity of the stalk is gradually obliterated. The optic stalk is thus transformed into the optic nerve cell layer

'Interitai limiting membrane

The Sclerotic and Chorioid Layers, and their Derivatives

After the mesenchyme grows in between the ectoderm and the lens (Fig. 377), the lens and optic cup are surrounded by a condensed layer of mesenchymal tissue, which gives rise to the supporting and vascular layers of the eyeball. By condensation and differentiation of its outer layers, a dense layer of white fibrous tissue is developed, which forms the sclerotic layer. This corresponds to the dura mater of the brain. In the mesenchyme of 25 mm. embryos a cavity appears distally which separates the condensed layer of mesenchyme continuous with the sclerotic from the vascular capsule of the lens (Fig. 379). This cavity is the anterior chamber of the eye and separates the anlage of the cornea from the lens capsule.

An inner layer of mesenchyme, between the anlage of the sclerotic and the pigment layer of the retina, becomes highly vascular during the sixth month. Its cells become stellate in form and pigmented, so that the tissue is loose and reticulate. This vascular tissue constitutes the chorioid layer, in which course the chief vessels of the eye. The chorioid layer corresponds to the pia mater of the brain. Distal to the ora serrata of the retinal layer the chorioid is differentiated: (1) into the vascular folds of the ciliary bodies; (2) into the smooth fibers of the ciliary muscle; (3) into the stroma of the iris. The proximal pigmented layers of the iris are derived from the pars iridis retinae and from a corresponding zone of the pigment layer. Of these, the pigment layer cells give rise to the sphincter and dilator muscles of the iris. These smooth muscle fibers are thus of ectodermal origin.

The Eyelids appear as folds of the integument in 20 mm. embryos. The lids come together and the epidermis at their edges is fused in 33 mm. embryos (Fig. 379). Later, when the epidermal cells are cornified, separation of the eyelids takes place. A third rudimentary eyelid, corresponding to the functional nictitating membrane of lower vertebrates, forms the plica semilunaris. The epidermis of the eyelids forms a continuous layer on their inner surfaces known as the conjunctiva^ which in turn is continuous with the anterior epithelium of the cornea.

The Eyelashes, or cilia y develop like ordinary hairs and are provided with small sebaceous glands. In the tarsus, or dense connective tissue layer of the eyelids, which lies close to the conjunctival epithelium, there are developed about 30 tarsal (Meibomian) glands. These arise as ingrowths of the epithelium at the edges of the eyelids, while the latter are still fused.

The Lacrimal Glands appear in embryos of about 25 mm., according to Keibel and Elze. They arise as five or six ingrowths of the conjunctiva, dorsally and near the ext.:mal angle of the eye. The aniages are at first knob-like, but rapidly lengthen into solid epithelial cords. They begin to branch in 30 mm. embryos. At stages between 50 and 60 mm. (C R) additional aniages appear which also branch.

In 38 mm, (C R) embryos a septum begins to partition the gland into orbital and palpebral portions. This septum is complete at 61) mm. (C R), the five or six aniages first developed constituting the peripheral orbital part. Lumina appear in the glandular cords in fetuses or 50 mm. {C R) by the degeneration of the central cells. Accessory lacrimal glands appear in 300 mm. (C R) fetuses. The lacrimal gland Ls not fully diflercntiated at birth, being only one-third the size of the adull gland. In old age marked degcueration occurs.

The NasO'lacrimal Duct arises in 12 mm. embryos as a ridge-like thickening of the epithelial lining of the naso-lacrimal groove (Fig. 149), which, it will be remembered, extends from the inner angle of the eye to the olfactory fossa. This thickening becomes cut off, and, as a solid cord, sinks into the underlying mesoderm (Schaeffer). Secondary sprouts growing out from this cord to the eyelids form the lacrimal canals. A lumen, completed at birth, appears during the third month {Fig. 372).

V. The Development of the Ear

The human ear consists of a sound-conducting apparatus and of a receptive organ. The conveyance of sound is the function of the external and middle ears.

Fig. .2. — Two stagei in the early dcvclo[jiiie i< I

Horizontal section through the head sjid opm m auditory placode and ganglion (X 27); B, section tlirougn human embryo (X 33).

The end organ proper is the inner ear with the cochlear duct. Besides this acoustic function tb ear acts as an organ of equilibration.

The Inner Ear

The epithelium of the internal ear is derived from the ectoderm. Its first anlage appears in embryos of 2 mm. as a thickened ectodermal plate, the auditory placode (Fig. 382 A). These are developed dorsal to the second branchial grooves, at the sides of the hind bram opposite the fifth neuromeres (Fig. 383). The placodes are invaginated to form hollow vesicles which close in embryos of 2.5 to 3 mm., but remain temporarily attached to the ectoderm (Fig. 382 B).

Fig. 383.— Four EIze). X about 30. r-c. vesicle; Near. 4, i\eur. 5, vesicle of a 4 mm. human embryo (after Keibel and tbe anlage of the endolymph duct and sac; a.v., otic four and five of the myelencephalon.

Wali oj mydencepkaion—

The auditory vesicle, or otocyst, when closed and detached, is nearly spherical, but approximately at the point where it was attached to the ectoderm a recess, the ductus enddymphUicus, is formed. The point of origin of this recess is shifted later from a dorsal to a mesial position (Figs. 384 and 385 o). The endolymph duct corresponds to that of selachian fishes, which remains permanently open to the exterior- In man, its extremity is closed and dilated to form the endolymphatic sac (Fig. 385 e).

The differentiation of the auditory vesicle has been described by His. Jr.. and more recently by Streetcr (Amer. Jour. Anat., vol, 6, 1906). In an embryo of about 7 mm. the vesicle has elongated, its narrower ventral process constituting the anlage of the cochlear duel (Fig. 385 a). The wider, dorsal portion of the otocyst is the vestibular anlage, which shows indications dorsaUy of the developing semicirciJar canals. These are formed in 11 mm. embryos as two pouches — the anterior and posterior canals from a single pouch at the dorsal border of the otocyst, the lateral canal later from a lateral outpocketing (Fig. 385 c). Centrally the walls of these pouches flatten and fuse to form epithelial plates. In the three plates thus produced canals are left peripherally, communicating with the cavity of the vestibiJe. Soon the epithelial plates are resorbed, leaving the semicircular canals as in Fig. 385 d, e. Dorsally a notch separates the anterior and posterior canals. Of these canals, the anterior is completed before the posterior. The lateral canal is the last to develop.

Fig. 384.— Right half of a tmisverse sect on through tbe hind-brain and ot c %es cle showing the position of the endolymph duct From a human embryo 69 mm long (His).

In a 20 mm. embryo (Fig. 385 e) the three canals are present and the cochlear duct has begun to coil like a snail shell. It will be seen that the anterior and posterior canals have a common opening dorsally into the vestibiJe, while their opposite ends and the cranial end of the lateral canal are dilated to form ampulla. In each ampulla is located an end organ, the crista ampuUaris, which will be referred to later. By a constriction of its wall the vestibule is differentiated into a dorsal portion, the utriculus, to which are attached the semicircular canals, and a ventral portion, the sacculus, which is connected with the cochlear duct (Fig. 385 e,f). At 30 mm. the adult condition is nearly attained. The sacculus and utriculus are more completely separated, the canals are relatively longer, their ampullae more prominent, and the cochlear duct is coiled about two and a half turns (Fig. 385/). In the adult, the sacculus and utriculus become completely separated from each other, but each remains attached to the endolymph duct by a slender canal which represents the prolongation of their respective walls. Similarly, the cochlear duct is constricted from the sacculus, the basal end of the former becomes a blind process, and a canal, the ductus reunienSy alone coimects the two.

The epithelium of the labyrinth at first is composed of a single layer of low columnar cells. At an early stage, fibers from the acoustic nerve grow between the epithelial cells in certain regions and these become modified to produce special sense organs. These end organs are the cristae ampullares in the ampullae of the semicircular canals, the maculm acusticce in the utriculus and sacculus, and the spiral organ (of Corti) in the cochlear duct.

The cristas and maculae are static organs, or sense organs for maintaining equilibrium. In each ampulla, transverse to the long axis of the canal, the epithelium and underlying tissue form a curved ridge, the crista. The cells of the epithelium are differentiated: (1) into sense cells with bristle-like hairs at their ends, and (2) into supporting cells. About the bases of the sensory cells nerve fibers from the vestibular division of the acoustic nerve branch. The macular resemble the cristae in their development save that larger areas of the epithelium are diflferentiated into cushion-like end organs. Over the maculae concretions of lime salts may form otoconia which remain attached to the sensory bristles.

Fig. 385.— l-'ii;. ,iS.i.— Six slap,-s in Ihe ilfvclnjinicnt of the itiltrnal tar (Sln-etcr). X 2.1. The fiKun's -hiuv Inlrnil views <if mix Ids < if llu- lid mt-mbni nous bl>yHmh--ci ul (>.6 mm.; ft at <> mm.; <-nl t1 mm.; dm 1.1 mm.: r m 20 mm., ami / al Mi mm. The nilors ycllun' ami ml are used 1<> iniliiatc n>|M.Ttivcly the (-(H'hirar nml \'cstil>uliir divtsions of the onnistic nene nnd its ininulia. ahunrf. fixur, .\iva of wall where al>-<iqition is etwnpl<-le; trus, ems eiimmuw: r. v. l-il., liuctus semieirruliiris luleralis: c .>i'. pi"/., (luelus -rmicireularis iHisterior: r. v..<ii^.. iluetus >emii-ireiilaris sujierior or anterinr; niUni, i\urt\\^ eiH-hlearis; i.kA. pinifA.iiHhlfar nnloKf; rwrfi'/vmf*., a])[>i'niiix eniiulymiihatieus: iini„ saeriiliis; tiir. ,«rfW., saceiis t'n<1»1ym[>hailrusi \inus h(. /.i(., siiuis iiirieuli Lili^ralis: iilrif., tilriitiliis.

The true organ of hearing, the spiral organ, is developed in the bascU epithelium of the cochlear duct, basal having reference here to the base of the cochlea. The development of the spiral organ has been studied carefully only in the lower mammals. According to Prentiss (Amer. Jour. Anat., vol. 14, 1913) in pig embryos of 5 cm. the basal epithelium is thickened, the cells becoming highly columnar and the nuclei forming several layers. In later stages, 7 to 9 cm., inner and outer epithelial thickenings are differentiated, the boundary line between them being the future spiral tunnel (Fig. 386 ^4). At the free ends of the cells of the epithelial swellings there is formed a cuticular structure, the membrana tectoria^ which appears first in embryos of 4 to 5 cm. The cells of the inner (axial) thickening give rise to the epithelium of the spiral limbus, to the cells lining the internal spiral sulcus J and to the supporting cells and inner hair cells of the spiral organ (Fig. 386 By C). The out^r epithelial thickening forms the pillars of Corti, the outer hair cells, and supporting cells of the spiral organ. Differentiation begins in the basal turn of the cochlea and proceeds toward the apex. The internal spiral sulcus is formed by the degeneration and metamorphosis of the cells of the inner epithelial thickening which lie between the labium vestibulare and the spiral organ (Fig. 386 B, C). These cells become cuboidal, or flat, and line the spiral sulcus, while the membrana tectoria loses its attachment with them. The membrana tectoria becomes thickest over the spiral organ and in full term fetuses is still attached to its outer cells (Fig. 386 C).

Hardesty (Amer. Jour. Anat., vol. 18, 1915), on the contrary, asserts that the membrana tectoria is not attached permanently to the cells of the spinal organ.

From what is known of the development of the spiral organ in human embryos, it follows the same lines of development as described for the pig. It must develop relatively late, however, for in the cochlear duct of a newborn child figured by Krause the spiral sulcus and the spiral tunnel are not yet present.

The mesenchyme surrounding the labyrinth is differentiated into a fibrous membrane directly surrounding the epithelium, and into the perichondrium of the cartilage which develops about the whole internal ear. Between these two is a more open mucous tissue which largely disappears, leaving the perilymph space. The membranous labyrinth is thus suspended in the fluid of the perilymph space. The bony labyrinth is produced by the conversion of the cartilage capsule into bone. In the case of the cochlea, large perilymph spaces form above and below the cochlear duct. The duct becomes triangular in section as its lateral wall remains attached to the bony labyrinth, while its inner angle is adherent to the modiolus. The upper perilymph space is formed first and is the scala vestibuliy the lower space is the scala tympani. The thin wall separating the cavity of the cochlear duct from that of the scala vestibuli is the vestibular membrane (of Reissner). Beneath the basal epithelium of the cochlear duct a fibrous structure, the basilar membrane, is differentiated by the mesenchyme. The modiolus is not preformed as cartilage, but is developed directly from the mesenchyme as a membrane bone. The development of the acoustic nerve has been described on page 358 with the other cerebral nerves.

Fig. 386. — Three staets in ihc dilTertntiation of the basai epithelium of the cochlear duct to form the s;>iral ori;an (of Corti), internal spiral sulcus and labium vestibulare. .1. Section through the cochlear duct of an 8,5 cm. pig fetus (X 120); B, the same from a 20 cm, fetus (X 140); C, from a 30 cm. fetus (near term] (X HO), Epithelium of spiral sulcus; /i,f., hair cells; i.cp.c, inner epithelial thickening; i.h.c., inner hair cells; i.pil.. inner pillar of Corti: l>ib. test,, labium vestibulare; limb, sp., limbus spiralis; m. hm., basilar membrane; m. lecl.. membrana tectorial m. veil., vestibular membrane; n. cwA., cochlear division of acoustic nerve; o.rp-c, outer epithelial thickeninR; o.h.c., outer hair cells; s.sp., sulcus spiralis; sc.lymp., scala timpani; si. II.. strij>e of Hcnsen: l.sp., spiral tunnel.

The Middle Ear

The middle ear cavity is differentiated from the first pharyngeal pouch which appears in embryos of 3 mm. The pouch enlarges rapidly up to the seventh week, is flattened horizontally, and is in contact with the Malleus ectoderm (Fig. 168). During the latter ^MMtrL.e) part of the second month, in embryos of 24 mm., the wall of the tympanic cavity is constricted to form the auditory Tympanum (Eustachian) tube. This canal lengthens ^ ^- O'fch II

(ReicherVs cartilage) and its lumen becomes slit-like during

Fig. 387 — Diagram snowing the branchial the fourth month. The tympanic cavity arch origin of the auditory ossicles.

is surrounded by loose areolar connective tissue in which the auditory ossicles are developed and for a time are embedded. Even in the adult, the ossicles, muscles, and chorda tympani nerve retain a covering of mucous epithelium continuous with that lining the tympanic cavity. The pneumatic cells are formed at the close of fetal life.

The development of the auditory ossicles has been described by Broman (Anat. Hefte, Bd. 11, 1899), with whose general conclusions most recent workers agree. The condensed mesenchyma of the first and second branchial arches gives rise to the ear ossicles.

The malleus and incus are differentiated from the dorsal end of the first arch (Fig. 387). The cartilaginous anlage of the malleus is continuous ventrally with Meckel's cartilage of the mandible. Between the malleus and incus is an intermediate disk of tissue, which later forms an articulation. When the malleus begins to ossify it separates from Meckel's cartilage. The incus is early connected with the anlage of the slopes, and the connected portion becomes the cms tongum. Between this and the stapes an articulation develops.

The stapes and Reichert's cartilage are derived from the second branchial arch (Fig. 387). The mesenchymal anlage of the stapes is perforated by the stapedial artery, and its cartilaginous anlage is ring-shaped. This form persists until the middle of the third month, when it assumes its adult structure and the stapedial artery disappears.

Fig .18S. — StaResin the development of the auricle. (Adapted in part after Hb). .4,11 mm.; B,

13.6 mm.; C, 15 mm.; D, adult. 1, 2, 3, clt:\-ations on the mandibular arch; 4, 5, 6, elevations on the hyoid urch; af, auricular fold; ov, otic vesicle; 1, tragus; 2, 3, helix; 4, 5, antihelii; 6, antitiagus.

The muscle of the malleus, the tensor tympani. is derived from the first branchial arch; the sliipfdial muscle from the second arch. The further fact that these muscles are innervated by ihe trigeminal and facial nerves, which arc the nerves of the first and second arches respectively, points toward a similar origin for the ear ossicles.

Fuths, studying rabbit embryos, on the contrary, concludes: (I) the stapes is derived from the capsule of the labyrinth; (2) the malleus and incus arise independently of the first branchial arch.

The External Ear

The external ear is developed from and about the first ectoticrmal branchial groove. The auricle arises from six elevations which appear, three on the mandibular, and three on the hyoid arch (Fig. 388). Modem accounts of the transformation of these hillocks into the adult auricle agree in the main.

Caudal to the hyoid anlages a fold of the hyoid integument is formed, the auricidar fold or hyoid helix. A similar fold forms later, dorsal to the first branchial groove, and imites with the auricular fold to form with it the free margin of the auricle. The point of fusion of these two folds marks the position of the satyr tubercle, according to Schwalbe. Darwin^s tubercle appears at about the middle of the margin of the free auriciJar fold, and corresponds to the apex of the auricle in lower mammals. The tragus is derived from mandibular hillock 1 ; the helix from mandibiJar hillocks 2 and 3; the antihelix from hyoid hillocks 4 and 5; the antitragus from hyoid hillock 6. The lobule represents the lower end of the auricular fold.

The external auditory meatus is formed as an ingrowth of the first branchial groove. In embryos of 12 to 15 mm. the wall of this groove is in contact dorsally with the entoderm of the first pharyngeal pouch. Later, however, this contact is lost, and during the latter part of the second month, according to Hammar, an ingrowth takes place from the ventral portion of the groove, to form a funnelshaped canal.

The lumen of this tube is temporarily closed during the fourth and fifth months, but later re-opens. During the third month a cellular plate at the extremity of the primary auditory meatus grows in and reaches the outer end of the tympanic cavity. During the seventh month a spaqe is formed by the splitting of this plate, and the secondary inner portion of the external meatus is thus developed.

The tympanic membrane is formed by a thinning out of the mesodermal tissue in the region where the wall of the external auditory meatus abuts upon the wall of the tympanic cavity. Hence it is covered externally by ectodermal, and internally by entodermal epithelium.

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Prentiss CW. and Arey LB. A laboratory manual and text-book of embryology. (1918) W.B. Saunders Company, Philadelphia and London.

Human Embryology 1918: The Germ Cells | Germ Layers | Chick Embryos | Fetal Membranes | Pig Embryos | Dissecting Pig Embryos | Entodermal Canal | Urogenital System | Vascular System | Histogenesis | Skeleton and Muscles | Central Nervous System | Peripheral Nervous System | Embryology History
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