Corti, and separating it from the endolymph of the ductus cochlearis, is the membrana tectoria, which springs from the crista spiralis close to the attachment of the membrane of Reissner, passes outwards superficial to the membrana reticularis, and ends externally at the spiral ligament.

The origin, course, and distribution of the auditory nerve in the labyrinth will now be considered. The auditory nerve is the portio mollis of the seventh cranial nerve. It appears at the base of the brain at the lower border of the pons Varolii. Traced to its origin its roots wind round the restiform body to the floor of the 4th ventricle, where they form the striæ acousticæ, and sink into the grey matter of the floor. Some of the fibres arise from an inner, others from an anterior collection of nerve cells, whilst others again are connected with the cells in the restiform body, and probably with the flocculus of the cerebellum. Where the nerve emerges at the lower border of the pons it contains a cluster of nerve cells. The auditory nerve passes down the internal meatus, and divides into a vestibular and a cochlear division. The vestibular division enters the vestibulc, and divides into five branches for the sacculus, utriculus, and three ampullæ of the membranous semicircular canals. Each branch enters a crista acoustica and forms a plexus, in the meshes of which nerve cells are imbedded. From this plexus fine non-medullated fibres arise, which enter the layer of cells on the surface of the crista, where they anastomose and form a very delicate plexus, from which fibres spring that in all probability join the central processes of the auditory cells.

The cochlear division enters a canal in the axis of the modiolus, and gives off lateral branches, which pass into the canals situated in the osseous spiral lamina. Here they radiate outwards to the membranous spiral lamina, and have connected with them collections of nerve cells forming the spiral ganglion. Beyond the ganglion they form a flat plexiform expansion, from which delicate nerves pass through a gap in the edge of the osseous lamina into the organ of Corti. In this organ the nerves, as Gottstein and Waldeyer have described, are arranged in two groups of fibres; the inner group become continuous with the deep end of the inner hair cells; the outer group pass across the canal of Corti and end in the outer hair cells. Hence these cells are the peripheral end-organs of the cochlear branch of the auditory nerve, or the auditory cells of the cochlea.

The perilymph of the labyrinth is set in vibration by the movements of the tympanic ossicles and the fenestra ovalis; motion is thus communicated to the membranous labyrinth and the endolymph which it contains. The auditory hairs and cells would thus be set in motion, and the vestibular branches of the auditory nerve would be stimulated to conduct sound-impulses to the brain. The movements of the perilymph in the scala tympani and of the endolymph in the ductus cochlearis would set in vibration the basilar membrane, and the auditory cells resting on it, by which the cochlear branches of the auditory nerve would be stimulated to conduct sound-impulses to the brain. It has been customary for physiologists to regard the vestibule as the part of the labyrinth by which sound or mere noise is determined; the cochlea, as the part which determines variations and degrees of sound, as musical notes or harmony; the semicircular canals, as determining the directions from which sound proceeds. But within the last two years experiments and arguments have been advanced almost simultaneously by Crum Brown and Mach in favour of the view that the semicircular canals act as peripheral end-organs for the sense of rotation, by which sense the axis about which rotation of the head takes place, the direction of that rotation, and its rate, are determined.

In the account of the development of the skeleton, p.

|

ear.

831, it was stated that the external meatus, tympanum, Developand Eustachian tube are the remains of the first branchial ment of cleft of the embryo, that the tympanic ossicles are formed in the first and second visceral arches, and that the petrous bone is ossified in the cartilaginous basis cranii. The membranous labyrinth apparently arises as an invagination of the integument at the upper end of the second branchial cleft. The invaginated fold then closes in to form a shut sac, the primary auditory vesicle. Out of this vesicle the three divisions of the labyrinth are successively produced, and become enclosed by the petrous cartilage, which when ossified forms the osseous labyrinth. The epidermal investment of the invaginated tegumentary sac becomes transformed into the special cell structures within the membranous labyrinth, and the sub-epidermal connective tissue forms its fibrous wall. The cochlear and vestibular nerves form at the same time as the labyrinth, and become connected through the trunk of the auditory nerve with the brain.

The TONGUE, situated on the floor of the cavity of the Tongue. mouth, is the chief organ provided for the excitation of the special sense of taste, but the under surface of the soft palate participates to some extent in this property. The tongue is also highly endowed with the sense of touch. The structures concerned in the excitation of taste and touch are situated in the mucous membrane which envelopes the tongue. The tongue is also a muscular organ, and plays an important part in articulation, mastication, and deglutition. Its shape is flattened from above downwards, so that it presents an upper surface or dorsum and a lower surface. Its posterior part is broad, forms the base or root of the organ, and is attached to the hyoid bone. Its anterior extremity or tip is more or less pointed, and its lateral margins or sides are rounded.

The muscles connected with the tongue are arranged in pairs, and form three distinct groups, viz., accessory, extrinsic, and intrinsic muscles. The accessory muscles are the stylo-hyoid, digastric, mylo-hyoid, genio-hyoid, omo-hyoid, sterno-hyoid, and thyro-hyoid, already referred to on page 836, which act upon the hyoid bone, and thus indirectly are concerned in the movements of the tongue. The extrinsic muscles pass from adjacent parts into the substance of the tongue, and are as follows:-The stylo-glossus arises from the tip of the styloid process and the stylo-maxillary ligament; it runs forwards along the side of the tongue to the tip. The hyo-glossus is divided into three parts; a, basiglossus, which arises from the body of the hyoid; b, ceratoglossus, from the great cornu of the hyoid; c, chondroglossus, from the small cornu of the hyoid. The fibres from these origins ascend into the side of the tongue. The genio-hyo-glossus arises from the upper tubercle of the symphysis of the lower jaw, its fibres radiate into the substance of the tongue along its whole length from base to tip; this muscle is separated from the corresponding muscle of the opposite half of the tongue by a mesial septum of fibrous tissue. The palato-glossus arises in the substance of the soft palate, and descends to the tongue in the anterior pillar of the fauces. The intrinsic muscles lie in the substance of the tongue itself, and are as follows :-The lingualis superior (noto-glossus), consisting of longitudinal fibres, which extend from the base to the tip beneath the mucous membrane of the dorsum; the lingualis inferior, consisting of longitudinal fibres, which extend from the base to the tip along the under surface between the hyo-glossus and geniohyo-glossus; transverse muscular fibres, which spring from the mesial fibrous septum and curve outwards and upwards to the sides of the tongue; vertical fibres, which pass through the substance of the tongue from the dorsum to the under surface. The extrinsic and intrinsic muscles can not only

move the entire tongue within the cavity of the mouth, protrude it between the lips, and again retract it, but can modify its form; thus the dorsum can be flattened, made convex or concave, the margins can be raised or depressed, and the tip elevated or depressed.

The mucous membrane of the tongue forms a part of the general mucous lining of the mouth; it covers the dorsum, tip, sides, and under surface; is reflected from the under surface to the floor of the mouth, where it forms the frænum or bridle of the tongue, and is reflected also from the base to the epiglottis as the frana of the epiglottis, as well as over the tonsils and anterior palatine pillars. This membrane has its free surface elevated into multitudes of fine processes, called the papillæ of the tongue, some of which are simple, others compound. The simple papillæ are situated on the back part of the dorsum and the under surface of the mucous membrane, as well as scattered between | the compound papillæ; they are simple conical elevations of the membrane. The compound papillae are arranged in three groups, named filiform, fungiform, and circumvallate papillæ. The filiform papilla, elongated and thread-like, are the smallest and most numerous, and cover the dorsum in front of the circumvallate papillæ. The fungiform or club-shaped are scattered over the anterior and middle parts of the dorsum, and at the tip and sides. The circumvallate papilla, seven to twelve in number, form a V-shaped figure on the dorsum towards its base; a depression in the mucous membrane, called foramen cacum, marks the apex of the V. These are the largest papillæ; each is sunk in a vallum or trench-like depression of the mucous membrane, which isolates it from the surrounding surface. The compound character of these papilla is due to each having projecting from it numerous small secondary papillæ. The epithelial covering of the filiform papilla is characterised by the peculiar modification which the tessellated epithelium of the mouth has undergone'; the cells have become cornified and elongated into dense, imbricated brush-like processes. In the carnivora the epithelium is so hardened as to form sharp spines, with the points turned backwards, which give to the tongues of these animals a rough prickly character. In the fungiform and circumvallate papillæ the inequalities between the secondary papillæ, which project from them, are filled up by the tessellated epithelium, so that the surface of the compound papillæ has a smooth appearance. Both the simple and compound

Ep

00000

FIG. 85.-Section through a gustatory lamella of the rabbit's tongue. G, gustatory bulbs situated in Ep, the epithelial layer of the mucous membrane; V, capillary blood-vessels in the sub-epithelial connective tissue. (From a preparation by A. B. Stirling.)

papillæ are highly vascular; the lingual artery not only supplies the muscular substance of the tongue, but gives off fine branches to the mucous membrane. These branches end in capillaries, which form simple loops in the simple papillæ, but in the compound papillae the capillaries are so multiplied that each secondary papilla has a capillary loop within it. The tongue is provided with several nerves. The hypo-glossal nerve supplies its muscular structure, but

8

FIG. 86.-s, superficial bulb; gustatory cell,

c,

the inferior lingualis apparently receives a branch from the chorda tympani of the facial. The lingual branch of the fifth is distributed to the mucous membrane of the anterior two-thirds of the tongue: it breaks up into minute branches, which enter the fungiform and filiform papillæ, but their exact mode of termination has not been precisely ascertained, though end-bulbs and gustatory bodies are said to have been seen in connection with some of the terminal branches. The glossal branch of the glosso-pharyngeal is distributed to the mucous membrane of the root of the tongue and of the circumvallate papillæ. In connection with its terminal branches peculiar flask-shaped organs, called gustatory bulbs or bodies, have recently been described by Lovén, Schwalbe, and Engelmann, in the sides of the circumvallate papillæ. These have been found in large numbers in lamellated folds of the mucous membrane of the posterior part of the side of the rabbit's tongue, which folds may appropriately therefore be called gustatory lamella. When sectious are made through one of these folds, or through a circumvallate papilla and the trench which surrounds it, numerous flask-shaped gustatory bulbs may be seen in the epithelium, which covers the side of the papilla and the opposite side of the trench. The bottom of each flask is next the sub-epithelial tissue, whilst its short neck opens on the surface by a mouth, the gustatory pore; similar bodies, though in much smaller numbers, have also been seen in the fungiform papillæ. Each gustatory body consists of two different forms of cells, named covering cells covering cells of a and gustatory cells. The covering cells gustatory cell with are elongated, nucleated spindles, which, P, its peripheral, and arranged in layers, form the envelope of its central process. each gustatory bulb, and reach from the bottom to the mouth of the flask; they enclose the gustatory cells. The gustatory cells are attenuated, homogeneous, and highly refractile cells, which possess an elliptical nucleated body with two processes, a central and peripheral. These cells occupy the axis of the gustatory bulb. The peripheral process, broader than the central, sometimes ends in a short hair-like tip, which almost reaches the gustatory pore; the central process extends to the base of the flask, and often divides into small branches. This Nerve of process is varicose, and not unlike the axial cylinder taste. of a nerve fibre. The branches of the glosso-pharyngeal nerve, which are distributed to the back of the tongue, enter the circumvallate papillæ, and form a minute plexus, with groups of nerve cells interspersed in it, from which bundles both of medullated and non-medullated fibres pass to the basis of the gustatory bulbs; and it is believed that the finest non-medullated fibres are continuous with the peripheral processes of the gustatory cells, which are therefore regarded as the peripheral end-organs of the nerve of taste, and by the excitation of these bodies gustative or taste sensations are produced. As the glosso-pharyngeal is the nerve distributed to the circumvallate papillæ, where these gustatory bulbs are especially found, it is therefore the special nerve of taste; but as these bulbs have also been sparingly seen in the other papilla, where the lingual nerve is distributed, that nerve probably acts in a minor degree as a nerve of taste, though its special function is undoubtedly that of a nerve of touch. The gustatory bulbs are not penetrated by blood-vessels, but, as Fig. 85 shows, the vascular sub-epithelial tissue is prolonged upwards along the sides of the bulbs almost as far as the plane of the gustatory pore. Key, Beale, and other observers have described special modifications of the epithelium in connection with the terminations of the gustatory nerves in the frog. The mucous membrane of the

tongue contains numerous smal tubular or branched glands, more especially on the dorsum near its root, which secrete mucus. Depressions also occur in this part of the mucous membrane, around the walls of which groups of lymphoid cells are collected in the sub-epithelial connective tissue, which have an arrangement closely resembling the structure of the adjacent tonsils, and form an example of adenoid tissue.

The SKIN, or Integument, invests the entire outer surface of the body, and contains structures by the excitation of which the properties of things are determined by the sense of touch. The skin also contains accessory structures, as the nails, hairs, sebaceous glands, and sweat glands. The skin consists of a non-vascular. cuticle or epidermis, and of a vascular and sensitive corium, or cutis vera.

The Cuticle, Epidermis, or scarf skin, forms the outer covering of the skin, and protects the cutis. It is a laminated structure, and consists of numerous layers of cells superimposed on each other. As these cells cover a free surface exposed to the air, they belong to the epithelium group. The thickness of the cuticle varies in different localities from th to both inch; where the skin is frequently exposed to pressure, as in the soles of the feet, the cuticle is the thickest and hardest; and the hands of those accustomed to manual labour have a hard and horny cuticle. The increase in thickness in these localities is for the purpose of protecting the highly sensitive cutis from injury. The outer surface of the cuticle in many parts of the body, especially the palm of the hand and the fingers, is marked by ridges and furrows; the ridges indicate the position and arrangement of the papillæ of the cutis, whilst the furrows are due to the sinking of the cuticle into the spaces between the rows of papillæ. The mouths of the sweat glands open on the surface of these ridges. The cuticle is divided into two strata. The superficial horny stratum consists of layers of flat, polygonal scales like a tessellated epithelium; the cells in the superimposed layers firmly adhere to each other by their surfaces, and in vertical sections this stratum presents a fibrous appearance; but the cells may be readily isolated by digestion in a caustic alkali. The deeper or mucous stratum, or rete Malpighii, lies next the cutis, and closely follows the undulations of its papillary surface. The cells forming the layer next the cutis are columnar in shape, those in the layers immediately succeeding are rounded or cubical, whilst those next in order are polygonal, and not unfrequently possess pointed processes or prickles projecting from them, hence the name, prickle cells, employed by Schultze. The cells which lie next the horny stratum assume the scale-like form. It is in the cells of the mucous stratum that the colouring matter of the skin is found, which in the fair races of men forms the isolated coloured spots called freckles and moles, but in the dark races the pigment granules are uniformly distributed through the cells of this stratum. The superficial cells of the horny stratum of the cuticle are continually being shed, so that the cells of the deeper layers gradually approach the surface, and new cells are continually being formed in the deeper part of the rete Malpighii. The cuticle is closely adherent to the cutis in the healthy living skin, but on the application of a blister, or when putrefaction sets in after death, it separates from it.

The Cutis vera.-When the cuticle is removed the surface of the cutis is seen to be studded with multitudes of minute elevations, the papillæ of the skin. These papillæ are either simple conical structures, or compound with two or three branches. They are largest in the palm and sole, being from th to th of an inch high, and are arranged in ridges, but more usually they are much shorter and irregularly distributed. The cutis is formed of connec

tive tissue, in which stellate connective tissue corpuscles and elastic fibres are abundant. The deeper surface of the connective tis

sue of the cutis

is reticulated, and is continuous with the bundles of connective tissue that form the areolar subcutaneous tissue. In the papillæ themselves the fibres of the connective tissue are not so well marked, and the surface of the papillæ possesses more of a homogeneous aspect, which gives rise to the appearance described as a basement membrane. The cutis is highly vascu

FIG. 87.-Vertical section through the skin and subcutaneous tissue. hs, horny stratum, and rm, rete Malpighii of cuticle; pp, papillæ of cutis; t, a touch corpuscle with n, a nerve fibre; bc, a blood and lc, a lymph capillary; ct, connective subcutaneous tissue; f, fat lobule; s, a sweat gland with its duct.

touch.

lar; the small arteries which go to the skin give off branches to the lobules of fat in the subcutaneous tissue, then penetrate the cutis, and form a plexus from which capillaries arise, which enter the papillæ, and form vascular loops within them. The lymphatic vessels of the skin are numerous; they form a plexus in the cutis, which lies beneath the vascular plexus, forms, as Neumann's injections show, a network around both the sebaceous and sweat glands, and gives off capillary loops into the papillæ. The nerves of the skin are the cutaneous branches both of the spinal and of certain of the cranial nerves, the origin and distribution of which have already been described. They run through the subcutaneous tissue, and enter the deep surface of the cutis, where they divide into branches. As these pass Nerves of towards the papillæ they unite to form a nerve plexus, from which smaller branches arise to enter the papillæ, and terminate, more especially in the skin of the palm of the hand, fingers, and sole, which are the surfaces most sensitive to touch impressions, in the tactile or touch corpuscles. The touch corpuscles discovered by Wagner and Meissner are the peripheral end-organs of the nerves of touch. They may be single or compound; are usually ovoid in form, not unlike a minute fir cone; and are transversely marked, from the transverse direction of the nuclei of fusiform cells which form an investing capsule. Each single corpuscle and each division of a compound corpuscle is penetrated by one, and, according to Thin, by never more than one, medullated nerve fibre, but the exact mode of termination of the axial cylinder of the fibre has not been ascertained. Virchow and other German observers have stated that the papillae which contain capillaries do not contain nerves or touch corpuscles, and vice versa; Dalzell and Thin have shown that certainly the majority of papillæ that contain nerve fibres and touch corpuscles are also vascular papillæ. Non-medullated nerve fibres ascend to the surface of the cutis, and, according to Langerhans, pass into the rete Malpighii between the cells of the mucous layer.

but

Nails. On the back of the last phalanx of each thumb, Nails. I. 113

finger, and toe is situated a firm horny curved plate, the nail. Each nail rests on a bed, the surface of which is formed of the cutis, which also overlaps the side and root of the nail; thus the nail fits into a groove formed of the cutis something after the manner in which a watch-glass fits into its rim. A nail is merely a special modification of the cuticle, the cells of the superficial stratum of which are more horny, harder, and more firmly adherent to each other than in the cuticle proper. Deeper than the horny stratum is the rete Malpighii of the nail, the cells of which are soft, as in the cuticle itself. The cutis forming the bed of the nail is studded with papillæ, which are arranged in almost parallel rows, and are highly vascular. Nails grow both in length and thickness: the increase in thickness is due to the formation of nerve cells on the bed of the nail; the increase in length takes place through the formation of nail cells at its root, and as the nail is thus slowly pushed forward it requires to be cut at intervals. At the root, sides, and below the free border of the nail the cuticle is continuous with the substance of the nail itself. Hair.-Projecting from the surface of the skin are multitudes of elongated cylindrical horny structures, the hairs. In the skin of the scalp, the armpits, and the pubis, they are long and numerous; but in the eye-brows, eye-lashes, vibrissae of the nostrils, and surface of the body generally, they are short. They are stronger and thicker in the skin of man than of woman, more especially on the cheeks, lips, and chin. Hairs do not grow from the skin of the palms and soles, the back of the ungual phalanges, and the surface of the upper eye-lids. Each hair is partially embedded in a depression of the skin, called a hair follicle. The deeper end of the follicle is somewhat dilated, and has in it a papilla, the hair papilla. The wall of the hair follicle is formed of the constituent structures of the skin; the outer part of the wall belongs to the cutis, and has been described as arranged in three layers, the external, middle, and inner layer of the hair follicle. The external and middle layers are formed of connective tissue, with blood-vessels; whilst the inner, sometimes called the vitreous layer, is transparent and homogeneous, and continuous with the so-called basement membrane of the cutis. The inner part of the wall of the hair follicle, or the root-sheath, belongs to the cuticle, and consists of two layers, the outer and inner root-sheaths. The outer root-sheath is continuous with the rete Malpighii, and consists of cells similar to those of that stratum. inner root-sheath is continuous with the horny stratum of the cuticle, and consists of elongated scale-like translucent cells in which no nuclei can be seen.

The

A hair possesses a root, a shaft, and a tip; the root is embedded in the hair follicle, whilst the shaft and tip form the free projecting part of the hair. In the human hair the substance of the hair is composed of a fibrous-looking horny material, which by the action of strong sulphuric acid is resolved into elongated, closely compacted, fusiform cells, which in coloured hairs contain pigment granules. In the thicker hairs the cells in the axis of the hair are polygonal, contain air, and form a central pith or medulla. The hair is invested by imbricated scale-like cells, which form the hair cuticle. In different animals the size and relative proportion of the cells of the cuticle, medulla, and fibrous part of the hair present many modifications. The wool of the sheep has its cuticle scales, with well-defined serrated margins, so that the hair of this animal is well adapted for felting into cloth; in the bat, also, the cuticle cells are large and strongly serrated. The bristles of the pig, again, have the fibrous part of the hair largely developed. In the deer tribe the hair consists of polygonal medulla-like cells, which contain air. The root of the hair dilates at its deeper end into a bulb which embraces the hair papilla. It is softer in texture than the shaft, so that the cellular

structure of the hair is more easily demonstrated. Next the papilla the cells are like those of the rete Malpighii, but when traced onwards to the shaft they are seen to become differentiated, both in structure and composition, into the proper hair cells. The root is enveloped in a special sheath, termed the sheath of Huxley, composed of nucleated cells, which sheath, in the more superficial part of the follicle, blends with the internal root-sheath. The hair papilla bears to the hair the same relation as a papilla of the cutis has to its investing cuticle, so that a hair is to be regarded as a specially modified cuticular structure. human hair papilla is vascular, but no nerves have been traced into it. In the tactile hairs of the mammalia, however, nerves have been traced into their papillæ.

The

The bristles, feathers, claws, hoofs, the horny envelope of the horn cores in the hollow horned ruminants, and various tegumentary spines and scales, present in many animals, are, like hairs and nails, special modifications of the tegumentary system.

Each hair follicle has opening into it the excretory duct of a small gland, named a sebaceous gland. This gland consists of the excretory duct, and of from two to twenty grape-like saccular expansions which open into the duct. The wall of the sacculi and of the duct is continuous with the vitreous layer of the outer wall of the hair follicle. Capillary blood-vessels are distributed on the outer wall of the sacculi. The sacculi are almost entirely filled with polygonal cells containing drops of fat, which cells are continuous with the epithelial lining of the gland duct and the cells of the outer root-sheath. These glands secrete a fatty material, which lubricates the surface of the hair. Sometimes a small parasite, called Acarus folliculorum, is found in a sebaceous gland.

Some years ago Kölliker described one or two bundles of smooth muscular fibres extending from the wall of the hair follicle to the deep surface of the cutis; these muscles, named arrectores pili, by their contraction erect the hairs, that is, cause them to become more prominent, and produce the condition of skin, called cutis anserina or goose skin, well known to occur when cold is applied to the surface of the body.

Hairs are developed about the 4th month of embryo life, within depressions in the cutis, which form the future hair follicles, filled with cells similar to, and continuous with, those of the rete Malpighii. A papilla forms at the bottom of this depression, around which the cells become arranged in a bulbous expansion. The cells, in line with the bulb, elongate and harden, and group themselves so as to form the shaft of the young hair, which at this stage is completely buried within the follicle. A rapid production of new cells takes place at the bulb, the hair consequently increases in length, and is pushed outwards through the superficial horny stratum of the cuticle, which had closed in the mouth of the depression or follicle in which the hair is produced. At the same time, the more external cells within the follicle are pushed outwards towards its wall, and form the cells of the root-sheath. When a hair is pulled out of its follicle the cells of the root-sheath are drawn out along with it. A new hair will be developed at the bottom of a follicle from which the hair has been shed as long as cells continue to be formed around the papilla. When the growth of cells ceases within the hair follicles then permanent baldness is the result.

The sebaceous glands are developed as bud-like offshoots from the hair follicles, filled, like the follicles themselves, with cells continuous with those of the rete Malpighii. Instead of the cells in these buds differentiating into a hair, they become filled with fatty particles, and the wall of the bud assumes the characteristic sacculated form of the gland.