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endolymphaticus (see note, p. 79). About the fifth week, the lower part of the vesicle is prolonged forwards as a diverticulum, the future ductus cochlearis. This is at first straight,
but as it elongates it curves on itself, so that at the twelfth week all three coils are differentiated. From the upper part of the vesicle the semicircular ducts are developed, and appear as three hollow, disc-like evaginations; the central parts of the walls of each disc coalesce and disappear, leaving only the peripheral ring or canal. The three ducts are free about the beginning of the second month, and are developed in the following order, viz.: superior, posterior, and lateral. The intermediate part of the otic vesicle represents the vestibule, and is divided by a constriction into an anterior part, the saccule, communicating with the ductus cochlearis, and a posterior portion, the utricle, receiving the extremities of the semicircular ducts. The constriction extends for some distance into the ductus endolymphaticus, and
thus the utricle and saccule are connected by a Y-shaped tube. Another constriction makes its appearance between the saccule and the vestibular end of the ductus cochlearis and forms the canalis reuniens. The epithelial lining is at first columnar, but becomes cubical throughout the whole labyrinth, except opposite the terminations of the acoustic nerve, where it forms the columnar epithelium of the macula of the utricle and saccule, of the crista ampullæ, and of the organon spirale. On the floor of the ductus cochlearis two ridges appear, of which the inner forms the limbus lamine spiralis, whilst the cells of the outer become modified to form the rods of Corti, the hair cells, and the supporting cells of Deiters and Hensen.
A, Left labyrinth of a human embryo of about four weeks; B,
The mesoderm surrounding the otic vesicle is differentiated into: (1) a fibrous layer, the wall of the membranous labyrinth; (2) a cartilaginous capsule, the future petrous bone; and (3) an intervening layer of gelatinous tissue, which is ultimately absorbed, leaving the perilymphatic space between the bony and membranous labyrinths.
The development of the external and middle parts of the ear are described on pp. 50-53.
The peripheral gustatory organ consists of groups of modified epithelial cells, termed calyculi gustatorii or taste buds, found on the tongue and in its immediate neighbourhood.
Taste buds are present in large numbers around the circumference of the papillæ vallatæ, while some are found also on their opposing walls (Fig. 730). They are very numerous over the foliate papillæ, which correspond with the papille foliate of the tongue of the rabbit, and are found also over the posterior part and sides of the tongue, either on the papillæ fungiformes or throughout the stratified epithelium. They exist, also, on the oral surface of the velum palatinum and on the posterior surface of the epiglottis.
Structure of Taste Buds (Fig. 731).-The taste buds are oval or flask-shaped, and occupy nests in the stratified epithelium of the regions mentioned. The deep extremity of each is expanded and rests upon the corium; the free end is perfor ated by a minute pore, termed the gustatory pore. They consist of two kinds of epithelial cells (a) supporting cells, and (b) gustatory cells (Fig. 732). The supporting cells are elongated, nucleated spindles, and are mostly arranged like the staves of a cask to form the outer envelope of the bud; but some are found in the
interior of the bud, amongst the gustatory cells. The gustatory cells occupy the centre
A, Three-quarter surface view of taste bud from the papilla foliata of a rabbit (highly magnified).
3. Taste buds.
6. Duct of serous gland.
of the bud, and each consists of a nucleated cell-body, prolonged into a peripheral
5. Taste buds.
B, Section through a part of the papilla foliata
of a rabbit.
B, Vertical section of taste bud from the papilla foliata of a rabbit (highly magnified).
and a central process. The peripheral process is rod-like and almost hyaline, and terminates at the gustatory pore in a slender filament, the gustatory hair. The central process passes towards the deep extremity of the bud, where it ends free, as a single or branched varicose filament.
Nerves of Taste.-The nerve supplying the taste buds over the anterior part of the tongue is the chorda tympani, which is derived from the sensory root of the facial nerve; that for the posterior part is the glossopharyngeal. The nerve fibrils, having lost their medullary sheaths, ramify partly be
tween the gustatory cells and partly amongst FIG. 732.-ISOLATED CELLS FROM TASTE BUD the supporting cells of the taste buds.
OF RABBIT (Engelmann).
a, Supporting cells. b, Gustatory cells.
The ducts of Ebner's glands open into the bottom of the valleys surrounding the papilla
vallatæ, and the serous-like secretion of these glands probably washes the free
hair-like extremities of the gustatory cells, and so renders them ready to be stimulated by successive substances. It should be added that there is a close association between the senses of smell and taste. This can be best appreciated by considering the defective taste perceptions resulting from inflammatory conditions of the nasal mucous membrane, or the common practice of holding the nose in order to minimise the taste of nauseous drugs.
The development of the tongue is described on pp. 45-46.
The integument or skin covers the body, and is continuous, at the orifices on its surface, with the mucous lining of its alimentary and other canals. It contains the peripheral terminations of many of the sensory nerves, and serves as an organ of protection to the deeper tissues. It is the chief factor in the regulation of the body temperature, and by means of the sudoriferous and sebaceous glands, which
open on its free surface, constitutes an important excretory structure. Its superficial layers are modified to form appendages in the shape of hairs and nails.
The skin is very elastic and resistant, and its colour, determined partly by its own pigment and partly by that of the blood, is deeper on exposed parts and in the regions of the genitals, axillæ, and mammary areolæ, than elsewhere. The colour varies also with race and age, the different races of the world being roughly classified, according to the colour of their skin, into the
three groups of white, yellow, and black. Pinkish in colour in childhood, the skin assumes a yellowish tinge in old age, while in certain diseases (e.g. icterus and melasma Addisonii) the colour undergoes marked alteration.
The surface of the skin is perforated by the hair follicles and by the ducts of the sudoriferous and sebaceous glands, and on the palms, soles, and flexor aspect of the digits it presents numerous permanent ridges, the crista cutis, which correspond with rows of underlying papillæ. Over the terminal phalanges these ridges form distinctive patterns, which are retained from youth to old age, and are utilised for purposes of identification. Retinacula of the skin are seen in the neighbourhood
Papilla of hair
FIG. 733.-VERTICAL SECTION OF THE SKIN (schematic).
Subcutaneous fatty tissue
of the joints, and it can be thrown into wrinkles by the contraction of the subcutaneous muscles, where those exist. Over the greater part of the body it is freely Inovable; but on the scalp and lateral surfaces of the auriculæ, as well as on the palms and soles, it is bound down to the subjacent tissues.
The skin consists of two strata, viz.: a deep, termed the corium, and a superficial, the epidermis (Fig. 734).
The corium or cutis vera is derived from the embryonic mesoderm, and consists essentially of a felted interlacement of connective tissue and elastic fibres. In its deeper part, or stratum reticulare, the fibrous bundles are coarse and form an open network, in the meshes of which are vessels, nerves, pellets of fat, hair follicles, and glands. This reticular stratum passes, as a rule, without any line of demarcation, into the panniculus adiposus or subcutaneous fatty tissue, but in some parts it rests upon a layer of striped or unstriped muscular fibres-the latter in the case of the scrotum. In the superficial layer, or stratum papillare, of the corium, the connective tissuebundles are finer and form a close network. Projecting from superficial surface are numerous finger-like, single, or branched elevations, termed papillæ (Fig. 734), which are received into corresponding depressions on the under surface of the epidermis. These papillæ vary in size, being small on the eyelids, but large on the palms and soles, where they may attain а length of 225 μ, and
where they produce the FIG. 734.-VERTICAL SECTION OF EPIDERMIS AND PAPILLE OF CORIUM (highly magnified). permanent curved ridges already referred to. Each ridge usually contains two rows of papillæ, between which the ducts of the sudoriferous glands pass to reach the surface. The papillæ consist of fine connective tissue and elastic fibres, mostly arranged parallel to the long axis of the papillæ; the majority contain capillary loops, but some contain the terminations of nerves. The superficial surface of the corium is covered with a thin, homogeneous basement membrane.
The epidermis or cuticle is derived from the embryonic ectoderm and covers the corium. Its thickness varies in different parts of the body and ranges from 0-3 mm. to 1 mm. or more; it is thickest on the palms of the hands and soles of the feet, and thinnest on the eyelids and penis. It is non-vascular and consists of stratified epithelium; its superficial layers are modified to form the stratum corneum, which may be separated by maceration or blistering from the deeper, softer portion, or stratum mucosum (Malpighi). The epidermis consists from within outwards of the following five strata (Fig. 734):
1. The stratum germinativum is a single stratum of nucleated columnar cells planted by denticulated extremities on the basement membrane of the corium.
Vascular papilla of corium
papilla of corium
Blood-vessels and nerves
2. The stratum mucosum consists of six or eight layers of polygonal, nucleated "prickle" or "finger" cells, the processes of which join those of adjacent cells. Between the cells of this layer are minute channels, in which leucocytes or pigment
granules may be seen. The cells of the stratum mucosum are characterised by the presence of numerous epidermic fibrils, which are coloured violet by hæmatoxylin and red by carmine. These fibrils are unaffected by boiling, but swell up under the action of acids and alkalies, and form the filaments of union between adjacent cells. On account of their presence, L. Ranvier has named this layer the stratum filamentosum. The dark colour of the negro's skin is caused by the presence of numerous pigment granules in the deeper layers of the stratum mucosum; the pigment of which melanin forms an important constituent is absent from the more superficial layers of the epidermis.
3. The stratum granulosum comprises two or three layers of horizontally arranged, flattened cells, scattered around the nuclei of which are elliptical or spherical granules of eleidin, a substance staining deeply with carmine and hæmatoxylin, and probably representing an intermediate stage between the protoplasm of the deeper cells and the keratin of the superficial layers.
4. The stratum lucidum, an apparently homogeneous layer, is in reality made up of several strata of flattened or irregular squames, which contain granules or droplets of keratohyalin, a hyaline substance, staining less deeply than eleidin.
5. The stratum corneum comprises several layers of flattened non-nucleated squames, the more superficial of which assume the form of horny scales and are from time to time removed by friction. The deeper cells contain granules of a fatty material having the consistency and plasticity of beeswax, and staining with osmic acid. The peripheral parts of the cells consist of keratin, a highly resistant substance which is unaffected by mineral acids, and is indigestible in pepsinhydrochloric acid.
L. Ranvier has pointed out that the stratum lucidum is really double, and has named the deeper of its two layers the stratum intermedium; this he describes as consisting of two or three layers of clear cells with atrophied nuclei, while in the cell-walls the epidermic fibrils "are rolled up like the threads of a cocoon."
Regeneration of the epidermis is generally regarded as taking place by cell proliferation in the stratum germinativum, the young cells gradually passing through the polyhedral and
granular stages, and ultimately becoming the flattened squames of the stratum corneum, while the eleidin granules of the stratum granulosum are converted into the keratin of the stratum corneum.
FIG. 735.-TACTILE CORPUSCLES.
A, End bulb (Krause).
B, Corpuscle of Pacini
Vessels and Nerves of the Skin. In the subcutaneous tissue the arteries form a plexus from which branches extend into the corium, where they supply the hair follicles and glands, and form a second plexus under the papillæ, to which small loops are given. The veins and the lymphatic vessels commence in the papillæ, and, after forming subpapillary plexuses, open into their respective subcutaneous vessels.
} (after Ranvier).
The nerves of the skin vary in number in different parts
of the body; they are extremely numerous where the sense of touch is acute, e.g. on the palmar surfaces of the terminal phalanges, while in the skin of the back, where the sensibility is less, they are fewer in number. Their different modes of ending are described on pp. 863-866.
APPENDAGES OF THE SKIN.
The appendages of the skin are the nails, the hairs, the sebaceous glands, and the sudoriferous or sweat glands.
Ungues. The nails (Figs. 736, 737) are epidermal structures, and represent the hoofs and claws of the lower animals. The root of the nail is hidden