Part 91

4. The =cochlear duct= (Ductus cochlearis) is a spiral tube situated within the cochlea. It begins by a blind end (Cæcum vestibulare) in the cochlear recess of the vestibule, and ends by a second blind end (Cæcum cupulare), which is attached to the cupola of the cochlea. The vestibular part is connected with the saccule by the ductus reuniens. The duct is triangular in cross-section, and it is usual to regard it as having three walls. The vestibular wall or roof, which separates the cochlear duct from the scala vestibuli, is formed by the very delicate =membrana vestibularis= (of Reissner), which extends obliquely from the lamina spiralis ossea to the outer wall of the cochlea. The tympanic wall or floor intervenes between the cochlear duct and the scala tympani; it is formed by the periosteum of the marginal part of the lamina spiralis and the =membrana basilaris=, which stretches between the free edge of the lamina and the outer wall of the cochlea. The outer wall consists of the fibrous lining of the cochlea, which is greatly thickened to form the =ligamentum spirale cochleæ=.

=Structure.=[223]—The membranous labyrinth consists in general of an outer thin fibrous layer, a middle transparent tunic, and an internal epithelium, composed of flattened cells. But in certain situations special and remarkable structures occur, among which are the following: (1) The =maculæ acusticæ= appear as small whitish thickenings of the inner walls of the saccule and utricle. The epithelium here consists of two kinds of cells, viz., supporting cells and hair cells. The latter are flask-shaped and are surrounded by the fusiform supporting cells. The free end of each hair cell bears a stiff, hair-like process composed of a bundle of cilia. Fibers of the saccular and utricular branches of the vestibular nerve form arborizations about the basal parts of the hair cells. Adherent to the surface of the maculæ are fine crystals of lime salts, embedded in a mucoid substance, and termed otoconia. (2) The =cristæ acusticæ= are linear thickenings of the wall of each ampulla of the membranous semicircular canals. Their structure is similar to that of the maculæ. (3) The =spiral organ of Corti= (Organon spirale) is an epithelial elevation which is situated upon the inner part of the membrana basilaris, and extends the entire length of the ductus cochlearis. It is very complicated in structure, but consists essentially of remarkable supporting cells and hair cells. Fibers of the cochlear nerve ramify about the basal parts of the hair cells.

=Vessels and Nerves.=—The =artery= of the internal ear is the internal auditory artery, a very small vessel which usually arises from the posterior cerebellar artery, and enters the internal auditory meatus. The =veins= go to the inferior petrosal sinus. The =vestibular nerve= is distributed to the utricle, saccule, and membranous semicircular canals, and mediates equilibration. The =cochlear nerve= gives a branch to the saccule and enters the central canal of the modiolus. Along its course it gives off fibers which radiate outward between the two plates of the lamina spiralis ossea, and ramify about the hair cells of the organ of Corti. The =ganglion spirale= or ganglion of Corti is situated in the spiral canal of the modiolus near the fixed border of the lamina spiralis. The cochlear nerve mediates the sense of hearing.

THE SKIN

The =skin= or =common integument= (Integumentum commune) is the protective covering of the body, and is continuous at the natural openings with the mucous membranes of the digestive, respiratory, and urogenital tracts. It contains peripheral ramifications of the sensory nerves, and is thus an important sense organ. It is the principal factor in the regulation of the temperature of the body, and by means of its glands it plays an important part in secretion and excretion. Some of its special horny modifications or appendages are used as organs of prehension or as weapons.

The =thickness= of the skin varies in the different species, on different parts of the body of the same animal, and also with the breed, sex, and age. The color also varies greatly, but this is masked in most places by the covering of hair or wool. The skin is in general highly elastic and resistant.

Permanent =folds= of the skin (Plicæ cutis) occur in certain situations, and in some places there are cutaneous =pouches= or =diverticula= (Sinus cutis).

The skin is attached to the underlying parts by the =subcutis= or =superficial fascia= (Tela subcutanea). This consists of connective tissue containing elastic fibers and fat. When the fat forms a layer of considerable thickness, it is termed the =panniculus adiposus=. Over a considerable part of the body the subcutis contains striped muscle, the =panniculus carnosus= (M. cutaneus); in some regions the fibers of the muscle are inserted into the skin, and their contraction twitches the skin or produces temporary folds.[224] The amount of subcutaneous tissue varies widely; in some places it is abundant, so that the skin can be raised considerably; in other situations it is practically absent and the skin is closely adherent to the subjacent structures. =Subcutaneous bursæ= often develop over prominent parts of the skeleton where there is much pressure or friction.

=Structure.=—The integument consists of the cutis or skin proper and its epidermal appendages, _e. g._, hairs, hoofs, claws, horns, etc. The cutis consists of two distinct strata, viz., a superficial epithelial layer, the =epidermis=, and a deep connective-tissue layer, the =corium=.

The =epidermis= is a non-vascular, stratified epithelium of varying thickness. It presents the openings of the cutaneous glands and the hair-follicles, and its deep surface is adapted to the corium.[225] It is divisible into a superficial, harder, drier part, the =stratum corneum=, and a deeper, softer, moister part, the =stratum germinativum=. The cells of the latter contain pigment, and by their proliferation compensate the loss by desquamation of the superficial part of the stratum corneum. In many places further subdivision into strata is evident on properly prepared cross-sections.

The =corium= consists essentially of a feltwork of connective-tissue and elastic fibers. It is well supplied with vessels and nerves, and contains the cutaneous glands, the hair-follicles, and unstriped muscle. The deeper part of the corium, the =tunica propria=, consists of a relatively loose network of coarse bundles of fibers, and in most places there is no clear line of demarcation between it and the subcutis. The superficial part, the =corpus papillare=, is of finer texture and is free from fat. Its superficial face is thickly beset with blunt conical prominences, the =papillæ=, which are received into corresponding depressions of the epidermis. They contain vascular loops and nerves, or, in certain situations, special nerve-endings.

The papillæ are best developed where the epidermis is thick and hairs are small or absent. On thickly-haired regions they are small or even absent. On certain parts of the body (anus, vulva, prepuce, scrotum, eyelids, etc.) the corium contains pigment in its connective-tissue cells.

The =glands= of the skin (Glandulæ cutis) are chiefly of two kinds, sudoriferous and sebaceous.

The =sudoriferous= or =sweat glands= (Glandulæ sudoriferæ) consist of a tube, the lower, secretory part of which is coiled in the deep part of the corium or in the subcutis to form a round or oval ball (Corpus glandulæ sudoriferæ). The excretory duct (Ductus sudoriferus) passes almost straight up through the corium, but pursues a more or less flexuous course through the epidermis, and opens into a hair-follicle or by a funnel-shaped pore (Porus sudoriferus) on the surface of the skin.

The =sebaceous glands= (Glandulæ sebaceæ) are in great part associated with the hairs, into the follicles of which they open. Their size varies widely, and is in general in inverse ratio to that of the hair. The larger ones are easily seen with the naked eye, and appear as small, pale yellow or brownish bodies. In certain situations (_e. g._, the labia vulvæ, anus, prepuce) they are independent of the hairs and are well developed. In form they may be branched alveolar, simple alveolar, or even tubular in type. They secrete a fatty substance, the =sebum cutaneum=, which serves as a protective against moisture, and may also (by its aromatic constituents) play an important part in the sexual life of animals.

The two kinds of glands described above are those which are most widely distributed, but many special types occur. Some of these are to be regarded as modified sweat glands, _e. g._, the naso-labial glands of the ox, the glands of the snout of the pig, and the glands of the plantar cushion of the horse. Others, _e. g_., the tarsal (or Meibomian) glands of the eyelids, are modified sebaceous. Still others are not yet classified satisfactorily. Some of these special types have been referred to in previous chapters, and others will receive attention in the special descriptions which follow. The mammary glands are highly modified cutaneous glands, which are intimately associated in function with the genital organs, and have been described with the latter.

=Vessels and Nerves.=—The =arteries= of the skin enter from the subcutis, where they communicate freely. In the deeper part of the corium they form a plexus, and another network is formed under the papillæ. Small vessels from the deep plexus go to the fat and sweat glands, and the subpapillary plexus sends fine branches to the papillæ, hair-follicles, and sebaceous glands. The =veins= form two plexuses, one beneath the papillæ, and another at the junction of the corium and subcutis. The =lymph vessels= form subpapillary and subcutaneous plexuses.

The =nerves= vary widely in number in different parts of the skin. The terminal fibers either end free in the epidermis and in certain parts of the corium, or form special microscopic corpuscles of several kinds.

THE APPENDAGES OF THE SKIN

The appendages of the skin are modifications of the epidermis, and comprise the hairs, hoofs, claws, horns, etc.

The =hairs= (Pili) cover almost the entire surface of the body in the domesticated mammals, and some parts which appear at first sight to be bare are found on close inspection to be provided with sparse and very fine hair. The hairs are constantly being shed and replaced, but at certain periods in the horse, for example, they fall out in great numbers, constituting the shedding of the coat. It is customary to distinguish the ordinary hairs (the coat), which determine the color of the animal, from the special varieties found in certain places. Among the latter are the long =tactile hairs= about the lips, nostrils, and eyes; the =eyelashes= or =cilia=; the =tragi= of the external ear; and the =vibrissæ= of the nostrils. Other special features will be noted in the discussion of the skin of the various species. The hairs are directed in such a way as to form more or less definite =hair-streams= (Flumina pilorum), and at certain points these converge to form =vortices= (Vortices pilorum).

The part of the hair above the surface of the skin is the =shaft= (Scapus pili), while the =root= (Radix pili) is embedded in a depression termed the =hair-follicle= (Folliculus pili). A vascular =papilla= (Papilla pili) projects up in the fundus of the follicle and is capped by the expanded end of the root, the =bulb= of the hair (Bulbus pili). The hair-follicles extend obliquely into the corium to a varying depth; in the case of the long tactile hairs they reach to the underlying muscle. Most of the follicles have attached to them small unstriped muscles known as the =arrectores pilorum=; these are attached at an acute angle to the under side of the deep part of the follicle, and their contraction causes erection of the hair and compression of the sebaceous glands, one or more of which open into the follicle.

The hairs are composed of epidermal cells, and consist from without inward of three parts. The =cuticle= is composed of horny, scale-like cells which overlap like slates on a roof. The =cortex= consists of horny fusiform cells which are packed close together and contain pigment. The =medulla= is the central core of softer, cubical or polyhedral cells; it contains some pigment and air-spaces.

The hair-follicles, being invaginations of the skin, are composed of a central epidermal part, and a peripheral layer which corresponds in structure to the corium. The follicles of the tactile hairs have remarkably thick walls which contain blood-sinuses between their outer and inner layers; in ungulates the sinuses are crossed by trabeculæ and assume the character of cavernous or erectile tissue.

The =hoofs=, =claws=, =horns=, and other horny structures consist of closely packed epidermal cells which have undergone cornification. In structure they might be compared to hairs matted together by intervening epidermal cells. They cover a specialized corium known as their =matrix=, from which the stratum germinativum derives its nutrition.

THE SKIN OF THE HORSE

The =thickness= of the skin of the horse varies from 1 to 5 mm. in different regions, and is greatest at the attachment of the mane and the ventral surface of the tail.

The =glands= are numerous and are larger than those of the other domesticated animals. The sebaceous glands are specially developed on the lips, the prepuce, mammary glands, perineum, and labia of the vulva. The sweat glands are yellow or brown in color. They occur in almost all parts of the skin, but are largest and most numerous on the outer wing of the nostril, the flank, mammary glands, and free part of the penis.

In addition to the ordinary and tactile =hairs= certain regions present coarse hairs of great length. The mane (Juba) springs from the dorsal border of the neck and the adjacent part of the withers; its anterior part, which covers the forehead to a variable extent, is termed the foretop (Cirrus capitis). The tail, with the exception of its ventral surface, bears very large and long hairs (Cirrus caudæ). The tuft of long hairs on the flexion surface of the fetlock (Cirrus pedis) gave rise to the popular name of this region.

The development of these special hairs varies widely, and is in general much greater in the draft breeds than in others. In Shire and Clydesdale horses, for instance, the hair on the volar aspect of the metacarpus and metatarsus and fetlock is often so long and abundant as to account for the term “feather,” which is commonly applied to it by horsemen.

THE HOOF

The =hoof=[226] (Unguis) is the horny covering of the distal end of the digit. It is convenient to divide it for description into three parts, termed the wall, sole, and frog.

1. The =wall= is defined as the part of the hoof which is visible when the foot is placed on the ground.[227] It covers the front and sides of the foot, and is reflected posteriorly at an acute angle to form the =bars=. The latter (Pars inflexa medialis, lateralis) appear on the ground surface of the hoof as convergent ridges, which subside in front and are fused with the sole; they are united with each other by the frog. For topographic purposes the wall may be divided into an anterior part or “toe” (Pars ungulæ dorsalis), lateral parts or “quarters” (Pars ungulæ medialis, lateralis), and the angles or “heels” (Pars ungularis medialis, laterales). It presents two surfaces and two borders.

The outer surface is convex from side to side and slopes obliquely from edge to edge. In front the angle of inclination on the ground plane is about 50° for the fore limb, 55° for the hind limb; on the sides the angle gradually increases and is 100° at the angles. The curve of the wall is wider on the external than on the internal side, and the external quarter is more oblique than the internal one. The surface is smooth and is crossed by more or less distinct ridges, which are parallel with the coronary border and indicate variations in the activity of the growth of the hoof. It is also marked by fine parallel striæ, which extend from border to border in an almost rectilinear manner.

The =slope= of the wall varies considerably in apparently normal hoofs. Lungwitz found by careful measurements of 56 fore and 36 hind feet the following average angles:

FORE FOOT HIND FOOT Toe 47.26° 54.1° Inner angle 101.57° 96.50° Outer angle 101.37° 96.1°

The wall of the fore foot may even be more upright than that of the hind, and may have an angle of 60°. The =length= of the wall at the toe, quarters, and heels is in the ratio of about 3 ∶ 2 ∶ 1 in the fore foot and about 2 ∶ 1½ ∶ 1 in the hind hoof.

The inner surface is concave from side to side, and bears about six hundred thin =primary laminæ= (Cristæ ungulæ), which extend from the coronary groove to the junction of wall and sole. Each bears a hundred or more =secondary laminæ= on its surfaces, so that the arrangement is pennate on cross-section. These horny laminæ are continued on the inner surface of the bars, and alternate with corresponding laminæ of the matrix.

The =proximal= or =coronary border= (Margo occultus) is thin. Its outer aspect is covered by a layer of soft horn known as the =periople=; this appears as a ring-like prominence above and gradually fades out below; at the angle it forms a wide cap or bulb and blends centrally with the frog. The inner aspect of the border is excavated to form the =coronary groove= (Sulcus coronalis ungulæ), which contains the thick coronary matrix. The groove is deepest in front, narrows on the sides, and is wide and shallow at the heels.[228] It is perforated by innumerable small, funnel-like openings which are occupied by the papillæ of the coronary matrix in the natural state.

The =distal= or =ground border= (Margo liber) of the unshod hoof comes in contact with the ground. Its thickness is greatest in front and decreases considerably from before backward on the sides, but there is a slight increase at the angles. Its inner face is united with the periphery of the sole by horn of lighter color and softer texture, which appears on the ground surface of the hoof as the =white line=.

In the case of horses at liberty the wall is usually worn off to the level of the adjacent sole, but if the ground is too soft the wall is likely to become unduly long and split or break or undergo deformation. On very hard or rough ground, on the other hand, the wear may be in excess of the growth. In the case of shod horses it is necessary to remove the excess of growth of the wall at each shoeing. The =thickness= of the wall at the toe, quarters, and heels is about in the ratio of 4 ∶ 3 ∶ 2 for the fore foot and about 3 ∶ 2½ ∶ 2 for the hind foot.

2. The =sole= (Solea ungulæ) forms the greater part of the ground surface of the hoof. It is somewhat crescentic in outline, and presents two surfaces and two borders.

The =superior= or =inner surface= is convex, and slopes with a varying degree of obliquity downward to the convex border. It presents numerous small funnel-like openings which contain the papillæ of the sole matrix in the natural state.

The =inferior= or =ground surface= is the converse of the preceding. It is normally arched—and more strongly in the hind than in the fore foot—but the curvature is subject to wide variation; in heavy draft horses the sole is commonly less curved than in the lighter breeds and may even be flat. The surface is usually rough, since the horn exfoliates here in irregular flakes.

The =convex border= is joined to the wall by relatively soft horn, previously referred to as forming the white line on the ground surface of the hoof. The angle of junction is rounded internally and presents a number of low ridges and specially large openings for the papillæ of the matrix. There is frequently a ridge of larger size at the toe.

The =concave border= has the form of a deep angle which is occupied by the bars and frog. By its junction with these it forms two pronounced ridges in the interior of the foot. The parts of the sole between the wall and bars may be termed its angles.

3. The =frog= (Furca ungulæ) is a wedge-shaped mass which occupies the angle between the bars and sole, and extends considerably below these on the ground surface of the foot. It may be described as having four surfaces, a base, and an apex.

The =superior= or =internal surface= is deeply concave from side to side. It bears a central ridge, the =spine= or “=frog-stay=” (Spina furcæ ungulæ), which is high posteriorly and subsides abruptly in front. On either side of this is a deep depression, bounded outwardly by the rounded ridge formed by the junction of the frog with the bar and sole. This surface presents fine striæ and openings for the papillæ of the frog matrix.

The =inferior= or =ground surface= presents a =central furrow= (Sulcus intercruralis), which is bounded by two ridges or crura (Crura furcæ ungulæ); these converge and form the apex.

The =sides= (Facies medialis et lateralis) are united at the upper part with the bars and sole, but are free below and form the central wall of the deep =lateral furrows= (Sulci cruroparietales), which are bounded outwardly by the bars.

The =base= is wide and high. It is depressed centrally and prominent at the sides, where it unites with the angles of the walls; the junction here is covered by the expanded periople and constitutes the bulb of the hoof.