Chapter V: Muscles of the Leg and Foot (24)

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).

FIG. 572.—LATERAL VIEW OF HORSE TO SHOW HAIR-STREAMS AND VORTICES.
(After Ellenberger-Baum, Anat. für Künstler.)
]

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.

FIG. 573.—DIGIT OF HORSE, SHOWING SURFACE RELATIONS OF BONES AND
JOINTS. THE LATERAL CARTILAGE IS EXPOSED.

_a_, First phalanx; _b_, second phalanx; _c_, third phalanx; _d_,
lateral cartilage; _e_, third sesamoid or navicular bone; _f_,
pastern joint; _g_, coffin joint; _h′_, cut edge of wall of hoof
(_h_); _i_, laminar matrix. (After Ellenberger, in Leisering’s
Atlas.)
]

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.

FIG. 574.—CROSS-SECTION OF FOOT OF HORSE, CUT PARALLEL WITH THE
CORONARY BORDER.
]

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.

FIG. 575.—HALF OF HOOF OF HORSE, INTERNAL SURFACE
]

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.

FIG. 576.—INTERNAL SURFACE OF PART OF HOOF OF HORSE. (After Schmaltz,
Atlas d. Anat. d. Pferdes.)
]

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.

FIG. 577.—HOOF OF HORSE, RIGHT FORE FOOT, GROUND SURFACE.

_37_, Inner quarter; _38_, outer quarter; _39_, ground border of wall
(toe); _40_, white line; _41_, sole; _42_, apex of frog; _43_,
lateral ridge of frog; _44_, central furrow of frog; _45_, lateral
furrow between frog and bar; _46_, bar. (After Ellenberger-Baum,
Anat. für Künstler.)
]

FIG. 578.—HORIZONTAL SECTION OF HOOF OF HORSE.

The section is cut just above the ridges of the frog and bars and
parallel with the ground surface. _1_, Wall; _2_, sole; _3_, median
ridge of frog; _4_, lateral ridge formed by junction of frog and
bar; _5_, median furrow over apex of frog; _6_, laminæ of wall; _7_,
laminæ of bar.
]

The =apex= occupies the central angle of the concave border of the sole, and forms a blunt, round prominence a little in front of the middle of the ground surface of the hoof.

=Structure of the Hoof.=—The hoof is composed of epithelial cells which are more or less completely keratinized except in its deepest part, the =stratum germinativum=; here the cells have not undergone cornification, and by their proliferation maintain the growth of the hoof. The cells are in part arranged to form tubes which are united by intertubular epithelium, and inclose medullary cells and air-spaces.

FIG. 579.—FRONTAL SECTION OF HOOF OF HORSE, POSTERIOR PART VIEWED FROM
IN FRONT.

_1_, Wall; _2_, sole; _3_, bar; _4_, frog; _5_, median ridge of frog;
_6_, lateral ridge formed by junction of frog and bar; _7_, laminæ
of bar; _8_, laminæ of wall; _9_, coronary groove; _10_, periople of
heel.
]

The =wall= may be regarded as consisting of three layers. The external layer consists of the periople and the stratum vitreum. The =periople= is composed of soft, non-pigmented tubular horn. It is continuous with the epidermis above, and extends downward a variable distance. Usually it forms a distinct band somewhat less than an inch wide, except at the heels, where it is much wider, and caps the angle of inflection of the wall proper. The =stratum vitreum= is a thin layer of horny scales which gives the wall below the periople its smooth, glossy appearance. The =middle= or =tubular layer= forms the bulk of the wall, and is the densest part of the hoof. Its horn tubes run in a straight direction from the coronary to the ground border. In dark hoofs it is pigmented except in its deep part. The internal or laminar layer consists of the =primary= and =secondary laminæ= and is non-pigmented. The laminæ are narrow and thin at their origin at the lower margin of the coronary groove, but become wider and thicker below. At the junction of the wall and sole they are united by interlaminar horn to form the white line. Only the central part of the laminæ becomes fully keratinized. They are composed of non-tubular horn in the normal state.

The =sole= consists of tubular and intertubular horn. The tubes run parallel with those of the wall and vary much in size.

The =frog= is composed of relatively soft horn, which is much more elastic than that of the wall or sole, and is not fully keratinized. The horn tubes in it are slightly flexuous.

The hoof is non-vascular and receives its nutrition from the matrix. It is also destitute of nerves.

THE MATRIX OF THE HOOF

The =matrix= of the hoof (Matrix ungulæ) is the specially modified and highly vascular part of the corium of the common integument which furnishes nutrition to the hoof. It is convenient to divide it into five parts which nourish corresponding parts of the hoof.

1. The =perioplic matrix= or =ring= (Margo matricis ungulæ) is a band 5 to 6 mm. in width which lies in a groove between the periople and the coronary border of the wall. It is continuous above with the corium of the skin, and is marked off by a groove from the coronary matrix. At the heels it widens and blends with the matrix of the frog. It bears very fine, short papillæ which curve downward and are received in depressions of the periople, to which it supplies nutrition.

2. The =coronary matrix= or =cushion= (Corona matricis ungulæ) is the thick part of the corium which occupies the coronary groove, and furnishes nutrition to the bulk of the wall. It diminishes in width and thickness posteriorly, and along the upper border of the bar it is not very clearly defined from the matrix of the frog. The convex superficial surface is thickly covered with papillæ 4 to 6 mm. in length, which are received into the funnel-like openings of the coronary groove. At the heels and along the bars the papillæ are arranged in rows, separated by fine furrows. The deep surface is attached to the extensor tendon and the lateral cartilages by an abundant subcutis which contains many elastic fibers and a rich venous plexus.

3. The =laminar matrix= (Latus matricis ungulæ)—also termed the sensitive laminæ—bears primary and secondary laminæ which are interleaved with the horny laminæ of the wall and bars in the natural state. It is attached to the wall surface of the third phalanx by a modified periosteum (Stratum periosteale) which contains a close-meshed network of vessels, and to the lower part of the lateral cartilages by a subcutis which contains a rich venous plexus. The laminæ are small at their origin above, become wider below, and end in several papillæ 4 to 5 mm. in length. The sensitive laminæ supply nutrition to the horny laminæ and to the interlaminar horn of the white line.

4. The =sole matrix= (Latus volare matricis ungulæ)—also termed the sensitive sole—corresponds to the horny sole, to which it supplies nutrition. It is often more or less pigmented and bears long papillæ, which are specially large along the convex border and at the angles. Centrally it is continuous with the matrix of the frog and bars. The deep surface is attached to the sole surface of the third phalanx by a modified and highly vascular periosteum.

FIG. 580.—LATERAL VIEW OF FOOT OF HORSE AFTER REMOVAL OF HOOF AND PART
OF SKIN. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
]

FIG. 581.—GROUND SURFACE OF FOOT OF HORSE AFTER REMOVAL OF HALF OF
HOOF. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
]

5. The =frog matrix= (Furca matricis ungulæ)—also called the sensitive frog—is moulded on the deep surface of the frog and bears small papillæ. Its deep face is blended with the plantar cushion. The germinal cells of the frog derive their nutrition from this part of the matrix.

The =digital= or =plantar cushion= (Torus digitalis) is an important part of the elastic apparatus of the foot. It is a wedge-shaped mass which overlies the frog. It presents for description four surfaces, a base, and an apex. The =superior surface= faces upward and forward and is connected with the distal fibrous sheath of the deep flexor tendon. The =inferior surface=, covered by the matrix of the frog, is moulded on the upper face of the frog. The =lateral surfaces= are related chiefly to the lateral cartilages; inferiorly the cushion is closely attached to the cartilages, but higher up a rich venous plexus intervenes. The =base=, situated posteriorly, is partly subcutaneous, and is divided by a central depression into two rounded prominences termed the bulbs of the plantar cushion. The =apex= lies under the terminal part of the deep flexor tendon. The cushion is poorly supplied with vessels. It consists of a feltwork of fibrous trabeculæ and elastic fibers, in the meshes of which are masses of fat. The bulbs are soft and loose in texture and contain a relatively large amount of fat, but toward the apex the cushion becomes denser and more purely white fibrous in structure. Branched =coil glands= occur chiefly in the part of the cushion which overlies the central ridge of the frog. Their ducts pursue a slightly flexuous course through the matrix and pass in a spiral manner through the frog. Their secretion contains fat.

FIG. 582.—CROSS-SECTION OF DIGIT OF HORSE, THROUGH DISTAL END OF
SECOND PHALANX.

_a_, Second phalanx; _b_, deep flexor tendon; _b′_, digital synovial
sheath; _c_, plantar cushion; _d_, lateral cartilage; _e_, skin;
_f_, hoof; _g_, anterior extensor tendon; _h_, cavity of coffin
joint; _i_, suspensory ligament of navicular bone; _k_, lateral
ligament of coffin joint; _l_, digital artery; _m_, digital nerve.
(After Ellenberger, in Leisering’s Atlas.)
]

=Vessels and Nerves.=—The matrix is richly supplied with blood by the =digital arteries=. The =veins= are valveless, and form remarkable plexuses which communicate freely with each other and are drained by the digital veins. The =lymph vessels= form subpapillary plexuses in the matrix of the sole and frog, and a wider-meshed plexus at the base of the plantar cushion. A considerable lymph vessel lies in the attached edge of each of the laminæ. The =nerves= are branches of the digital nerves; some fibers end in lamellar corpuscles and end-bulbs.

The =ergot= is a small mass of horn which is situated in the tuft of hair at the flexion surface of the fetlock. It is the vestige of the second and fourth digits of extinct equidæ and hence is absent in cases in which these digits are developed. A small tendinous band, 3 to 5 mm. in width, extends downward and slightly forward from the fibrous basis of the ergot on each side, crosses over the digital vessels and nerves very obliquely, and blends below with the digital fascia and the plantar cushion. It is known as the ligament or tendon of the ergot.

The term =chestnut= is applied to the masses of horn which occur on the inner surface of the forearm about a handbreadth above the carpus, and on the lower part of the inner face of the tarsus. They have an elongated oval form and are flattened. They are regarded usually as vestiges of the first digit. That of the hind limb is absent in the donkey and very small in the mule.

These horny vestiges are quite variable in form and size and are
correlated with the fineness or coarseness of the integument in
general. The supracarpal chestnut is about 1½ to 2½ inches long, oval
in outline, the proximal end being pointed; it overlies the flexor
carpi internus at a quite variable distance above the carpus, and
hence should not be used as a surgical landmark. The tarsal chestnut
lies at a point behind the lower part of the internal lateral ligament
of the hock. When well developed it is about 2 to 2½ inches long,
broad below and produced above to form a long pointed end, with a
short blunt anterior process. They are composed of horn somewhat like
that of the frog.

ORGAN OF SMELL

The peripheral part of the olfactory apparatus or organ of smell (Organon olfactus) is that part of the nasal mucous membrane which was referred to in the description of the nasal cavity as the olfactory region; this (Regio olfactoria) is limited to the ethmoturbinals and the adjacent part of the superior turbinal and the septum nasi, in which the fibers of the olfactory nerve ramify. It is distinguished by its yellow-brown color, thickness, and softness. It contains characteristic tubular =olfactory glands= (of Bowman), which are lined by a single layer of pigmented cells, and a neuro-epithelium, the =olfactory cells=, the central processes of which extend as non-medullated fibers to the olfactory bulb.

The epithelium is non-ciliated and is covered by a structureless
limiting layer. It consists essentially of three kinds of cells,
supporting, basal, and olfactory. The supporting cells are of long
columnar form above and contain pigment granules; below they taper and
often branch, and their central processes unite with those of adjacent
cells to form a protoplasmic network. The basal cells are branched and
lie on a basement membrane. The olfactory cells are situated between
the supporting cells; they have the form of long narrow rods, with an
enlarged lower part which is occupied by the nucleus. The peripheral
end pierces the limiting membrane and bears a tuft of fine hair-like
cilia (olfactory hairs). A central process extends from the nucleated
pole of the cell to the olfactory bulb as a non-medullated olfactory
nerve-fiber.

The =vomero-nasal organ= (of Jacobson) lies along each side of the anterior part of the lower border of the septum nasi. It communicates with the nasal cavity through the naso-palatine canal. It consists of a tube of hyaline cartilage lined with mucous membrane; a small part of the latter along the inner side is olfactory in character.

THE ORGAN OF TASTE

The peripheral part of the gustatory apparatus (Organon gustus) is formed by the microscopic =taste buds= (Calyculi gustatorii), which occur especially in the foliate, fungiform, and vallate papillæ, in the free edge and anterior pillars of the soft palate, and the oral surface of the epiglottis. The taste buds are ovoid masses, which occupy corresponding recesses in the ordinary epithelium; each presents a minute opening, the gustatory pore. The buds consist of fusiform supporting cells grouped around central =gustatory cells=. The latter are long and narrow; the peripheral end of each bears a small filament, the gustatory hair, which projects at the gustatory pore; the central end is produced to form a fine process which is often branched. The taste buds are innervated by fibers of the glosso-pharyngeal nerve and the lingual branch of the trigeminus.

SENSE ORGANS AND SKIN OF THE OX

THE EYE

The =eyelids= are thick, prominent, and less pliable than those of the horse. The lower lid bears a considerable number of cilia, which are, however, finer than those of the upper lid. The tarsal glands are more deeply embedded and therefore not so evident. The =conjunctiva= of the lower lid presents folds or ridges, and one or two prominences which contain numerous leukocytes; the conjunctival epithelium is transitional in type. The superficial part of the cartilage of the =third eyelid= is leaf or shovel-shaped and thicker than in the horse; the edge bears a narrower process, on which there is a transverse bar, giving the arrangement some resemblance to an anchor. The =gland= of the third eyelid is very large—an inch or more in length—and may be divided into two parts. The deep part is pink and consists of loose lobules, but the much larger superficial part is more compact. There are two large and several smaller excretory ducts. The subconjunctival tissue of the third eyelid contains lymph nodules, which are specially numerous on the bulbar side.

FIG. 583.—EYE OF OX. (After Ellenberger-Baum, Anat. für Künstler.)
]

The =lacrimal gland= is thick and distinctly lobulated. It is more or less clearly divided into a thick upper and a thinner lower part (Glandula lacrimalis superior, inferior). There are six to eight larger excretory ducts and several smaller ones. The =naso-lacrimal duct= is shorter than in the horse, and is almost straight. Its terminal part is inclosed between two plates of cartilage, and it opens near the nostril on the outer wall; the orifice is placed on the inner side of the alar fold of the inferior turbinal, and is therefore not easily found.

FIG. 584.—LEFT EYEBALL OF OX _in situ_, EXTERNAL VIEW.

_19_, Orbital fat. (After Ellenberger-Baum, Anat. für Künstler.)
]

The =eyeball= resembles that of the horse in shape, but is considerably smaller.

The average transverse diameter is about 42 mm., the vertical diameter
41 mm., and the axis 36 mm. The angle between the axes is about 119°.

The =sclera= is often more or less pigmented; where non-pigmented it may have a bluish tinge. The point of entrance of the optic nerve is about 2 mm. external to the vertical meridian, and about 7 mm. below the horizontal meridian.

The thickness of the sclera is about 2 mm. at the posterior pole, 1
mm. at the equator, and 1.2 to 1.5 mm. near the cornea.

The =cornea= is thicker than that of the horse.

The =tapetum= of the chorioidea is extensive at the outer side, but is a narrow strip internally. It has a metallic luster, shading from a brilliant green to a deep blue; centrally it has a reddish sheen.

The =iris= is usually very dark colored. The granula form a continuous series of small nodules along the upper margin of the pupil and are most prominent centrally; they are very small on the lower margin.

The =optic papilla= is much smaller and is not so sharply defined on ophthalmoscopic examination as that of the horse. Several relatively large retinal arteries radiate from the center of the papilla to the periphery of the retina. They may be seen to wind spirally around the veins, which are very large.

The arteria centralis retinæ arises from a posterior ciliary artery
and divides close to the optic papilla into three branches usually; of
these the upper is larger than the two lateral branches. The retina is
more vascular than that of the horse; it has a round area centralis at
the inner side, which is continued outward by a narrow strip of a
similar histological structure.

The =vitreous body= is less fluid than that of the horse.

THE EAR

The =external ear= is inclined outward. The middle part is much wider and less curved than in the horse. The apex is also wide and does not curve forward. The lower part of the anterior border is bent backward and bears long hairs. The posterior border is thin and is regularly convex, except below, where it is indented. The opening is wide and the concave surface presents four ridges; the convex surface presents corresponding depressions. The styloid process of the conchal cartilage is short and blunt. The edges of the annular cartilage are in contact at the inner side and the upper border is notched externally. The scutiform cartilage is a very irregular quadrilateral plate; it is situated on the lateral aspect of the cranium with its concave surface in contact with the fat and temporalis muscle. The osseous external auditory meatus is directed practically straight inward; it is about twice as long as that of the horse, and tapers very gradually from without inward. The chief special characters of the =auricular muscles= are as follows: (1) The scutularis blends with the frontalis muscle. The interscutularis arises from the base of the processus cornu (or the corresponding area in polled cattle) and from the frontal crest. (2) The cervico-scutularis arises from the posterior surface of the cranium below the frontal eminence, and receives slips from the parieto-auricularis and cervico-auricularis superficialis. (3) The zygomatico-auricularis and scutulo-auricularis superficialis inferior are fused. (4) The scutulo-auricularis superficialis superior is distinct from the interscutularis; it arises from the superficial face of the scutiform cartilage. (5) The cervico-auricularis superficialis arises from the ligamentum nuchæ and ends on the convex surface of the conchal cartilage. (6) The parieto-auricularis arises from the nuchal surface of the frontal bone, the adjacent part of the parietal bone, and the ligamentum nuchæ; it is inserted below the preceding muscle. (7) The scutulo-auricularis superficialis accessorius arises from the superficial face of the scutiform cartilage and is inserted by a round tendon into the anterior part of the convex surface of the conchal cartilage. (8) The cervico-auricularis profundus minor consists of two fasciculi; the inner, narrow part is deep red in color, and arises from the fascia of the cervical muscles external to the occipital attachment of the ligamentum nuchæ; the outer, wider part is pale, and arises beneath the cervico-auricularis superficialis, indirectly from the scutiform cartilage. The two unite and are inserted into the lower aspect of the base of the conchal cartilage. (9) The scutulo-auricularis profundus minor arises from the temporal crest above the external auditory meatus and is inserted into the anterior part of the deep face of the scutiform cartilage.

The =cavum tympani= is small; it communicates ventrally with the air-cells of the bulla ossea. The tympanic membrane is nearly circular and is not so oblique as in the horse. The auditory ossicles are smaller than those of the horse; the malleus is more curved, the body of the incus is longer, and there is a small prominence on the front of the head of the stapes for the attachment of the stapedius muscle.

The =Eustachian tube= is small and is only about two inches long. The pharyngeal opening is small and is situated on the side of the fornix of the pharynx close to the base of the cranium. There is no flap-like expansion of the cartilage, but the inner border of the orifice is formed by a fold of mucous membrane.

The =internal ear= has essentially the same arrangement as in the horse.

COMMON INTEGUMENT

The thickness of the =skin= of the ox is greater than that of any of the other domesticated animals; in general it is about 3 to 4 mm., but at the root of the tail and the point of the hock it is about 5 mm. and on the brisket 6 to 7 mm. The variably developed prominence at the anterior part of the pectoral region known as the brisket consists of a fold of skin (Plica colli ventralis longitudinalis), which contains posteriorly a mass composed of coarse fibrous trabeculæ and fat.

The =cutaneous glands= are fewer and less developed than in the horse. Except about the natural openings, at the point of the hock, and the flexion surface of the fetlock, the =sweat glands= do not form a coil, but are enlarged at the deep end and are variably flexuous. The =sebaceous glands= are best developed about the natural openings and on the udder, but there are none on the teats. The =naso-labial glands= form a thick layer under the bare skin of the muzzle. They are compound tubular glands and are lined with cubical epithelium. The openings of their excretory ducts are easily seen.

The =hairs= are extremely variable in color and size in the different breeds and in different individuals. The hair of the frontal region is often curly, especially in the bull. There is no mane, and the long hairs of the tail occur only at the end, where they form the “brush” (Cirrus caudæ).

FIG. 585.—CLAWS OF OX, VOLAR ASPECT. (After Ellenberger-Baum. Anat.
für Künstler.)
]

The =claws=, four in number on each limb, cover the ends of the digits. Those of the chief digits conform in a general way to the shape of the third phalanges, and each may be regarded as having three surfaces. The abaxial or outer surface is convex from side to side, and is marked by ridges parallel with the coronary border. Its anterior part is concave from edge to edge, and the angle which it forms with the ground is about 30°. The interdigital or inner surface is concave and grooved; it touches the opposite claw only at its ends. The inferior or ground surface consists of two parts, viz., a slightly concave sole, which is pointed in front and widens behind, and a prominent bulb of soft thin horn, which is continuous above with the skin. The claw may be regarded as consisting of three parts—periople, wall, and sole. The periople surrounds the coronary border in the form of a flat band, which is about half an inch wide, except at the heels, where it widens to cover the entire surface. The wall forms most of the abaxial or outer part of the claw and is reflected in front upon the interdigital surface. It thins out toward the bulb or heel, which appears, as stated above, to consist of the thin expansion of the periople. The sole occupies the angle of inflection of the wall; it is continuous without demarcation with the periople of the bulb. The perioplic matrix bears relatively long papillæ. The coronary matrix is much less developed than in the horse and its papillæ are short. The sensitive laminæ are much narrower and are more numerous than in the horse; secondary laminæ are not present. The matrix of the sole is not marked off behind from that of the periople; its papillæ are very small and close together. The matrix of the bulbs is separated from the flexor tendon by a mass of elastic, fatty tissue, which is analogous with the plantar cushion of the horse. The papillæ here are long and often compound.

FIG. 586.—SAGITTAL SECTION OF DISTAL PART OF FORE LIMB OF OX.
]

The accessory digits bear short conical horn capsules which resemble in a general way those of the chief digits, and have a similar matrix, which covers one or two nodular vestigial phalanges. From these a fibrous band descends obliquely on the volar aspect of each chief digit and is attached below to the distal phalanx and sesamoid bones, sending fibers also to the elastic pad of the heel.

The =horns= (Cornua) inclose the horn processes of the frontal bones (except in the polled breeds). They vary very greatly in size, form, and curvature. The root or base of the horn (Radix cornus) has a thin edge which is continuous with the epidermis. It is covered by a thin layer of soft horn similar to the periople of the hoof of the horse. Near the root the horn is encircled by variable rings. Toward the apex (Apex cornus) the thickness of the horn increases till it becomes practically a solid mass. The horn consists mainly of tubes which are very close together, except at the rings, where there is more intertubular horn. The =matrix= of the horn (Matrix cornus) is united to the horn process by periosteum which is traversed by numerous blood-vessels. The matrix at the root of the horn is thick and bears long, slender papillæ; in the body of the horn it becomes thin and the papillæ are smaller, but increase in size apically. Rudimentary papillated laminæ also occur.

The =skin= of the =sheep= varies in thickness from 0.5 to 3 mm., but differs greatly in fineness and in other respects in various breeds. In Merinos considerable folds occur on the neck. Cutaneous pouches (Sinus cutis) are constantly present in certain situations. The =infraorbital= or =lacrimal pouch= (Sinus cutaneus infraorbitalis) is an invagination about half an inch in depth, which is situated in front of the inner angle of the eye. It bears scattered fine hairs, into the follicles of which large compound sebaceous glands open; coil glands are also present. The secretion of the glands is fatty and forms a yellow sticky covering on the skin when dry. The =inguinal= (or mammary) =pouch= (Sinus cutaneus inguinalis) is much more extensive, and is situated in the inguinal region in both sexes. The skin of the pouch bears scattered fine hairs and contains well-developed sebaceous glands and exceedingly large coil glands. The =interdigital pouch= (Sinus cutaneus interdigitalis) is a peculiar tubular invagination of the integument which opens at the anterior part of the interdigital cleft. On sagittal section it appears as a bent tube, an inch or more (ca. 2.5 to 3 cm.) in length, and about a fourth of an inch (ca. 6 to 7 mm.) in diameter. The pouch extends downward and backward, and then curves sharply upward between the distal ends of the proximal phalanges. Its deep blind end is somewhat ampullate. The subcutis forms a capsule around it. The skin of the pouch is thin and pale; it bears fine colorless hairs, the follicles of which receive the secretion of several sebaceous glands. The coil glands here are compound and very large; they open into the hair-follicles or directly into the pouch. The secretion of the glands is a colorless fatty substance. The skin is covered in great part by wool, which is similar in structure to hair, but is finer, curly, and usually contains no medullary cells. The follicles of the wool are curved, and are arranged more or less distinctly in groups of ten or a dozen, several of which open in common on the surface. A considerable part of the face and the limbs is covered with short, stiff hair, and long hairs occur more or less interspersed among the wool. The claws resemble those of the ox. The horns, when present, are more or less prismatic, distinctly ringed, and vary in length and curvature in different breeds. Their structure is similar to those of the ox.

The olfactory and gustatory organs resemble in general those of the horse.

SENSE ORGANS AND INTEGUMENT OF THE PIG

THE EYE

The posterior part of the orbital margin is formed chiefly by the orbital ligament, which extends from the extremely short supraorbital process to the small eminence at the junction of the malar and the zygomatic process of the temporal.

Cilia occur only on the upper eyelid. The inner canthus is prolonged by a groove on the infraorbital region. The tarsal glands are very short and curved. The conjunctival epithelium is of the transitional type with many goblet cells.

The deep part of the cartilage of the third eyelid is broad and spoon-like, the superficial part somewhat anchor-shaped. In addition to the gland which surrounds the deep part of the cartilage, there is a deeper gland of Harder (Glandula palpebræ tertiæ profunda), which is surrounded by a distinct capsule and a blood-sinus. This gland is brownish or yellowish-gray in color, elliptical in outline, and about an inch (2 to 3 cm.) in length. It is situated deeply below the attachment of the inferior oblique muscle.

The caruncula lacrimalis has the form of a ridge; it divides into two branches which join the skin at the inner commissure. It is red in color and contains numerous large coil glands.

The lacrimal gland is mucous in type. There is no lacrimal sac, and the two lacrimal ducts pass through separate openings at the infraorbital margin. The naso-lacrimal duct is usually short and opens into the inferior meatus at the posterior end of the inferior turbinal.[229]

The eyeball appears small, partly on account of the narrowness of the palpebral opening; it closely approaches the spherical form. The cornea, although in reality almost circular, appears oval when viewed from the front, the inner end being much the broader. The chorioid has no tapetum. The iris is usually dark grayish-brown or yellowish-brown, but sometimes has a blue tinge. The pupil is a transverse oval in ordinary light, but almost circular when dilated; granula are not present. The optic papilla is nearer the posterior pole than in the horse and ox, and has a distinct central artery and vein.

THE EAR

The external ear differs considerably in size, thickness, and position. It may be carried vertically, inclined inward, or hang downward. It is relatively wide and is little curved except at the base. The concave surface presents several cutaneous ridges which correspond approximately with the long axis of the ear. The anterior border of the concha is strongly recurved in its lower part, and divides into two branches, one of which passes almost horizontally backward in the cavum conchæ, while the other continues the general direction of the border. The posterior border is slightly concave above, strongly convex below, forming a prominence somewhat analogous to the lobule of the human ear. The osseous external auditory meatus is very long and is directed downward and inward. The tympanic membrane is almost circular.

The tympanic cavity proper is small, but it is continuous with the numerous cells of the large bulla ossea. The Eustachian tube is short; its pharyngeal opening is situated in the upper part of the wall of the pharynx immediately behind the posterior nares. It is somewhat infundibular, and is bounded internally by a thick fold of mucous membrane (Torus tubarius).

THE SKIN AND APPENDAGES

The thickness of the skin in improved breeds is 1 to 2 mm. Fat usually accumulates in the subcutis and forms a distinct and often extremely thick panniculus adiposus over the greater part of the body.

The sebaceous glands are in general small and much fewer than in the other animals. The sweat glands, on the other hand, are large, yellow or brownish in color, and are in many places visible to the naked eye. At the inner side of the carpus there are small cutaneous diverticula, the so-called carpal glands, into which numerous compound coil glands open. Large glands also occur in the skin of the digits and interdigital space. Compound tubular glands are present in the skin of the snout. Large sebaceous and sweat glands are found at the entrance to the preputial diverticulum.

The hairs are sparsely scattered in improved breeds—indeed, in some cases the skin is almost bare. The long hairs or bristles are arranged usually in groups of three. They are most developed on the neck and back. The bristles about the snout and the chin are sinus hairs.

The =claws= and their matrix resemble those of the ox, but the bulbs are more prominent and form a greater part of the ground surface; they are also better defined from the sole, which is small. The claws of the accessory digits are more developed and their parts better differentiated than those of the ox.

The olfactory region is extensive in correlation with the large size of the olfactory bulbs; the mucous membrane here is brown in color.

SENSE ORGANS AND INTEGUMENT OF THE DOG

THE EYE

The posterior margin of the orbit is formed by the orbital ligament, as in the pig. The orbital axes if produced backward would include an angle of about 79°, the optic axes an angle of about 92.5° (Koschel).

The lower eyelid has no distinct cilia. The inner angle is wide, the lacus lacrimalis shallow, and the caruncula lacrimalis small and yellowish-brown. The conjunctival epithelium is cylindrical, with many goblet cells. The gland which surrounds the deep end of the cartilage of the third eyelid is large and pink; it is a mixed gland.

The lacrimal gland is flat and lies chiefly under the orbital ligament. It is pink in color and mixed in structure. The naso-lacrimal duct is variable in arrangement and may differ on the two sides. It may open into the inferior meatus immediately after its emergence from the osseous lacrimal canal; thence it continues forward and opens on the lateral wall of the nostril below the inferior turbinal fold.

The eyeball is almost spherical and is relatively large, especially in the small breeds. The sclera is thick in the ciliary region, where it contains a well-developed venous plexus; in the vicinity of the equator it is thin, and the chorioid shows through it. The cornea is almost circular. The chorioid is richly pigmented, and presents a well defined =tapetum cellulosum=; this is somewhat triangular in outline and has a metallic luster. Its color is often golden green, shading to blue peripherally, but the blue tone may predominate, and in other cases it is mainly golden yellow or reddish-yellow. The appearance is due to several layers of peculiar flattened polygonal cells. The ciliary muscle is more developed than in the other species. The iris is commonly light or yellow brown, but not rarely has a blue tinge; the color often differs in the two eyes. The pupil is round; its edge is either smooth or bears minute round prominences. The retina presents a round area centralis external to the optic papilla. The latter is variable in outline; commonly it has the form of a triangle with the angles rounded off, but is often round or oval. Branches of the central vessels of the retina radiate from the middle of the papilla. The surfaces of the lens are not so strongly curved as in the other domestic animals, and there is little difference in the radius of curvature of the two surfaces.

THE EAR