Part 232

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

The external ear differs greatly in size and form in the various breeds; in most it is relatively wide and thin and the greater part of it hangs down over the opening. The anterior border has a prominence (Spina helicis) at its lower part, and below this it divides into two branches (Crura helicis). One of these continues down to a blunt point, while the other turns in to the cavity of the concha. The posterior border forms a pouch below its middle, and is thick and rounded in its basal part. Here the conchal cartilage bears a pointed process (Processus uncinatus), which projects upward and backward. The outer wall of the cavum conchæ is supported by a quadrilateral plate (Tragus) above and a semiannular cartilage below. The concave surface presents several cutaneous folds, and in the cavum conchæ there are ridges and irregular prominences. There are long hairs on the borders, apex, and the folds of the concave surface, but the skin of the cavum conchæ is bare. The annular cartilage is a complete ring. The osseous external auditory meatus is wide and extremely short.

The auricular muscles are thin. The scutularis is extensive and is divisible into interscutularis and fronto-scutularis. It partly covers the occipitalis muscle, which is oval and unpaired and ends in the fascia of the frontal region. The scutulo-auriculares superficiales, inferior et accessorius, and the cervico-scutularis are not differentiated as separate muscles. The cervico-auricularis superficialis and the parieto-auricularis both bifurcate and are inserted into the base of the concha and the scutiform cartilage. The parotido-auricularis is narrow and almost long enough to meet the opposite muscle. The tragicus consists of two parts; of these the lateral one is long and arises from the border of the mandible above the angular process. The helicis and the verticalis auriculæ are well developed.

The tympanic cavity is very roomy, extending ventrally into the large but simple cavity of the bulla ossea. The tympanic membrane is extensive and oval. The promontory is very pronounced. The auditory ossicles are large. The Eustachian tube is short, and has a fibrous outer wall. Its pharyngeal opening is an oblique slit situated on the lateral wall of the naso-pharyngeal meatus, and is margined internally by a thick mucous fold.

The cochlea has three and a half turns and is more sharply pointed than in the ungulates.

THE SKIN AND APPENDAGES

The skin varies greatly in thickness in different breeds. It is remarkably loose on the dorsal aspect of the neck and trunk, where it can be raised in extensive folds. The coat is also subject to extreme variation in length, thickness, color, etc. The hairs are arranged in groups of three to a large extent. The sebaceous glands are best developed in the short and rough haired breeds. They are largest and most numerous at the lips, anus, dorsal surface of the trunk, and sternal region. The sweat glands are relatively better developed in the long and fine haired breeds. The largest are found in the digital pads. Coil glands occur in the skin of the muzzle, the perineum, and the paranal pouches.

On the flexion surface of the carpus and digit there are hairless, cushion-like pads. The carpal pad (Torus carpalis) is situated medial and distal to the accessory carpal bone. The metacarpal and metatarsal pads (Torus metacarpalis, metatarsalis) are the largest and are situated behind the distal ends of the metacarpal or metatarsal bones and the greater part of the first phalanges. They are somewhat heart-shaped, with two unequal lateral lobes, and the apex distal. The metapodiophalangeal joints rest on them when the paw supports the weight. The digital pads (Tori digitales) are oval and are much smaller; they similarly support the second digital joints. That of the first digit is small on the fore limb, and usually absent from the hind limb. Each receives two suspensory bands from the tubercles at the distal end of the corresponding second phalanx. The pads have a basis of fibro-elastic tissue and fat, which is intimately adherent to the skin and is connected by trabeculæ with the bones and tendons. The epidermis is thick and largely keratinized, and is studded with numerous rounded papillæ which are readily seen with the naked eye. The corium has large papillæ, and contains sweat glands and lamellar corpuscles.

The =claws= (Ungues) correspond in form to the ungual part of the distal phalanges which they inclose. The horn of the claw consists of a body or wall and a sole. The former is strongly curved in both directions, and is compressed laterally. Its root or coronary border fits into the depression under the bony collar or ungual crest of the third phalanx and is covered by skin. The lateral borders converge and inclose the sole anteriorly. The matrix presents a coronary part which bears papillæ only near the coronary border. On the dorsal surface it is thickened to form a ridge, and the corresponding part of the horny covering is thick and maintains the pointed character of the claw. The matrix of the sides of the wall bears small laminæ which converge to the matrix of the sole. The latter is papillated. When the flexor muscles are inactive, the distal phalanges and the claws are maintained in dorsal flexion by two elastic ligaments. These arise from the tubercles at the proximal end of the second phalanx and converge to the dorsal aspect of the ungual crest of the third phalanx.

FIG. 587.—PADS OF PAW OF DOG.

a, Carpal pad; b, metacarpal pad; c^1-c^5, digital pads. (From Leisering’s Atlas.) ]

FIG. 588.—HALF OF CLAW OF DOG, INNER SURFACE.

a, a′, Coronary part, b, volar part of wall; c, sole; d, section of c; e, section of wall; 1, coronary border; 2, apex; 3, dorsal surface. (From Leisering’s Atlas.) ]

OLFACTORY AND GUSTATORY ORGANS

The =olfactory region= lies above the lamina transversa, which separates it from the naso-pharyngeal meatus. The mucous membrane is distinguished by its thickness and yellow color, and is very extensive. It covers about half of the large ethmoturbinals, and one-third to one-half of the septum nasi. The ethmoturbinal which projects into the frontal sinus and the adjacent inner wall of the sinus is olfactory in character. Olfactory nerve-fibers ramify in the vomero-nasal organ (of Jacobson).

The gustatory apparatus presents no special features of importance.

INDEX

Abdomen, muscles of, 245 of dog, 326 of pig, 313 muscles of, 301, 313

Abdominal aorta, 536 branches of, 566 of dog, 638 artery, anterior, 541 posterior, 579 subcutaneous, 580 cavity of horse, 352 of ox, 390 fascia of horse, 245 muscles of dog, 326 of horse, 245 of pig, 301, 313 panniculus, 245 plexuses, 713 tunic of horse, 245 of pig, 313 veins, 598