900 THE SENSE ORGANS AND COMMON INTEGUMENT OF THE PIG 
ordinary 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 corium 
of the horn (Corium cornus) is united to the horn process by periosteum which is 
traversed by numerous blood-vessels. The corium at the root of the horn is thick 
and bears long, slender papillae; in the body of the horn it becomes thin and the 
papillze are smaller, but increase in size apically. Rudimentary papillated lamin 
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 large folds 
occur on the neck. Cutaneous pouches (Sinus cutis) are constantly present 
in certain situations. The infraorbital or lacrimal pouch (Sinus cutaneus infra- 
orbitalis) is an invagination about half an inch in depth, which is situated in front 
of the medial 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 interdigi- 
talis) 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. 2.5-3 em.) in length, and about a fourth of an inch (ca. 6- 
7 mm.) in diameter. The pouch extends downward and backward, and then 
curves sharply upward between the distal ends of the first 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 to 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 oecur 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. 
THE SENSE ORGANS AND COMMON INTEGUMENT OF THE PIG 
THE EYE 
The posterior part of the orbital margin is formed chiefly by the orbital liga- 
ment, 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 medial 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. 
