764 THE SENSE ORGANS AND SKIN OF THE HORSE 



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 trabecule 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, Ijut 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 (Jul^a) 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 caudse). 

 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 (Unguis) is the horny covering of the distal end of the digit. It is 

 convenient to divide it for dc^scription 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.- 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 surfac(> of the hoof as convergent ritlges, which sub- 

 side 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 ungulse dorsalis), lateral parts or "quarters" (Pars ungulse medialis, later- 

 alis), 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 alwut 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 



1 The hoof may be loosened and removed intact by allowing the foot to macerate or by 

 boiling it. 



2 The term foot is used here in the popular sense, i. e., to designate the hoof and the structures 

 inclosed within it. 



