STRUCTURE OF THE SKIN. 433 



mediately superposed cell layers (mucous) that become morbidly 

 increased in the earlier stages of inflammation; it is on the surface 

 of the papillary layer that the liquid is thrown out which raises the 

 cuticle in the form of a blister, and it is at this point mainly that pus 

 forms in the ordinary pustule. 



The fibrous bundles of the true skin contain plain muscular fibers, 

 which are not controlled by the will, but contract under the influence 

 of cold and under certain nervous influences, as in some skin dis- 

 eases and in the chill of a fever, and lead to contraction, tightening, 

 or corrugation of the skin, contributing to produce the " hidebound " 

 of the horseman. Other minute muscular filaments are extended 

 from the surface of the dermis to the hair follicle on the side to which 

 the hair is inclined, and under the same stimulating influences pro- 

 duce that erection of the hair which is familiarly known as " staring 

 coat." Besides these, the horse's skin is furnished with an expansion 

 of red voluntary muscle, firmly attached to the fibrous bundles, and 

 by which the animal can not only dislodge insects and other irritants, 

 but even shake 'off the harness. This fleshy envelope covers the sides 

 of the trunk and the lower portions of the neck and head, the parts 

 unprotected by the mane and tail, and serves to throw the skin of. 

 these parts into puckers, or ridges, in certain irritating skin diseases. 



The hairs are cuticular products growing from an enlarged papilla 

 lodged in the depth of a follicle or sac, hollowed out in the skin and 

 extending to its deepest layers. The hair follicle is lined by cells of 

 epidermis, which at the bottom are reflected on the papilla and become 

 the root of the hair. The hair itself is formed of the same kind of 

 cells firmly adherent to each other by a tough intercellular substance, 

 and overlapping each other like slates on a roof in a direction toward 

 the free end. 



The sebaceous glands are branching tubes ending in follicles or sacs 

 and opening into the hair follicles, lined by a very vascular fibrous 

 network representing the dermis, and an internal layer of cells repre- 

 senting the mucous layer of the cuticle. The oily secretion gives gloss 

 to the hair and prevents its becoming dry and brittle, and keeps the 

 skin soft and supple, protecting it at once against undue exhalation of 

 water and undue absorption when immersed in that medium. Besides 

 those connected with the hair follicles there are numerous isolated 

 sebaceous glands, opening directly on the surface of the skin, produ- 

 cing a somewhat thicker and more odorous secretion. These are found 

 in large numbers in the folds of the skin, where chafing would be 

 likely if the surface were dry, as on the sheath, scrotum, mammary 

 glands, and inner side of the thigh, around the anus and vulva, in the 

 hollow of the heel, beneath the fine horn of the frog, on the inner side 



H. Doc. 705, 50-2 28 



