STRUCTURE OF THE HORSE. 95 



with villi or papillae. The membrane of one system becomes 

 continuous with that of another; for an illustration, the mem- 

 brane which lines the digestive canal becomes continuous with 

 that lining the respiratory canal. In a similar manner the mu- 

 cous membrane of the urinary canal becomes continuous wdth 

 that of the genital canal in the vagina in the female, in the 

 urethra in the male; the membrane becomes continuous with the 

 skin at the orifice of these organs. The closed cavities of the 

 body ai-e lined by serous membranes. The following are the 

 serous membranes and their position in the body: The perito- 

 neum lines the abdominal cavity; the pleura covers the lungs 

 and lines the thoracic cavity; the pericardium, that contains and 

 supports the heart; the endocardium, which is continued through 

 the veins and arteries, lymphatics, etc. 



THE SKIN AND ITS APPENDAGES. 



The skin and its appendages form the external casing of 

 the body. The skin consists of two parts — the epidermis or 

 cuticle, superficially placed, and the dermis, corium or cutis vera, 

 which forms the deep layer. Its chief appendages are glands 

 and the epithelial modifications, hair, horn, and hoof. The epi- 

 dermis forms a protective covering over the whole surface of the 

 body. It varies in thickness in different parts, being especially 

 thick wherever the skin is exposed to friction. It is moulded 

 over the surface of the corium, covering the ridges, depressions, 

 and papillae. It is made up of three principal layers — the horny 

 layer, or stratum corueiim, is the most superficial, and consists 

 of layers of flattened cells, which are dry and horny, without any 

 nucleus; the stratum lucidum, composed of several layers of nu- 

 cleated cells, which are more or less indistinct and in section 

 appear as an almost homogeneous layer; the rete mucosum or 

 malpighian layer consists, in its upper part, of layers of prickle 

 cells, and its inferior layer consists of a single stratum of 

 columnar cells. In these cells the pigment exists which gives 

 color to the skin. The cuticle is itself insensible, but one of its 



