CHAPTER XI 

 DISEASES OF THE SKIN 



General Discussion. — The two layers that form the skin are 

 the epidermis and the derma. The cells of the enter layer or epi- 

 dermis are of two kinds. The snperficial portion is formed by 

 horny, flattened cells and the deeper by softer cells. This layer 

 of the skin varies greatly in thickness in the different species. 

 The derma is composed of some mnscnlar fibres interwoven with 

 the connective-tissne fibres. It contains the roots of the hair fol- 

 licles, sweat and oil glands. The external face which is covered 

 by the epidermis shows a mnltitnde of little elevations. These 

 are the vascnlar and nervons papilla. In addition, it shows 

 openings through which the hairs and the skin glands pass. 

 The inner surface is united more or less closely to the muscular 

 or underlying tissue by a layer of fibro-fatty tissue. 



The appendages of the shin are the hairs and horny produc- 

 tions. The horny productions comprise the horns, chestnuts, 

 ergots, claws and hoofs. 



The hair varies in length, thickness and coarseness in the 

 different species, and the different regions of the body. In addi- 

 tion, breeding, care, heat and cold may cause marked variations 

 in the thickness of the coat. Exposure to cold causes the coat 

 to thicken. High temperatures cause the short hairs to drop 

 out and the coat to become thin. 



Diseases of the skin may be classified as parasitic and non- 

 parasitic. Parasitic skin diseases are caused by animal and 

 vegetable parasites. ISTon-parasitic skin diseases are caused by 

 irritation to the skin and internal causes. Irritation to the skin 

 may be either chemical, thermic or mechanical. The internal 

 causes may be due to an individual predisposition together with 

 digestive disturbances and the eating of feeds too rich in protein. 



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