SKIN 583 



with its development in hair, and of the layer of sub- 

 cutaneous fat, are manifest. 



Hair. — The hairy coat of animals maintains a layer of air 

 next the skin at a more equable temperature than that of the 

 surrounding air, and so plays an important part in the regula- 

 tion of temperature (p. 269). 



The strong hairs developed about the muzzle and in the 

 eyelashes are tactile organs (p. 102). Attached to each hair 

 follicle is a band of non-striped muscle, the arrector pili, which 

 can erect the hair by contracting. These muscles are under the 

 control of the central nervous system, and the nerve fibres have 

 been demonstrated in the cat to take much the same course as 

 the vaso-constrictor fibres of somatic nerves. They belong to 

 the true sympathetic set of nerves. 



A hair after a time ceases to grow, and the lower part in 

 the follicle is absorbed and the hair is readily detached. From 

 the cells in the upper part of the follicle, a new down-growth 

 occurs, a papilla forms and the hair is regenerated. In the 

 horse this process occurs twice a year, and the thickness of the 

 coat grown depends upon the degree of exposure to cold. The 

 hair of the mane and tail and the tactile hairs are not shed 

 with the rest of the coat. 



(2) The Sensory functions have been studied under the 

 Receptors (p. 99 et seq.). 



(3) The Respiratory action of the skin in mammals is of 

 little importance. 



(4) The Excretory Function of the Skin. 



Two sets of glands develop in the skin — sweat glands and 

 sebaceous glands. 



A. Sweat Glands. 



1. Structure. — The sweat glands are simple tubular glands 

 coiled up in the subcutaneous tissue with ducts opening on the 

 surface of the skin. The secreting epithelium somewhat 

 resembles that of the convoluted tubules of the kidney. Sweat 

 glands are widely distributed over the skin of the horse. In 

 oxen and sheep they are less abundant, being most developed 

 on the muzzle. In the dog and cat they are found in the nose 

 and in pads of the feet. 



