428 VETERINAKY PHYSIOLOGY 



In man, in early life, micturition is a purely reflex act, and 

 in the dog it is perfectly performed when the spinal cord is cut 

 in the back. As age advances the reflex mechanism conies to 

 be more under the control of the higher centres, and the 

 activity of the sphincters may be increased or abolished as 

 circumstances indicate. 



3. EXCRETION BY THE SKIN 



The skin is really a group of organs, and some of these 

 have been already studied. (The structure of the skin and its 

 appendages must be studied practically.) 



(1) The Protective functions of the horny layer of epidermis, 

 with its development in hair and nail, and of the layer of 

 subcutaneous fat, are manifest. 



Hair. Attached to each hair follicle is a band of non-striped 

 muscle, the arrector pili, which can erect the hair by con- 

 tracting. 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 (see p. 280). 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 many of the lower animals 

 this process occurs twice a year. 



(2) The Sensory functions have been studied under the 

 Special Senses. 



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

 little importance. 



(4) The Excretory Function of the Skin. Three sets of 

 glands develop in the skin sweat glands and sebaceous glands, 

 which are common to both sexes and are constantly active 

 and mammary glands, which are active in the female during 

 the period of suckling. 



A. Sweat Secretion 1. Structure. The sweat glands are 

 simple tubular glands coiled up in the subcutaneous tissue 



