408 HUMAN PHYSIOLOGY 



the centre may prevent the expulsion of urine, while later in 

 the disease, when the nerve structures have been destroyed, 

 the urine is not retained and dribbles away on account of the 

 absence of the tonic contraction of the muscles. 



The expulsion of the last drops of urine is carried out by 

 the rhythmic contraction of the bulbo-cavernous muscle, 

 while the peristaltic contraction of the bladder wall is assisted 

 by the various muscles which can press upon the contents of 

 the bladder. 



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 

 comes 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 epi- 

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

 layer of subcutaneous fat, are manifest. 



Hair. In man the hair has largely ceased to have the im- 

 portant protective function it fulfils in many of the lower 

 animals, but the muscular mechanism by which the position 

 of the hairs can be modified still persists. Attached to each 

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

 which by contracting can erect the hair. 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. 264). 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. 



