4i6 A MANUAL OF PHYSIOLOGY 



stimuli as sponging of the skin round the anus (Goltz). 

 Here, of course, the act is entirely reflex ; and the centre 

 is situated at the level of the fifth lumbar nerves. The 

 efferent nerves of the bladder, like those of the rectum, 

 come partly from the cord directly through the sacral nerves, 

 and partly through the lumbar sympathetic chain (second to 

 sixth ganglia). The sacral fibres are connected with nerve 

 cells in the hypogastric plexus, and the sympathetic, partly 

 at least, in the inferior mesenteric ganglia. This anatomi- 

 cal coincidence acquires interest in view of the striking 

 physiological similarity between the processes of micturition 

 and defsecation, a similarity which is emphasized by the fact 

 that the latter is almost invariably accompanied by the 

 former. An important difference, however, is that the will 

 can far more readily set in motion the machinery of micturi- 

 tion than that of defsecation ; a man can generally empty 

 his bladder when he likes, but he cannot empty his bowels 

 when he likes. 



Sometimes in disease, and especially in disease of the 

 spinal cord, the mechanism of micturition breaks down ; 

 the bladder is no longer emptied ; it remains distended with 

 urine, which dribbles away through the urethra as fast as 

 it escapes from the ureters. To this condition the term 

 incontinence of urine is properly applied. 



Reflex emptying of the bladder, without an act of will or 

 during unconsciousness, is not true incontinence. The in- 

 voluntary micturition of children during sleep, for example, 

 is a perfectly normal reflex act, although more easily excited 

 and less easily controlled than in adults. 



II. Excretion by the Skin. 



Besides permitting of the trifling gaseous interchange 

 already referred to (p. 258), the skin plays an important part 

 in the elimination of water by the sweat-glands. 



Sweat is a clear colourless liquid of low specific gravity 

 (1003 to 1006). consisting chiefly of water with small 



