5 o 4 EXCRETION 



series of co-ordinated movements, that can be started by impulses 

 passing to a centre in the spinal cord from above or from below 

 from the brain or from the bladder. In dogs, with the spinal cord 

 divided at the upper level of the lumbar region, micturition takes 

 place regularly when the bladder is full, and can be excited by such 

 slight stimuli as sponging of the skin around 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 sympa- 

 thetic chain (second to sixth ganglia). The sacral fibres are con- 

 nected with nerve cells in the hypogastric plexus, and the sympa- 

 thetic, partly at least, in the inferior mesenteric ganglia. This 

 anatomical coincidence acquires interest in view of the striking 

 physiological similarity between the processes of micturition and 

 defalcation, a similarity which is emphasized by the fact that the 

 latter is almost invariably accompanied by the former. An im- 

 portant difference, however, is that the will can far more readily 

 set in motion the machinery of micturition than that of defaecation ; 

 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 involuntary mic- 

 turition of children during sleep, for example, is a perfectly normal 

 reflex act, although more easily excited and less easily controlled 

 than in adults. Section either of both nervi erigentes, or of both 

 hypogastrics, is never followed by more than quite temporary dis- 

 turbance of function of the bladder in dogs, both male and female. 

 In a few days the urine is normally passed. In bitches the same 

 is true when both pairs of nerves are divided. But in male dogs 

 true incontinence of urine follows section of the four nerves, as well 

 as intense tenesmus due to paralysis of the lower part of the large 

 intestine. 



SECTION IV. EXCRETION BY THE SKIN. 



Besides permitting of the trifling gaseous interchange already 

 referred to (p. 292), the skin plays an important part in the elimina- 

 tion 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 quantities of salts, 



