472 A MANUAL OF PHYSIOLOGY 



into contraction. This is aided by an expulsive effort of the 

 abdominal muscles. The sphincter vesicae is relaxed ; and the 

 urine is forced along the urethra, its passage being facilitated 

 by discontinuous contractions of the ejaculator urinse muscle, 

 which also serve to squeeze the last drops of urine from the 

 urethral canal at the completion of the act. 



Regurgitation into the ureters is to a great extent prevented 

 by their compression between the mucous and muscular coats 

 of the bladder, where they run for more than half an inch before 

 opening at the posterior angle of the trigone. But it has been 

 shown that a certain amount of back flow can take place. Small 

 bodies like diatoms suspended in water and pigments dissolved 

 in it have been found in the pelvis of the kidney, the renal 

 tubules, and even the circulation after being injected into the 

 bladder. 



The pressure in the bladder of a man may be made as high as 

 10 cm. of mercury during the act of micturition ; about half this 

 amount is due to the contraction of the vesical walls alone, the 

 rest to the contraction of the abdominal muscles. A pressure 

 of 1 6 to 26 mm. of mercury is required to open the sphincter of 

 a rabbit's bladder in life. 



Although the whole performance seems to us to be completely 

 voluntary, there, are facts which show that it is at bottom a 

 reflex 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 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 ana- 

 tomical 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 important difference, however, is that the will can far more 

 readily set in motion the machinery of micturition than that 

 of deiaecation ; 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 



