viii THE EXCRETION OF URINE 475 



lumbar vertebrae. The immediate effect of this operation is 

 paralysis of the bladder, which, however, ceases in a few days, 

 while the operative sequelae disappear, and purely reflex micturi- 

 tion takes place, owing to the tension of the bladder which acts on 

 the centripetal nerves. Micturition can also be caused by holding 

 the animal in a vertical posture, and exciting the skin of the 

 perineum with slight mechanical stimuli. 



Even when the spinal centres are occluded, the paralysis of the 

 bladder, although it lasts longer, gradually dies out, until the 

 normal function of the bladder is completely re-established. This 

 appears from the marvellous results obtained by Goltz and Ewald 

 (1896) on dogs deprived of the lumbo-sacral cord, which long 

 survived this serious operation. The bladder was much distended 

 at first by accumulation of urine, and had to be emptied artificially 

 by pressure on the peritoneal walls; but the paralysis of the 

 detrusor slowly disappeared, until a few months after the operation 

 it was found that the bladder emptied itself periodically without 

 adventitious aid. The animal passed long periods without urinat- 

 ing, during which time the urine accumulated, and was evacuated 

 when the stimulus to the bladder walls became sufficient to induce 

 reflex micturition. 



A very simple proof of this fact, showing that the bladder is 

 able to function normally without the help of the cerebrospinal 

 centres, by co-ordinated reflexes from the intravesicular ganglion 

 plexus, was obtained by von Zeissl (1896). He divided on a dog all 

 the nerves that run to the bladder, and found that after this 

 operation (which is much simpler than that of Goltz and Ewald) 

 the animal was able both to retain the urine and to expel it 

 periodically in a normal manner. 



That in this case also the intravesicular ganglia function as 

 reflex centres appears very probable from the experimental demon- 

 stration of Sokowin, Nussbaum, Nawrocki and Skabitschewsky, 

 which show that the inferior mesenteric ganglion is capable of 

 functioning as a reflex centre for the bladder. 



But the reflex action of the ganglion plexus of the bladder does 

 not cut out the rhythmical automatic activity of its own muscle 

 cells, independent of the ganglia. Rhythmic contractions are seen 

 in fact in the frog's bladder when it is excised from the animal, 

 as also in fragments of bladder which are subsequently found under 

 the microscope to show a total absence of ganglion cells (Pfalz). 



The question has been much discussed as to whether the 

 mucous membrane of the bladder has any capacity for absorption, 

 and whether the urine accumulated in the bladder becomes 

 concentrated from absorption of water or any of the soluble 

 substances. The capacity of the bladder for absorption is readily 

 shown by the injection into it of toxic substances; these take 

 effect after a certain time. The fact that the urine collected 



