430 THE ELIMINATION OF WASTE PRODUCTS. 



times occurs in the adult as the result of emotions, or at least sensory im- 

 pressions, appears to be the result of reflex action. In these several cases 

 we may fairly suppose that the centre in the spinal cord is affected by affer- 

 ent impulses reaching it along various sensory nerves or descending from 

 the brain. Hence we are led to the conception that when we make water 

 by a conscious effort of the will, what occurs is not a direct action of the will 

 on the muscular walls of the bladder, but that impulses started by the will 

 descend from the brain after the fashion of afferent impulses and thus in a 

 reflex manner throw into action the micturition centre in the spinal cord. 

 We may draw an analogy between the micturition apparatus and the re- 

 spiratory mechanism. We saw reasons in the latter case to think that when 

 the will interfered with the respiratory movements, it did so by acting upon 

 the nervous mechanism in the central nervous system and not by acting di- 

 rectly on the muscular fibres of the diaphragm and other respiratory muscles. 

 And the case of the plain muscular fibres of the bladder seems even stronger 

 than that of respiratory muscles so largely skeletal in nature. We might 

 also draw an analogy with the heart. We are not able to throw into action, 

 by any direct effort of the will, the cardiac augmentor mechanism. Were 

 we able to do so powerfully and suddenly, we might throw into violent ac- 

 tion a weakly beating heart much in the same way that we empty an ob- 

 scurely contracting bladder. Nor is this view negatived by the fact that 

 paralysis of the bladder, or rather inability to make water either voluntarily 

 or in a reflex manner, is a common symptom of cerebral or spinal disease 

 or injury. Putting aside the cases in which the reflex act is not called forth 

 because the appropriate stimulus has not been applied, the failure in mic- 

 turition under these circumstances may be explained by supposing that the 

 shock of the spinal injury or some extension of the disease has rendered the 

 spinal centre unable to act. 



The so-called incontinence of urine in children is simply an easily ex- 

 cited and frequently repeated reflex micturition. In cases of cerebral or 

 spinal disease a form of incontinence is frequently met with which seems to 

 be of a different nature. The bladder becoming full, but, owing to a failure 

 in the mechanism of voluntary or reflex micturition, being unable to empty 

 itself by a complete contraction, a continued dribbling of urine takes place 

 through the urethra, the fulness of the bladder being sufficient to overcome 

 the resistance at the neck of the urethra. It is probable, however, that 

 even in these cases the flow is partly caused by obscure, unfelt, intrinsic 

 contractions of the bladder. 



365. Whether, under normal conditions, the urine undergoes any 

 notable change during its stay in the bladder has been much debated. 

 Experiments show thai poisonous substances injected into the bladder with 

 all due care to avoid any abrasion of the epithelium are absorbed and pro- 

 duce their usual effects. It has also been stated that if a solution of urea 

 be injected into the bladder after ligature of both ureters, and allowed to 

 stay for some hours, part of the urea disappears. But at present there is 

 no very decided proof that under ordinary conditions either the water or 

 other constituents of urine are to any appreciable extent absorbed by the 

 bladder. 



Under abnormal conditions, as in inflammation or irritation of the blad- 

 der, the urine may have undergone marked changes during its stay in the 

 bladder, one of the most common being a change of some of the urea into 

 ammonium carbonate, by which the urine also becomes alkaline. Under 

 abnormal conditions also, the mucus of the urine, which in a healthy man 

 is insignificant, though in some animals, for instance the horse, it occurs in 

 considerable quantity, is largely increased during the stay in the bladder. 



