CH. XXXVIII.] MICTURITION 581 



must be situated below this point. In such animals there is no 

 consciousness of the afferent impulse, and the same is true for the 

 human subject with corresponding injuries to the spinal cord. 

 Such animals or men have also no voluntary control over the act; it 

 occurs in them purely reflexly. 



The efferent nerves to the bladder fall into two sets : (1) The 

 nervus erigens ; this is undoubtedly the more important of the two. 

 Stimulation of this nerve causes contraction of the bladder, and 

 relaxation of its sphincter, the two necessary acts by which the 

 urine is expelled. (2) The hypoyastric nerves; pre-ganglionic fibres 

 leave the cord in the lumbar region, pass thence to the inferior 

 mesenteric ganglion, from the cells of which the post-ganglionic fibres 

 ultimately reach the bladder by the hypogastric nerves. Much 

 difference of opinion has been expressed regarding the action of these 

 nerves, but in most animals they cause constriction of the sphincter, 

 and in some cases relaxation of the bladder walls also. The hypo- 

 gastric nerves would therefore appear to be the functional anta- 

 gonists of the nervi erigentes. In many animals the bladder 

 constantly exhibits rhythmic contractions. 



In theory, therefore, micturition is a reflex action ; but in practice 

 it is a voluntary act, and the voluntary muscles of the abdomen press 

 upon the bladder and assist its emptying. It is only in the cases of cord 

 injury or disease already alluded to that the voluntary factor is absent. 



The simplest view to take of voluntary micturition is the follow- 

 ing : The will causes the abdominal muscles to contract, and the 

 increased pressure on the bladder so produced is the signal for the 

 reflex to occur. It is further probable that the mere thought of 

 micturition may influence the sacral vesical centre, and heighten its 

 sensitiveness. This certainly is the case in the neighbouring 

 centre for the erection of the penis ; erection can ' be evoked as a 

 reflex act, yet it is a matter of experience that it also takes place 

 as a result of the emotions. 



If urine is voided too frequently, the cause may be (1) peripheral, 

 as in inflammation of the bladder ; here the organ is unduly sensitive 

 to the pressure of fluid; and (2) central, as in cases of fear and 

 excitement ; here the sensibility of the vesical centre is heightened. 

 In children where control of the vesical centre is often not fully 

 established while they are young, frequent and involuntary micturi- 

 tion may occur. 



Deficiency of power to expel the urine may be due to actual 

 obstruction, from an enlarged prostate or a stricture in the urethra. 

 It may also be due to weakness of the bladder, as in cases where 

 the organ is much distended and its musculature attenuated; this 

 condition is often secondary to obstruction produced by stricture, or 

 other causes. 



