THE URINE 341 



evacuation of the bladder contents, the organ gradually advances 

 to the brim of the pelvis, and then impinges on the abdominal 

 cavity ; in a state of extreme distension it may project for some 

 distance into the cavity, the weight of the fluid having a tendency 

 to cause the organ to incline towards the floor of the abdomen 



The entrance to the urethra is controlled by a circular layer 

 of unstriped muscle, part of the bladder muscle, but outside 

 this is a band of voluntary muscle, which must be regarded as 

 part of the urethra. Physiologists are not agreed as to the 

 mechanism involved in the act of micturition. Ordinarily it is 

 a voluntary act, but the dog with its spinal cord divided in 

 the lumbar region, will carry out the process perfectly, though 

 there can be no question of consciousness involved. In this case 

 the afferent impulses conducted to the cord by the second and 

 third sacral nerves stimulate a vesical centre — the grey matter 

 of the sacral cord. There can be no doubt that, under ordinary 

 circumstances, the act is a voluntary one. The efferent nerves 

 supplying the bladder are derived from two sources — viz., 

 directly from the sacral spinal nerves, and a second supply 

 through the sympathetic. The sacral nerves furnish the nervus 

 erigens, which is connected with the hypogastric plexus, while 

 the sympathetic supply is furnished by the second to the sixth 

 lumbar ganglia ; these pass to the inferior mesenteric ganglion, 

 and issue from it as the hypogastric nerves. Stimulation of the 

 nervus erigens causes relaxation of the sphincter and contraction 

 of the wall of the bladder, while the hypogastric nerves, though 

 relaxing the wall of the bladder, contract the sphincter. In the 

 latter action it is antagonistic to the spinal nerve supply. It is 

 almost certain that the most important source of supply to the 

 bladder is that furnished by the nervus erigens. 



At the moment the bladder wall begins to contract, it is 

 assisted by the abdominal muscles and a fixed diaphragm, and 

 the flow is never as powerful in the female as in the male, the 

 final expulsion of the last drops from the urethra of the latter 

 being given by the rhythmical contraction of the perineal muscles 

 and accelerator urince. During the act both the horse and mare 

 stand with the hind-legs extended and apart, resting on the toes 

 of both hind feet, thereby sinking the posterior part of the body ; 

 the male animal also often advances the fore-legs in order to 

 avoid getting them splashed ; in this position the penis is pro- 

 truded, and the tail raised and quivering. The stream which 

 flows from the two sexes is very different in size, depending on 

 the relative diameters of the urethral canal. The mare after 

 urinating spasmodically erects the clitoris, the use of which it 

 is difficult to see ; it may be due to the passage of a hot alkaline 

 fluid over a remarkably sensitive surface. The horse can, under 



