THE QUANTITY AND COMPOSITION OF THE URINE 337 



muscular tissue is arranged in several layers, some fibres being longitudinal, 

 others transverse, and others again oblique, and by their contraction the 

 contents of the cavity can be completely expelled through the urethra. 

 Just beyond the point where the bladder and urethra are continuous, one 

 with the other, the muscular fibres are so arranged as to form a strong 

 muscular band encircling the urethra, to which the name of sphincter of 

 the bladder is applied. The office of this muscle, which is in part under 

 the influence of the will, is to close the orifice of exit from the bladder, and 

 it therefore acts in antagonism to the general muscular coat of the bladder. 

 It is necessary that it should relax before any urine can be expelled. If it 

 loses its tone, the urine can no longer be retained, and incontinence of 

 urine occurs. 



The internal coat of the bladder is the mucous membrane. This is 

 a soft, pale rose-coloured layer, which is continuous before and above with 

 the membrane lining the ureters, and behind and below with that lining the 

 urethra. In the empty bladder it presents folds or rugse, which are obliter- 

 ated as the urine accumulates and distends the organ. It is composed 

 of an external layer of connective tissue, which is in accurate contact with 

 the muscular coat, and of several layers of epithelial cells. The bladder 

 is well supplied with blood-vessels, nerves, and lymphatics. The arteries 

 are derived from the branches of the internal pudic, and the veins return 

 the blood to the internal pudic veins. The lymphatics form a close net- 

 work on and internal to the muscular layers, and discharge their contents 

 into the sublumbar glands. The nerves are derived from the hypogastric 

 plexus, and partly proceed from the spinal cord and partly from the great 

 sympathetic nerve. The nerve centre controlling the act of staling is 

 situated in the spinal cord at the lower part of the lumbar region, as 

 is shown by the fact that after division of the spinal cord in the dorsal 

 region the bladder can be stimulated to discharge its contents by the 

 application of cold to the buttocks; ordinarily, however, the sensation 

 of fulness of the bladder excites consciousness, and impulses proceed from 

 the brain which, on the one hand, cause the sphincter guarding the opening 

 from the bladder to relax and allow the urine to escape, and, on the 

 other hand, cause the muscular tissue forming the walls of the bladder 

 to contract and force it out. 



THE QUANTITY AND COMPOSITION OF THE URINE 



The quantity of urine secreted depends, of course, chiefly upon the 

 quantity of water that has been ingested, but it is in part determined 

 also by the activity of the skin. With free exercise, and abundant sweat- 



