4:26 COMPARATIVE PHYSIOLOGY. 



same side, to show insertion of ureters into bladder; I, prostate; J, Cowper's 

 glands; K, membranous or intra-pelvic portion of urethral canal; L, its bulbous 



Sortion; M, cavernous body of penis; m, m-, its roots; N, head of penis; 1, ab- 

 ominal aorta; 8,8, arteries (renal) giving off principal capsular artery; 3, sper- 

 matic artery; 4, common origin of umbilical and arteries of bulb; 5, umbilical 

 artery; 6, its vesical branch; 7, internal artery of bulb; 8, its vesico-prostatic 

 branch. 



ordinary control of the brain-centers over those in the spinal 

 cord. 5. It may be assumed that the normal tone of the 

 sphincter of the bladder is maintained reflexly by the spinal 

 cord. The unwonted muscular contraction associated with an 

 obstruction to the outflow of urine may be in part of nervous 

 origin, but is also, in all probability, owing in some degree to 

 the muscle-cells resuming an independent contractility, due to 

 what we recognize as the principle of reversion. The same is 

 seen in the heart, ureters, and similar structures. 



Pathological. — There may be incontinence of urine from pa- 

 ralysis, the cerebral centers being unable to control those in 

 the spinal cord. Dribbling of urine may be due to retention in 

 the first instance, thfe tone of the sphincter being finally over- 

 come, owing to increase of pressure within the bladder. Over- 

 distention of the bladder may arise in consequence of lack of 

 tone in the muscular walls, though this is rare. Strangury is 

 due to excessive action of the walls of the bladder and the 

 sphincter, brought about reflexly, when the organ is unduly 

 irritable, as in inflammation, after the abuse of certain drugs 

 (cantharides), etc. 



Comparative. — In man the last drops of urine are expelled 

 by the action of the bulbo-cavernosus muscle and perhaps some 

 others. In the dog and many other animals the regulated and 

 voluntary use of this muscle, marked in a high degree, produces 

 that interrupted flow so characteristic of the micturition of 

 these animals. 



Summary. — Urine is in mammals a fluid of variable speciflc 

 gravity and reaction, yellow in color, and containing certain 

 salts, pigments, and nitrogenous bodies. The chief of the latter 

 is urea. 



The kidneys and skin especially supplement one another, 

 and normally great activity of the one implies lessened ac- 

 activity of the other. This is availed of in the treatment of dis- 

 ease. 



Both the Malpighian capsules and the renal tubules have a 

 true secretory function, though the larger part of the water of 

 urine is secreted by the former. Blood-pressure is an important 



