182 EXCRETION. 



urethra, it presents a small projecting fold of mucous 

 membrane, which is sometimes called the uvula vesicse. 

 Over the whole of the surface of the trigone, the mucous 

 membrane is very closely adherent, and is never thrown 

 into folds, even when the bladder is entirely empty. 



The blood-vessels going to the bladder are ultimately 

 distributed to its mucous membrane. They are not very 

 numerous, except at the fundus, where the mucous mem- 

 brane is tolerably vascular. Lymphatics have been described 

 as existing in the walls of the bladder, but Sappey, whose 

 researches in the lymphatic system have been very extended 

 and successful, has failed to demonstrate them in this situa- 

 tion. 1 The nerves of the bladder are derived from the hypo- 

 gastric plexus. 



The urethra is provided with muscular fibres and is lined 

 by a mucous membrane, the anatomy of which w T ill be more 

 fully considered in connection with the function of genera- 

 tion. In the female the epithelium of the urethra is like 

 that of the bladder. In the male the epithelial cells are 

 small, pale, and of the columnar variety. 



Mechanism of the Discharge of Urine. In some of the 

 lower orders of animals, in which the urine is of a semisolid 

 consistence, the movement of vibratile cilia in the uriniferous 

 tubes probably aids in the discharge of the urine ; but in the 

 human subject, the existence, even, of cilia is doubtful, and 

 the urine is discharged into the pelvis of the kidneys and 

 the ureters by pressure due to the act of separation of the 

 fluid from the blood. Once discharged into the ureters, the 

 course of the urine is determined in part by the vis a tergo, 

 and in part, probably, by the action of the muscular coats 

 of these canals. Miiller has found that the ureters can be 

 made to undergo a powerful local contraction upon the ap- 

 plication of an intense galvanic current ; a and Bernard has 



1 SAPPEY, Traite cTanatomie descriptive, Paris, 1857, tome iii., p. 516. 

 9 MUELLER, Manuel de physiologic, Paris, 1851, tome L, p. 396. 



