THE UROGENITAL SYSTEM 101 



a, rhythmic, peristaltic contraction that forces the urine along. 

 The backward passage of th(> urine into the ureters is prevented by 

 the oblique manner in which they pierce^ the wall of the bladder. 



The urinary bladder is located on the floor of the pelvis. When 

 full it is egg shaped, and may extend over the brim of the pubis 

 into the abdominal cavity. Near its posterior or constricted part 

 are found the openings of the ureters. The wall is composed of an 

 inner mucous coat, a middle muscular coat of involuntary fibers, 

 and an outer serous covering which does not extend over the entire 

 organ. The bladder functions as a reservoir for the urine. The 

 act of expelling urine from the bladder is termed "micturition." 



The urethra is a single tube which carries away the urine from 

 the bladder. The orifice into the urethra is guarded by a ring-like 

 muscle which completely closes it when in a state of contraction. 

 In the male it is a common tube to the urinary and genital systems, 

 and will be described with the latter. The urethra of the mare is 

 about 2 inches long and connects the bladder with the vulva, into 

 the floor of which it opens by the meatus urinarius, about a hand 

 breadth from the external opening of the vulva. 



THE PHYSIOLOGY OF XJRINATION 



The blood enters the glomerulus of the malpighian body 

 under pressure, where it meets with opposition to its outflow. 

 Here most of the water and soluble salts found in urine are ex- 

 creted. The process is not a simple filtration, as was at one time 

 supposed, but involves a selective action of the epithelial cells lining 

 this portion of the uriniferous tubule. After they leave the mal- 

 pighian bodies the uriniferous tubules are lined with cells capable of 

 extracting from the blood the urinary pigments and solids that 

 were not allowed passage at the expanded end of the tubules. 

 These are flushed out by the fluid from above, and are carried to 

 the pelvis of the kidney by the collecting tubules, as previously 

 described. 



It is evident from this brief review of the activities of the 

 kidneys that the flow of urine will be increased (1) by raising the 

 blood-pressure; (2) by increasing the water content of the blood 

 through absorption from the intestines; (3) by retarding the elimi- 

 nation of fluids by other channels of the body, Hke the digestive 

 and respiratory tracts and the skin. When these conditions are 

 reversed the flow of urine will be reduced. 



