144 PRINCIPLES OP VETERINARY SCIENCE 



by other channels of the body, like the digestive and respiratory 

 tracts and the skin. When these conditions are reversed the 

 flow of urine will be reduced. 



The function of the kidneys is, concisely, to elaborate the urine, 

 yet with the exception of hippuric acid, the kidneys do not form 

 the substances urine contains. Their work is therefore mostly 

 to dispose of wastes formed in other parts of the body. They 

 are active in maintaining (1) the concentration of the blood by 

 regulating the salt content; (2) the volume of the blood by regulat- 

 ing the water content; (3) the neutral reaction of the blood by 

 excreting the excess sodium acid phosphate. When the liver fails 

 to function properly the kidneys compensate by eliminating larger 

 amounts of ammonium salts and other waste products than they 

 do normally. 



THE PHYSIOLOGY OF URINATION 



The process of expelling urine from the body after it has been 

 elaborated by the kidneys consists first, in the flow from the 

 pelvis of the kidney through the ureters to the bladder; second, 

 in the emptying of the bladder. The first act is involuntary and 

 continuous, like that of urine secretion, and is made possible by 

 the rhythmic waves of contraction of the involuntary muscle 

 fibers in the ureters. The emptying of the bladder, true urina- 

 tion, or micturition, occurs periodically through reflex stimula- 

 tion of the muscles in the bladder wall by impulses received from 

 the nerve-center for urination situated in the lumbo-sacral part of 

 the spinal cord. This act is either the result of pressure or 

 irritation in the bladder, or an impulse originating in the brain. 

 Urine retention and expulsion are ordinarily involuntary in 

 animals. 



THE REPRODUCTIVE ORGANS OF THE MALE 



Starting with the testicles as the central structures of the male 

 genital tract, the following organs are found in succession — the 

 testicles and their covering, the scrotum, the ductus deferens, 

 the seminal vesicles, the prostate and Cowper's glands, and the 

 penis (Fig. 48). 



The testicles are paired glandular organs situated in the inguinal 

 region. Each has a worm-like appendage of seminiferous tubules 



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