8 URINE. 



The urethra is a membranous canal, which in the male 

 extends from the base of the bladder to the extremity of the 

 penis. It possesses a mucous coat continuous with that of 

 the kidneys, ureters, and bladder, and is enveloped throughout 

 its whole length by longitudinal and circular muscular fibres. 

 At the commencement of its course the urethra is (in the 

 male) surrounded by a body possessing a structure partly 

 muscular and partly glandular, called the prostate gland. 

 This portion of the canal is called the prostatic portion of the 

 urethra. A little further in its course it is surrounded by 

 very powerful muscular arrangements, in addition to the 

 layers of muscular fibres enveloping its whole length ; this is 

 called the membranous portion of the urethra, and differs 

 much in length in different animals. It is most fully de- 

 veloped in those animals whose penis is either very short 

 or very long; thus, it is remarkable for its development 

 in the ruminantia and carnivora. In the former (e.g. in 

 the ox) the powerful muscular arrangement is required 

 to force the urine and semen through the very long penis; 

 in the latter (e. g. in the cat) the penis being very short, it 

 serves to propel these fluids beyond the extremity of that 

 organ. The membranous portion of the urethra is con- 

 tinued into the spongy portion, as that division of the 

 urethra is called, which is surrounded by spongy or erectile 

 tissue, a form of tissue which will be described under the 

 generative organs. 



The urine is a fluid varying very much in different ani- 

 mals; it contains, however, certain substances which seem 

 typical of the secretion. 



The urine contains a large amount of water ; a very im- 

 portant nitrogenous substance called urea; two important 

 acids, the uric and hippuric; a large proportion of inorganic 

 salts, and certain organic substances of an ill-defined nature, 



