224 ADVANCED PHYSIOLOGY 



is an oval sac and when moderately full is about five inches 

 in vertical measurement and three inches across, holding 

 about two thirds of a pint. The muscle fibres of its 

 walls are smooth extending in every direction about it, so 

 that when they contract during the process of emptying, the 

 bladder shrinks in all its dimensions. 



The Urethra. The single tube which leads from the middle 

 of the lower end of the bladder to' the exterior, is called the 

 urethra. The opening from the bladder into it is ordinarily 

 closed by strong circular muscles passing around the tube at 

 its point of exit. These muscles stay in a condition of con- 

 stant, involuntary contraction most of the time, relaxing 

 only on receipt of a special message from the b r ain. 



The Need of Drinking Fresh Water. In the chapter on foods 

 we learned that the body needs a constant and large supply 

 of water. The necessity for plenty of water in dissolving and 

 removing body wastes through the kidneys furnishes one of 

 the chief reasons for this. Most of the water that flows through 

 the kidney tubules must come originally from the water we 

 drink. If plenty is supplied, the whole liquid content of the 

 blood can be more easily kept in solution, and more water can 

 be given off in the urine to "flush" the system. The sewers 

 of a city must never be of too small capacity and they must 

 never become clogged; water must be kept running abun- 

 dantly in them to prevent solids from settling and decom- 

 posing; otherwise, pestilence and disease set in. Similarly, 

 every organ and cell in the body must have plenty of liquid 

 blood flowing through and past it to furnish fresh supplies, 

 and to receive all the wastes, liquid, solid or gaseous. 



GENERAL SUMMARY OF METABOLISM 



Since urea represents the form in which all the nitrogen of 

 the proteid material in our food leaves the body, the whole 

 process of metabolism may be appropriately mentioned here. 

 In an earlier chapter the definition of the term metabolism 



