42 NUTRITIONAL PHYSIOLOGY 



ing and tone change in such a case. A non-living, elastic 

 sac may be distended by increasing its contents, but will 

 react with a pressure proportional to the distention. A 

 living organ which adapts itself to increased contents by a 

 diminution of tone may exert no more pressure when full 

 than when nearly empty. This principle is well illustrated 

 by the urinary bladder. At one time this organ may have 

 a capacity of a pint, and again its cavity may be nearly 

 obliterated, but there is no strict correspondence between 

 its size and the internal pressure. Indeed, a strong degree 

 of tone and a high pressure may exist when the bladder 

 is quite small. 



Glands and Secretion. We have taken time and space 

 to deal with the elements of muscular activity, and we 

 must also give a place to another type of tissue and to its 

 work. Some appreciation of the physiology of glands is as 

 much a prerequisite of the study of the alimentary process 

 as is a knowledge of the mechanism of contraction. Every- 

 one understands that the nervous system throws the skele- 

 tal muscles into their orderly activity, but the fact that 

 the secretions of the body are often produced under 

 nervous influences is not so familiar. Yet we do not have 

 to look far for suggestive examples. The flow of tears 

 as an accompaniment of an emotional experience is clear 

 evidence that the small organs above the eyeballs which 

 elaborate the tears are in connection with the brain and 

 responsive to its changing conditions. A like relationship 

 can be demonstrated for the glands that produce saliva 

 and for those which secrete sweat. Secretion and con- 

 traction are two manifestations of metabolism which are 

 alike regulated by the nervous system. In fact, it is 

 doubtful whether we "have any other expression of the 

 working of the nerve-centers than these two, the phe- 

 nomena of consciousness being set aside for the present. 



What, then, is a gland? The word is used sometimes 

 to designate a large organ like the liver, the pancreas, or a 

 kidney. Sometimes it is used with reference to a micro- 

 scopic affair like an individual sweat-gland or one of the 



