92 PRINCIPLES OF PHYSIOLOGY 



met with in dietaries. Thus, butcher meat 

 abounds in protein and fat, potatoes in starch 

 (a carbo-hydrate) ; and vegetable oils and 

 animal fat are rich in fat. By combinations 

 of these, a suitable dietary is formed, and 

 experience has taught mankind, even in 

 savage conditions, empirically to combine 

 such substances. Further, science has shown 

 that a suitable dietary supplies the requisite 

 amount of carbon and the requisite amount 

 of nitrogen to make up for the daily loss of 

 carbon eliminated chiefly by the lungs, as car- 

 bonic acid, and of nitrogen thrown out by the 

 kidneys mainly as urea. 



47. In order to become incorporated with 

 the living tissues, food stuffs must pass 

 through a series of elaborate physical and 

 chemical processes, the object of which is 

 to render them soluble and suitable for ab- 

 sorption into the blood. These processes 

 constitute Digestion. The food is broken 

 down and mixed in the mouth with saliva 

 so as to form a pulpy mass. Such matters 

 as saline substances may be at once dissolved, 

 and the whole process is facilitated by the 



