THE ESSENTIAL PHENOMENA OF DIGESTION. 29 



The last arterial ramifications are of capillary dimen- 

 sions, that is, their diameter is hardly as great as that 

 of a hair : they penetrate to all parts of the organs, and 

 the work of nutrition is accomplished by their aid. They 

 terminate in other vessels which are not less delicate, 

 and these capillary veins receive the blood deprived of 

 its life-sustaining elements. 



The veins, like the arteries, generally take their names 

 from the parts of the body they traverse: thus, there are 

 jugular, renal, mesenteric veins, etc. Unlike the arteries, 

 they unite together in their course, and at last form only 

 two large vessels, the superior vena cava and the in- 

 ferior vena cava, There is, however, one exception : the 

 portal vein enters the liver and divides up as the pul- 

 monary artery does in the lungs, and the ramifications 

 afterwards unite to form two large vessels, called the 

 hepatic veins, just as the capillaries of the lungs unite to 

 form the pulmonary veins. 



CHAPTER IV. 

 The Essential Phenomena of Digestion. 



THE general name aliments is given to those substances, 

 whatever may be their nature, that are habitually used 

 or are capable of being used for nutrition. Digestion is 

 the operation by which these substances are introduced 

 into the organization, either directly or after having been 

 subjected to certain internal processes, which may be 

 chemical or simply mechanical, having for their object 

 the preparation of the substances for absorption. The 



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