358 PHILOSOPHY OF ZOOLOGY. 



termed exhaling or secerning vessels. In other cases, they 

 terminate in glands, whose office it is to separate some in- 

 gredients from the blood, for the benefit of the system, or to 

 be thrown out as useless. It is in these terminating 

 twigs of the arteries or the capillary vessels, that the changes 

 take place in the blood, by which its qualities are altered, 

 and it passes into venous blood. 



In the process of digestion, the food is mixed with 

 a variety of secreted fluids, by which it is gradually 

 prepared for the action of the absorbing vessels or 

 lacteals. These separate the nutritious portion, and con- 

 vey it to a particular receptacle. Another set of ab- 

 sorbents, the lymphatics, take up all the substances 

 which have been ejected from the circulation, and which 

 are no longer necessary in the particular organs, and com- 

 municate their contents to the store already provided by 

 the lacteals. The veins receive the altered blood from the 

 extremities of the arteries, or the glands in which they ter- 

 minate and proceed with it towards the lungs to be again 

 aerated. In their progress, they obtain the collected fluid 

 of the other absorbents, and, in the lungs, again prepare 

 the whole for the use of the system. Thus, during the 

 continuance of life, the arteries supply the materials by 

 which the system is invigorated and enlarged, and oppose 

 that tendency to decay, produced by the influence of ex- 

 ternal objects. The process continues during the whole of 

 life, new matter is daily added, while part of the old and 

 useless is abstracted. The addition is greatest in early life, 

 the abstraction is greatest in old age. 



This continued system of add it ion and abstraction has led 

 some to conclude, that a change in the corporeal identity of 

 the body takes place repeatedly during the continuance of 

 life, that none of the particles of which it consisted in youth, 

 remain in its composition in old age. Some have consider- 

 ed the change effected every three, others every seven 



