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 absorbmg 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 addition 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 
