THE HUMAN HEART. 
351 
their attainment ; and what are the contrivances for the avoid- 
ance of those dangers which would, mechanically, be most 
likely to occur ? 
First of all, a supply of nourishment has to be furnished to 
every part of the body, no structure or organ being omitted ; 
secondly, waste material has to be taken up and removed from 
the system after it has served its purposes there ; and both 
these objects are accomplished by the circulating vital fluid, the 
first by a transudation of the watery part of the blood through 
the pores of the walls of those minute capillaries which ramify 
through every portion of the body, and which are too minute to 
permit the red corpuscles of the blood to escape, the function of 
these latter apparently being- to vivify the watery portion or 
serum of the blood, which is the nutrient fluid : while the 
second of the objects above mentioned is attained by the blood 
circulating through various organs in the body, whose special 
function it is to remove used up material by peculiar vital 
processes, of which nothing farther is known than this, that 
they are effected by the same transudation of serum through 
the walls of the capillaries, or, in the case of the lungs, with 
which we are at present most concerned, by a similar passage 
of carbonic acid and water out through the walls of the air-cells 
of which the lungs are composed, while oxygen at the same time 
passes in by a similar process. 
Through the other secreting and excreting organs, the blood, 
or a portion at least of it, passes in the course of the general 
circulation of the body, but through the lungs the whole of it 
passes by a separate circulation quite distinct from that of the 
body, and called the pulmonic circulation. Since the heart, 
then, has to effect two distinct circulations, it is necessary that 
it should be in effect double ; and such is, in fact, the case, so 
that physiologically there are two quite distinct hearts in man 
and the higher animals, although anatomically the two are 
joined together. There are, therefore, four chambers, two for 
receiving the blood, the first on its return from the body, the 
second on its return from the lungs ; these are called “ auricles;” 
and two for expelling the blood through the circulation, the 
first through the pulmonic circulation, and the second through 
the systemic circulation, or that of the body ; these chambers 
are called “ ventricles.” In the heart of fishes there are but 
two chambers, one auricle and one ventricle ; in Batrachia, 
there are three chambers, two auricles and a ventricle ; while 
in reptiles the ventricle has a partition which is imperfect in the 
lower classes, so that their heart has virtually only three 
cavities, but which becomes perfect in the Crocodiles, so that 
theirs, like that of birds and mammalia, is composed of four 
cavities. 
