222 
OTHER USES OF THE CIRCULATION. 
which, collects the blood from these, and returns it to the 
heart. This circuit of the blood is sometimes called the lesser 
circulation; to distinguish it from that which it makes 
through the general system, which is called the greater 
circulation . 
254. Although carbonic acid is one of the chief impurities 
with which the blood becomes charged during its circulation, 
in consequence of the changes of composition which are con¬ 
tinually taking place in the living body, it is by no means the 
only one ; and other organs are provided, besides the lungs, 
for removing the noxious matters from the current of the cir¬ 
culation as fast as they are introduced into it. Thus, in the 
course of its movement through the general system, the blood 
is made to pass through the liver, the kidneys, and the skin, 
each of which has its special purifying office; these organs, 
however, have no such special circulation of their own as the 
respiratory apparatus of higher animals possesses, though the 
liver, as we shall hereafter see (§ 267), is peculiarly supplied 
by a sort of offset from the general circulation, so that the 
blood from which its secretion is formed is venous instead of 
arterial, like that transmitted to the lungs. 
255. The course which the blood takes, and the structure 
of the apparatus which is subservient to its movement, differ 
very greatly in the several classes of animals. The chief of 
these differences will be pointed out hereafter; and it will be 
preferable to commence with the highest and most complex 
form of the circulating system, such as we find in Man, that 
it may serve as a standard of comparison with which the 
rest may be contrasted. 
Circulating Apparatus of tlie Higher Animals. 
256. In Man, and those animals which approach him most 
nearly in structure, the heart is situated between the lungs in 
the cavity of the chest, which is termed by anatomists the 
thorax. Its form is somewhat conical; the lower extremity 
tapering almost to a point, and the upper part being much 
larger. The lower end is quite unattached, and points rather 
forwards and to the left; during the contraction of the heart, 
it is tilted forwards, and strikes against the walls of the chest, 
between (in Man) the fifth and sixth ribs. It is from the 
large or upper extremity that the great vessels arise; and 
