OTHER USES OP 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 the 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 



