Chap. IV.] GENERAL PHYSIOLOGY. 59 



The circulating medium is the blood. In the living state it is 

 warm and fluid. It consists of an almost colourless liquid, the 

 plasma, in which float minute solid bodies, the corpuscles. These 

 are of two kinds : (1) red corpuscles, minute, flattened, round, or 

 oval discs (see p. 64) ; (2) colourless, white, or pale corpuscles, 

 which change their form when living, and resemble a minute 

 organism called the amoeba (see Ch. xx.). Blood undergoes rapid 

 change when dying, causing clotting or coagulation. This is 

 due to the formation of fine filaments of fibrin. If the blood be 

 allowed to stand, the clot contracts, and a thin fluid, termed 

 serum, is expressed. This serum is not the same as plasma ; nor 

 is the fibrin during life merely held in solution in the plasma to 

 separate out on death. The fibrin is generated from the plasma 

 and the white corpuscles. These contain a substance, fibrinogen, 

 which, under the influence of a second substance, probably also 

 derived from the corpu&cles, gives rise to fibrin. 



The blood may be venous or arterial. Our blood, for example, 

 contains about 60 per cent of gaseous constituents by volume, 

 chiefly oxygen, nitrogen, and carbonic acid gas. The carbonic acid 

 gas is almost entirely held, in a state of very loose combination, 

 by the plasma. The oxygen is bound up in the hcemoglobin of 

 the red corpuscles. The proportion of oxygen aud carbonic acid 

 gas varies according to the venous or arterial condition of the 

 blood. In us venous blood contains 12 per cent, oxygen and 46 

 per cent, carbonic acid gas ; arterial blood, 20 per cent, oxygen 

 and 39 per cent, carbonic acid gas. Thus the carbonic acid gas 

 in the blood is always in excess of the oxygen. The operations 

 of giving up the carbonic acid gas and absorbing fresh oxygen 

 seem to be quite independent. Let us note then that the red 

 corpuscles are the carriers of oxygen throughout the system ; 

 while the plasma distributes the nutritive material and carries 

 off' the waste products. 



The organ for propelling the blood is the heart, which thus 

 provides for the continuity of the circulation. In the pigeon 

 and the rabbit it has two sides completely separated off from 

 each other without possibility (in the adult) of direct communi- 

 cation. Each side has two parts (1) a receiver, the auricle ; 



