304 DEVELOPMENT AND PURPOSE CHAP. 



whole blood stream along throughout the tissues. He will 

 thus follow it through the branching arteries into the capil- 

 laries, observe its interchange of substance with the cells 

 which it bathes, and thus account for its emergence from 

 the capillaries into the veins in the changed character of 

 venous blood. In the same way he will follow it back to 

 the right ventricle and thence through the pulmonary circu- 

 lation where it is restored to its arterial character, to the 

 left auricle, and by the valvular mechanism to the 

 left ventricle from which he started. Here the essential 

 features are mechanical or chemical, and for our purpose we 

 may assume that the chemical is, by methods which year by 

 year come more clearly into view, to be reduced to the 

 mechanical. Nor need we stay to enquire into certain 

 points of the explanation which might present some diffi- 

 culty to the mechanical view, by asking, for example, how 

 far the interchange of substance, which is the essential 

 point in the whole function, can be squared with the 

 physical laws of diffusion, or whether the behaviour of the 

 arteries can be wholly understood on the analogy of elastic 

 tubes. We may better attend to points which, not by 

 their obscurity but their clarity, emphasize the specific 

 character of an organism. This circulatory process, for 

 example, does not work with even regularity. If the body 

 is thrown into violent muscular exertion the metabolism 

 of the muscular tissue is proportionately heightened in 

 order to supply the requisite amount of kinetic energy. 

 This augmentation requires in turn an increased supply of 

 oxygen while it produces a surplusage of oxidised broken- 

 down proteids which have to be eliminated if the muscle is 

 to continue to do its work. These requirements can only 

 be met by an increase in the blood-stream, both to bring up 

 the oxygen and to remove the waste, and in the healthy 

 organism this supply is forthcoming through an accelera- 

 tion of the heart and a dilatation of the arteries, which dila- 

 tation is, moreover, localised if a particular set of muscles 

 have alone to be supplied. At the same time, respiration 

 is quickened, so that the blood is more rapidly oxidised. 

 The action of the heart and arteries then appears to be 

 determined by the function which they have to perform, 



