PHYSIOLOGICAL 289 



portion as the nerve-cells become centralised in structures like the 

 brain and the spinal cord. The life of a higher animal depends on 

 the timing and tuning, even up to orchestration, of countless 

 reflexes. Nervous stimuli from the outer world or the organs of the 

 body affect the cells of the centres, and evoke efferent impulses 

 which effectively control muscles and glands and other parts. 



With this nervous integration in the big-brained animals must 

 be associated, though the relation remains a riddle, a mental or 

 psychical integration; for it is difficult, some would say impossible, 

 to doubt that a dog, for instance, may be actuated by a perceived 

 purpose, and there is nothing that integrates life more powerfully. 

 The physiologist, like the physician, has often to take account 

 of the esprit de corps in a quite literal sense. 



But another important kind of integration is effected by the 

 blood, the common medium (along with the lymph) from which 

 all the cells of the body take, and to which they all give. As we 

 have already seen, the blood is a food-distributor, a gas-carrier, 

 and a waste-collector; it is also a bearer of the migrant amoeboid 

 cells called phagocytes, which play an important part in the internal 

 defences of the body, as is also true of many of the cells lining the 

 vast interior area of the capillaries. Moreover, the blood is able to 

 manufacture counteractives or anti-bodies, which act as antagonists 

 to poisons, though we have as yet no clear knowledge of how they 

 effect this. Thus the blood has a manifold regulative influence. 



It may be useful to think for a moment of the familiar reaction 

 of sweating, which is an automatic way of preventing the tempera- 

 ture of a mammal's body from rising too high in very warm weather 

 or in the course of very hard work. The sweat-glands in the skin 

 are stimulated to increased activity; they pour out more water, 

 the evaporation of which serves to lower the temperature. The 

 stimuli come to the sweat-glands by nerves which may accompany 

 the peripheral blood-vessels or may be independent of them; and 

 these nerves come from special centres in the spinal cord and in 

 the medulla oblongata. But how do these nerve-cells become alive 

 to the fact that sweating is called for? The answer illustrates what 

 we mean by the integrative action of the blood. The blood that 

 has become overheated flows through the central nervous system 

 and excites the specialised nerve-cells of the regulative ("thermo- 

 taxis") centre; and thence nervous impulses pass to the cells of 

 the sweat-glands, arousing their greater activity. This is not, indeed, 

 the whole story, for the blood which we have simply called "over- 

 heated" may differ from the normal in containing more than the 

 usual quantity of carbonic acid; and it does differ from the normal 

 in its content of hydrogen-ions. But the general fact of importance 

 here is simply the effectiveness of the integrative action of the blood. 



A notable extension of this general idea of the blood as a corre- 



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