THE ACTIVE FORCES OF LIVING ORGANISMS 37 



tissue. It is evident, therefore, that all forms of 

 movement, but especially those which are continuous, 

 may, and probably do, play a very important part in 

 the processes of metabolism. The influence of the 

 respiratory movements is especially worthy of con- 

 sideration in this respect. These produce a series of 

 variations of blood-pressure which are probably felt 

 throughout the whole organism, and can scarcely fail 

 to influence the mode of oxidation of the tissues. 

 When the skull of a living animal is removed, the 

 brain may be seen to rise up during expiration and 

 to sink during inspiration.* Expiration is held to 

 hinder and inspiration to facilitate the return of blood 

 from the brain, and it is evident that this applies 

 not only to the blood returning from that organ, but 

 also to that coming from other parts of the body. 

 At the same time, whilst venous pressure is diminished 

 during inspiration to the point of becoming negative, 

 arterial pressure is, on the contrary, increased, 

 though not in a manner which is exactly synchronous 

 with the inspiratory movement, t The former, in- 

 deed, follows close upon the latter, and attains its 

 maximum only at the beginning of the expiratory 

 movement. These variations of pressure in the 

 thorax and in the vascular system in general must 

 lead to corresponding variations in the degree of 

 oxidation in the tissues ; for, as the arterial blood- 

 pressure rises, so must the pressure of the lymph 

 and of the oxygen it may for the moment contain be 



* Foster, loc. cit, p. 613. f ^bid., p. 614. 



