374 GASEOUS EXCHANGES OF MUSCLES IN THE BODY. [BOOK I. 



and we have tacitly assumed that the medium in question is 

 the air. This is not strictly true. The medium enclosing the 

 elementary parts of contractile substance consists of the tissue 

 juices, which come into contact with the air only at the surface 

 of the muscle : the tissue juices mediate between the muscular 

 substance and the air. Now the fluid which bears to muscle 

 within the body the same relation which air has to excised 

 muscle, is the blood ; and as in the former case, the tissue juices 

 are those which deal directly with the muscle-substance, mediating 

 between this and the blood. By the ramification of blood-capillaries 

 between the fibres of muscle, the opposed surfaces of the muscle and its 

 medium become enormously more extended than in the case of air ; 

 and by how much the more extensively the muscle is presented to the 

 medium, by so much the more readily will exchanges be effected. 

 This is a circumstance favourable to exchanges between muscle and 

 blood apart from any peculiar fitness or endowment of blood for the 

 work of exchange, which are matters for discussion in the Chapters on 

 Respiration. 



In consequence of the organization of the body, and the necessity 

 which it is under of preserving a normal standard or equilibrium, there 

 are two methods of determining the influence which muscles exert upon 

 the blood. We may, in the first place, contrast the blood flowing to 

 and that flowing from the muscle, while the muscle is left in repose, 

 or is thrown into activity. This is the direct method, and is equivalent 

 to exposing muscles to an atmosphere of known constitution and after- 

 wards analysing the atmosphere. In the second place, we may observe 

 the changes of the general ingesta and excreta of the body which are 

 brought about when muscle is converted from one state to the other. 

 This is essentially an indirect method. Hitherto it has only been 

 employed to ascertain the chemical processes of muscle on passing from 

 the state of rest to that of activity, when the same animal is 

 compared, in respect of its ingesta and excreta, during repose and 

 during exertion. But a simultaneous comparison of the daily food and 

 excreta on the one hand, and the proportion of muscular and non- 

 muscular elements of the body on the other, in different kinds of 

 animals enjoying the same conditions of rest, might be employed to 

 ascertain the normal exchanges between muscle and its medium when 

 the former is at rest. 



It is only necessary here to point out in general terms the uncer- 

 tainty of the indirect method of analysis. Everything which is given 

 up by muscle to blood is not of necessity given up by the blood to 

 the general excreta. The method of excretion is only one of the 

 means employed by the body to preserve its equilibrium. Some 

 part of the substances cast by muscle into the blood may be appro- 

 priated by other organs or tissues, and never appear at the surface of 

 the body ; and some part of the substances excreted, though brought 

 to the surface during muscular activity, may not have arisen within the 

 active muscle. 



