THE MECHANICS OF THE LIVING MACHINE 



311 



-rc 



machinery devised on the same general structure as the heart 

 and blood vessels. 



It is evident, however, that one phase in the circulation 

 requires further explanation. The force that drives the blood 

 is the contraction of the walls of the heart. Unless we ex- 

 plain the beating of the heart, we have not explained cir- 

 culation. The explanation of this phenomenon belongs to 

 the study of muscles, for the walls of the heart are nothing 

 more than a chamber made up of a series of muscles. The 

 beat of the heart is, therefore, no more mysterious than 

 the contraction of other muscles. The contraction of the 

 muscles, it is true, we do 

 not yet fully understand, 

 but we do know that mus- 

 cles constitute a machine 

 which by physical laws 

 transforms the energy 

 stored in the foods into 

 motion. 



Not only is the distribu- 

 tion of the blood to be 

 explained by mechanical 

 principles, but the method 

 by which the blood sup- 

 plies the tissues with their 

 nourishment is fairly simple. 

 The blood first absorbs nour- 

 ishment from the alimen- 

 tary canal and is then car- 

 ried into the active tissues 

 of the body for example, 

 to the muscles where again it is placed in a position in which 

 osmotic pressure will be exerted. The blood passes through the 

 muscles in thin-walled capillaries, on the putside of which is a 

 liquid called the lymph (Fig. 137), and thus there is a membrane 



FIG. 137. DIAGRAM OF A FEW CAPIL- 

 LARIES FILLED WITH BLOOD CORPUS- 

 CLES AND SURROUNDED BY LYMPH. 

 THE ARROWHEADS SHOW THE DIALY- 

 SES FROM THE LYMPH INTO THE 

 TISSUES, AND FROM THE TISSUES 

 BACK INTO THE BLOOD 



we, white corpuscles; 

 rc, red corpuscles. 



