212 



GEx\ERAL BIOLOGY 



materials is effected. These supply the aquatic environ- 

 ment that is necessary for the maintenance of cell life: 



for cell life, in the beginning aqua- 

 tic, is in an important sense aqua- 

 tic still, even in terrestrial organ- 

 isms. Living protoplasm is a semi- 

 fluid substance, and metabolism 

 is compatible only with a liquid 

 state. 



The fluids of the body are moved 

 about, (that is, circulated) in part 

 by the movements of the tissues 

 which they bathe, and in the higher 

 organisms appear special organs of 

 propulsion. Blood vessels at first 

 appear as short open contractile 

 tubes, that communicate freely with 

 the coelom, and that merely serve to 

 keep the blood irregularly moving. 

 When, as in the higher vertebrates 

 they have become completely closed 

 channels, capable of retaining the 

 differentiated red corpuscles and 

 carrying them about the body in 

 continuous procession, they are 

 still supplemented by that inter- 

 cellular circulation that is due to the 

 contraction of the muscles and 

 movements of the organs. The 

 need of this propulsion of body fluids 

 by body movements is convincingly 

 evidenced in ourselves by the benefit of physical exercise 

 (even though performed by proxy, as in massage), and 

 conversely, by the stagnation induced by too exclusively 

 sedentary habits. 



Fig. 136. Diagram of the 

 two main channels by 

 which food enters the 

 ge neral circulation in 

 mammals. e, intestine 

 with villi, V, V, in its walls 

 r a, right auricle of the 

 heart, m, post cava; n, 

 precava; o, thoracic 

 lymph duct; ^.pancreas; 

 q, pancreatic duct; r, 

 hepatic vein; 5, portal 

 vein; t, bile duct from /, 

 liver: arrows indicate the 

 course of secretions en- 

 tering the intestine, and 

 of the absorbed food de- 

 parting therefrom. 



