230 THE BLOOD 



buted in a useful form over the country. There are, therefore, 

 two forms of transport which we may term external and internal. 

 As all material has finally to be carried by the inland transport 

 service and as the amount of traffic on this system to some extent 

 controls the rate of importation, it will be convenient to direct 

 attention to it in the first place. 



Inland Transport The Blood. 



The blood has been called the liquid tissue of the body. On 

 two counts this is a misnomer. Firstly, it tends to detract from 

 the liquidity of the tissues in general. Further, blood cannot 

 rightly be considered a tissue at all. No doubt a very pretty 

 picture could be drawn of blood and its containing membranes 

 as a tissue, clotting, as other tissues clot, on death, but when the 

 facts are examined they do not bear out such an idea. The evid- 

 ence too, culled from comparative studies of the development of 

 a circulatory system, is all at variance with the liquid tissue theory. 



1. Development. 



Much may be learned from a study of the evolution of 

 any system. Material exists for such a study in Comparative 

 Physiology. 



(a) Unicellular organisms require no circulatory system. Their 

 imports go direct to the sole factory of the place. They may be 

 landed at any part of the coast and are at once acted on. What 

 is suitable is accepted, the residue is rejected or left untouched. 

 Examination of a unicellular animal leads to the conclusion that 

 the cell contents are in a state of constant motion. Water, every 

 now and then, is engulfed, passes more or less directly through 

 the organism, and is excreted, carrying with it the by-products 

 of cell activity. 



(b) Some invertebrates have an open coelomic system. Their 

 more complex structure necessitates the production of a current 

 of fluid so that material may reach the inner cells. That is, some 

 of the water in which the animal lives is passed by means of canals 

 to the different parts of the body. The fluid is kept in circulation 

 by the rhythmic contractions of whip-like processes called cilia. 

 The ciliary waves force the water through the tubes into the 

 lacunae of the tissues. Such a system is difficult to control. It 

 is dependent on the nature of the bathing medium. It carries the 

 possibility of constant changes in the salt content of the cells of 

 the animal. Any change in the environment will be passed on 



