158 



THE BLOOD 



brought into play which cause the fluid to move in such a manner that 

 delicate streams or even oscillatory currents are produced. The more 

 efficient protection against outside influences, which is thus afforded 

 the "blood," enables it to maintain a much greater complexity with- 

 out materially hindering the osmotic interchanges. 



In the higher animals, the blood assumes all the characteristics 

 of a tissue, but in order that the cellular units of the body may be 

 brought into relation with their nutritive source in the shortest pos- 

 sible time, it is made to move rapidly through a system of intricate 

 and recurrent tubes, the driving force being furnished by a central 

 muscular organ, the heart. Besides, these animals are equipped with 

 a fluid known as the lymph 1 which serves the purpose of a medium 



FIG. 94. THE DEVELOPMENT OF THE CIBCULATOHY SYSTEM. 



Osmotic interchanges take place 1, between the medium and the substance of the 

 cell through the cell wall; 2, between the contents of the alimentary canal (AC) and the 

 tissue cells (B) ; 3, between the contents of the alimentary canal (AC) and its recesses, 

 and the tissue cells (B and C), and 4, at the same two places after the recesses have 

 become separated from the body canal. 



for the osmotic interchanges between the blood and the tissues. Thus, 

 as the amount of blood present in the body is relatively small whereas 

 its complexity is great, it must be evident that the lymph forms an 

 economic factor of greatest importance, because its copiousness and 

 watery consistency enable it to enter the smallest spaces and to come 

 in direct contact with practically every cell of the body. The cells, 

 it is commonly stated, are bathed in lymph, while the blood itself 

 does not actually touch them. This statement, however, is not in- 

 tended to imply that these nutritive fluids are quite independent of 

 one another. On the contrary, the lymph is derived originally from 

 the blood in accordance with physico-chemical laws. It is diluted 

 plasma which, however, is not lost to the blood, but is again returned 



1 A third circulating fluid is the chyle, but as this is merely lymph loaded with 

 the products of digestion, it need not be considered separately. The same holds 

 true of the intraocular fluid and of the liquor cerebrospinalis. 



