INTRODUCTION 19 



of local events just as surely and much more quickly than 

 can the material exports of another region. Reactions 

 produced through the nervous system are correspondingly 

 sharp and prompt in developing. 



Blood and Lymph. In the bodies of the higher animals 

 the internal medium may be described as existing in two 

 forms. In direct contact with the majority of the cells 

 there is a comparatively stagnant fluid, the lymph. 

 From this they draw their needful supplies of oxygen and 



Fig. 3. B is a blood-vessel of the smallest size a capillary 

 with walls of flattened cells like that in Fig. 2, d. The blood flow- 

 ing within is removed from direct contact with the cells (C, C), but 

 dissolved substances may pass from one to the other through the 

 capillary wall and the medium of the lymph (L). 



food; into it they discharge their waste. The limited 

 resources of the lymph at a given point would be quickly 

 exhausted were it not that the blood is passing close by in 

 vessels whose delicate walls permit the passage of material 

 in both directions. The blood is in rapid movement and 

 it is constantly renewing the oxygen of the lymph with 

 fresh portions just brought from the lungs. It is at the 

 same time receiving from the lymph the accumulated 

 waste. 



