304 



THE CIRCULATION 



there is no such thing either as "general blood-pressure" or universal 

 vaso-motion, so readily, so much, and so often do the local conditions 

 vary. Each organ and each functional group of cells even, probably 

 controls its own supply of blood either through enzymes or by the action 

 of local nerve-ganglion. (See also Expt. 82 in the Appendix). 



Concerning the precise relations of the tonal variation in the size of 

 the heart (Figs. 157 and 160), to the arterial and capillary vaso-motion, 

 nothing definite as yet, unfortunately, is known. There is probably 

 some direct reciprocal relationship. 



The Functions of the Blood Capillaries. Inasmuch as practically the 

 whole interchange between the blood and the tissues takes place through 

 the capillary-walls, it is obvious that the functions of this part of the 

 circulation should be thoroughly known. The details are as yet, however, 



FIG. 164 



Capillary Teasels**' 

 Vasa capillaria 



The venous and arterial networks as seen in the corium of the gastric mucosa. /i- (Toldt.) 



largely unrevealed. The small size of these tubes (less than a milli- 

 meter long and from T -oYo to y|-^ of a millimeter in diameter) together with 

 the fact that their functions are largely based on molecular movements, 

 are the recondite conditions which have kept the workings of the capil- 

 laries doubtful to us. Even their structure is not definitely known in 

 all its details, especially whether a plexus of nerve-fibrils surrounds them, 

 and what the nature is of the cement-substance joining together the 

 edges of the cells. The protoplasm composing them is nearly trans- 

 parent and of extreme thinness and apparent simplicity, yet either with 

 or without the influence of the nervous system it probably determines 

 more than almost any other one sort of tissue the metabolism of the body, 

 for through it passes the means by which the body lives. For the sake 

 of what goes on in the millimeter or half -millimeter of these billions of 

 capillaries all the remainder of the mechanism of the circulation exists. 



