THE CIRCULATION 301 



which blood-supply and blood-pressure depend. When a part becomes 

 active it requires more blood than while resting: the increase is supplied 

 largely by vaso-dilatation. Heat-regulation, as we have seen, is chiefly 

 accomplished by varying the amount of blood in the viscera and on the 

 body's periphery as the case demands ; this is one of the chief uses of vaso- 

 motion. The nutrition of the tissues by the lymph depends largely on 

 osmosis from the capillaries, and this in turn is dependent on the supply 

 of blood at the spot and on its pressure in the vessels. The case is 

 similar with glandular action. We see, therefore, that some of the most 

 fundamental of organic functions depend sooner or later on the relative 

 caliber of the arteries. 



The mechanism of vaso-motion (discovered by Claude Bernard in 1851) 

 is none too well known, but it is certain that there are (autonomic) 

 nerves connected with the arterial muscle-cells which influence the latter 

 to contract (the vaso-constrictors), and that there are others which some- 

 how occasion the relaxation of the arteries (the vaso-dilators). Each of 

 these sets of sympathetic nerves has moreover a center or centers in the 



Fia 162 



An artery and a vein from the stomach of a frog to show the sphincters about them. 



84 /j. (Mayer.) 



central nervous system, while a general directing vaso-motor centerlis 

 probably one of the numerous centers of the medulla oblongata. The 

 muscle-fibers of the arterial walls are chiefly circular, and their contrac- 

 tion narrows the arterial caliber. It has not been demonstrated as yet 

 how, if at all, nervous influence causes active relaxation of a muscle 

 cell. The process appears to occur not only here but in the heart-muscle, 

 the inhibitory influence of the vagus being perhaps essentially of this 

 nature (see above, page 295). Starting, then, with an average tone (or 

 degree of contraction) of the circular fibers, vaso-constrictor influence 

 from the nerves on Gaskell's theory would increase their katabolism 

 and their activity which is contraction. If, however, the influence be 

 vaso-dilator, katabolism would be partly checked (whether replaced by 

 anabolism or not) and the pressure of the blood from the heart would 

 force open somewhat the now relaxed arterial walls. It is possible, too, 

 that the arrangement of the muscular fibers in the arterial wall is such, 

 (some being longitudinal and some of all degrees of obliquity), that a 

 positive enlargement of the tube may be actively produced by their con- 



