296 CIRCULATION. 



an artery supplying a part with blood is removed from the 

 influence of the heart, the vessel will contract and force its 

 contents into the vein. This affords the most rational expla- 

 nation of the phenomena observed by Dr. Dowler. When 

 the blood is allowed to enter slowly, so as not to distend the 

 vessel, though it be supplied to the capillary system, it does 

 not there undergo any propelling influence, competent, at any 

 time, to increase the rapidity of the flow from the vein. 



Physiologists who, like Bichat, 1 have been unable to 

 explain the local variations in the capillary circulation with- 

 out the intervention of a force resident in these vessels or the 

 surrounding tissues, have not appreciated the action of the 

 arterioles. These little vessels are endowed to an eminent 

 degree with contractility and, by the contractions and re- 

 laxations of their muscular walls, regulate the supply of blood 

 to the capillaries of individual parts. Their action is com- 

 petent to produce all the variations which are observed in the 

 capillary circulation. 



It is evident, then, that the arterial pressure, which is 

 itself derived from the action of the heart, is competent to 

 produce the circulation of the blood, as we observe it, with 

 all its variations, in the capillary vessels ; that there is no 

 evidence of the intervention of any other force, but, on the 

 contrary, microscopic observations and experiments on the 

 arteries and veins, thus far, show that there is no other force 

 in operation. 2 



1 Loc. cit. 



2 It has been asserted that there is a circulation of the blood in the area vas- 

 culosa, the first blood-vessels that are developed, before the heart is formed ; but 

 there are no definite and reliable observations which show that there is any regular 

 movement of the blood, which can be likened to the circulation as it is observed 

 after the development of the heart, anterior to the appearance of a contractile 

 central organ. Another example of what is supposed to be circulation without 

 the intervention of the heart is in cases of acardiac foetuses. Monsters without a 

 heart, which have undergone considerable development and which present systems 

 of arteries, capillaries, and veins, have been described. All of these, however, 

 are accompanied by a twin, in which the development of the circulatory system is 



