CAUSES OF THE CAPILLARY CIRCULATION. 91 



contractions of the arteries, a property which has already been fully considered. Move- 

 ments which have been observed in membranes detached from the body are due to the 

 mere emptying of the divided vessels or to simple gravitation. It must be remembered 

 that, in microscopical examinations, the movements observed are immensely exaggerated 

 by the magnifying power, and we receive, at first sight, an erroneous impression of their 

 rapidity. The movements of the blood in detached membranes, due merely to gravitation, 

 have been so satisfactorily explained by the experiments of Poiseuille, that it is deemed 

 unnecessary to refer to the observations of those who have attributed this phenomenon 

 to other causes. 



Physiologists who, like Bichat, have been unable to explain the local variations in the 

 capillary circulation without the intervention of a force resident in these vessels or in 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 relaxa- 

 tions of their muscular walls, they regulate the supply of blood to the capillaries of in- 

 dividual parts. Their action is competent to produce all the variations which are ob- 

 served 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, microscopical observations and experiments on the 

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



It has been asserted that there is a circulation of the blood in the area vasculosa, 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 develop- 

 ment 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 cir- 

 culatory system is quite or nearly perfect. 



Influence of Temperature on the Capillary Circulation. Within moderate limits, a 

 low temperature, induced by local applications, has been found to diminish the quantity 

 of blood sent to the capillaries and retard the circulation, while a high temperature 

 increases the supply of blood and accelerates its current. The mechanism of this is 

 beautifully shown by the experiments of Poiseuille. This observer found that when a 

 piece of ice was applied to the web of a frog's foot, the mesentery of a small warm-blooded 

 animal, or to any part in which the capillary circulation can be observed, the number of 

 corpuscles circulating in the arterioles became very much diminished, " those which car- 

 ried two or three rows of corpuscles giving passage to but a single row." The circulation 

 in the capillaries first became slower and then entirely ceased in parts. On removing 

 the ice, in a very few minutes the circulation regained its former characters. If, on the 

 other hand, the part be covered with water at 104 Fahr., the rapidity of the current in 

 the capillaries is so much increased that we can hardly distinguish the form of the cor- 

 puscles. 



Influence of Direct Irritation upon the Capillary Circulation. Experimental re- 

 searches on the effects of direct irritation of the capillaries, in parts where the circulation 

 can be observed microscopically, have been quite numerous since Thompson studied the 

 effects of saline solutions on the web of the frog's foot, in 1813. The most noticeable 

 papers on this subject are those of Dr. Wilson Philip and Mr. Wharton Jones. The latter 

 paper, which receiv-ed the Astley Cooper prize for 1850, is based on very extended and 



