CAUSES OF THE CAPILLARY CIRCULATION. 295 



divided vessels or simple gravitation. It must be remem- 

 bered that in microscopic examinations, the movements which 

 are observed are immensely exaggerated bj the magnifying 

 power, and we receive, at first sight, an erroneous idea of 

 their rapidity. The movements of the blood in detached 

 membranes, due merely to gravity, 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. 1 



Dr. Dowler, of New Orleans, made some experiments on 

 the circulation in patients dead with yellow fever, in which 

 he found that the blood would flow in a tolerably full stream 

 from a punctured vein a few minutes after death. This he 

 attributes to an independent action of the capillaries, which 

 continues for a time after the action of the heart has 

 ceased. 2 These observations are met by the following experi- 

 ment performed years before by Magendie. 3 A ligature was 

 passed around the thigh of a dog, leaving only the crural 

 artery and vein. A ligature was then applied to the vein, 

 and a small opening made below it in the vessel, from which 

 the blood escaped in a jet. On compressing the artery, the 

 flow of blood was not immediately arrested in the vein, but 

 continued to gradually diminish in force until it stopped after 

 a few moments. On examining the artery below the point 

 of compression, it was found contracted, and completely 

 emptied of blood, while the vein was full below the punc- 

 ture. The pressure being removed from the artery, the blood 

 commenced to flow from the vein, and a jet was soon estab- 

 lished as before. When the artery was slightly compressed, 

 so as to allow the passage of a small quantity of blood, not 

 enough to distend the vessel, the blood flowed from the vein, 

 but no longer in a jet. This experiment shows that when 



1 POISEUILLE, Recherches sur les Causes du Mouvement du Sang dans les Vals- 

 seaux Capillaires, 1835, p. 127. 



2 DTJXGLISON, Human Physiology, Philadelphia, 1851, vol. i., p. 420. 



3 MAGENDIE, Precis tilementaire de Physiologic, Paris, 1836, tome ii., p. 390. 



