80 ON THE MOTION OF THE BLOOD. 



a punctured vein should afford blood very readily. It' the chief 

 vein of a limb is wounded, the blood will flow, because it re- 

 ceives the whole blood of the arteries, transmitted by the vis a 

 tergo, no other veins existing- into which it can be drawn when 

 the vacuum occurs in the right auricle : what is a parallel circum- 

 stance, if all the veins of a limb are tied, they swell, whereas 

 the ligature of one causes no tumefaction in it. These circum- 

 stances are no proof that the vis a tergo is sufficient of itself to 

 bring back the blood, because it is certain that such a vacuum 

 exists, and that such must be the effects of this vacuum upon the 

 movement of the blood : the hemorrhage in the former instance, 

 and the tumefaction in the latter, show a certain force only in the 

 blood, which, were it even sufficient to bring the blood back to 

 the heart, as an experiment of M. Majendie's almost proves it to 

 be,* would not probably long continue so after the assistance of 

 suction was removed. 



From the structure of the heart it is clear that the mere alter- 

 nate relaxation of its parietes enlarges its cavities and forms a 

 vacuum. Experiment proves the same. Dr. Carson put the hearts 

 of some frogs just extracted into water, blood-warm. They 

 were thrown into violent action, and, upon some occasions, pro- 

 jected a small stream of a bloody colour through the transparent 

 fluid. It was thought that a stream of the same kind continued 

 to be projected at every succeeding contraction ; but that, after 

 the first or second, it ceased to be observable, in consequence of 

 the liquid supposed to be imbibed and projected, losing its 

 bloody tinge and becoming transparent, or of the same colour 

 with the fluid in which the heart was immersed. The organ was 



* A ligature was passed around the whole of a dog's thigh excepting the 

 crural artery and vein. Another was fixed upon the vein. On puncturing the 

 rein, the blood was projected to some distance and continued to be so except 

 when the artery was compressed ; and as long as the circulation continued, the 

 stream through the wound in the vein could be regulated at pleasure by com- 

 pressing or liberating the artery. Precis Elcmcntairv de Physolog-ic. t. ii. 

 p. 323 sq. 



