CIRCULATION. 



is the alternate contraction and relaxation of the ventricles ; 

 but this is only one of the operations of the organ. In any 

 of the class of mammals the anatomy and action of the vas- 

 cular system are to all intents and purposes the same as in 

 the human subject ; and though the exposure of the heart by 

 opening the chest modifies somewhat the force and frequency 

 of its pulsations, the various phenomena follow each other 

 in their natural order, and present essentially their normal 

 characters. The operation of exposure of the heart may be 

 performed on a living animal without any great difficulty ; 

 and if we simply take care to keep up artificial respiration, 

 the action of the heart will continue for a considerable time. 1 

 We may keep the animal quiet by the administration of 

 ether, or by poisoning with woorara, the latter agent acting 

 upon the motor nerves, but having no effect upon the heart. 

 Having opened the chest, we see the heart enveloped in its 

 pericardium, regularly performing its functions ; and on 

 slitting up and removing this covering, the various parts are 

 completely exposed. The right ventricle and auricle, and 

 a portion of the left ventricle, can be seen without disturbing 

 the position of the parts; but the greater part of the left 

 auricle is concealed. As both auricles and ventricles act 

 together, the parts of the heart which are exposed are suffi- 

 cient for purposes of study. 



Action of the Auricles. Excepting the short time occu- 

 pied in the contraction of the auricles, these cavities are con- 

 tinually receiving blood on the right side from the system, 

 by the venae cavae, and on the left side from the lungs, by 

 the pulmonary veins. This continues until their cavities are 

 completely filled, the blood coming in by a steady current ; 

 and during the repose of the heart, the blood is also flowing 



1 For a full description of the operations for exposing the heart in living ani- 

 mals, the reader is referred to an article by the author in the American Journal 

 of the Medical Sciences, October, 1861, entitled Experimental Researches on points 

 connected with the Action of the Heart and with Respiration, 



