34 THE CIRCULATION OF THE BLOOD. [CHAP. XXVIII. 



fibrous zone which connects the artery to the infundibulum of the 

 ventricle ; and a free concave border divided by a small round body 

 of fibrous tissue (corpus Arantii) into two equal portions. As the 

 blood flows from the ventricle, these valves lie up against the wall of 

 the artery ; but immediately the blood regurgitates towards the ven- 

 tricle, they are pushed by it in towards the mouth of the artery, and 

 their free margins, as well as a considerable portion of their ventri- 

 cular surfaces coming into close apposition, an effectual barrier is 

 formed against the return of the blood. The semilunar valves of the 

 aorta are essentially the same in all points of form and structure as 

 those of the pulmonary artery; but those of the aorta are the stronger. 



The inner surface of the wall of the central cavity of each ven- 

 tricle is marked by very numerous fleshy columns (carnece columnce) 

 which project from it in relief. There are three orders of them ; 

 first, the simple column in relief, which adheres throughout its 

 whole length to the wall of the ventricle; secondly, the column, 

 attached at each extremity, but free in the interval, so that a probe 

 or other instrument may be passed beneath its middle part ; and 

 thirdly, the column attached at one extremity to the wall of the 

 ventricle, and projecting into its cavity by the other : these last are 

 distinguished from the others by the name of musculi papillares ; the 

 chordse tendinese spring from their free extremities, and are in- 

 serted into the mitral and tricuspid valves, and one or two into the 

 wall of the ventricle. The infundibular portion of the ventricle is 

 perfectly smooth on its inner surface, and quite free from columns 

 earnese. 



The auricles are thin-walled muscular bags of irregular somewhat 

 cuboidal shape. Each communicates by a wide orifice, with its 

 corresponding ventricle, and is separated from its fellow by a thin 

 fleshy septum, which at its middle is so thin as to be translucent. 

 At this situation an orifice existed during intra-uterine life, through 

 which a communication took place between the auricles (foramen 

 ovale, or Botalli). Each auricle has two distinct portions; the 

 sinus venosus which forms by far the greater portion of the bag, 

 and the proper auricle, or auricular appendage, which appears like 

 an offshoot from the former, somewhat in the shape of a dog's ear, 

 projecting forward on each side of the aorta and pulmonary artery. 

 The veins pour their blood into the sinus venosus ; the auricular 

 appendage receives no blood vessels, but its cavity communicates with 

 that of the sinus venosus. The right auricle receives the two great 

 veins of tie system, and the great venous trunk of the heart, the 

 coronary vein. The superior vena cava opens into the upper angle 



