PHYSIOLOGIC APPARATUS. 725 



The conus arteriosus is separated from the ventricle by three semilunar 

 valves. The interior of the conus is traversed by a longitudinally disposed 

 membranous valve attached to its dorsal surface; the ventral edge is, however, 

 free. The upper extremity of the conus passes into the bulbus aorta, from 

 which it is separated by a semilunar valve and the free extremity of the 

 longitudinal valve. From the bulbus aortae arise two large branches, a 

 right and a left, each of which is subdivided by two longitudinal partitions 

 into three vessels, the carotid trunk, the aortic arch, and the pulmo-cutaneous 

 trunk. (See Fig. 377.) The carotid and aortic trunks communicate 

 separately with the cavity of the bulbus, while the pulmo-cutaneous trunk 

 communicates with the conus arteriosus by a single orifice, just below the 

 free end of the longitudinal valve. After pursuing a short course these 



1C. 



FIG. 376. DORSAL SURFACE OF THE FROG HEART, (after Howes.) ra. Right auricle. 

 la. Left auricle, sv. Sinus venosus. sv' . Opening of sinus venosus into right auricle. 

 pv. Pulmonary vein. v. Right anterior caval vein, p' s' c' . The pulmo-cutaneous, aortic and 

 carotid trunks respectively. 



three vessels separate from one another to distribute blood to the various 

 organs of the body. The two aortic trunks wind around the esophagus 

 and unite posteriorly to form the dorsal aorta; the pulmo-cutaneous divides 

 into a pulmonary artery which is distributed to the lung and a cutaneous 

 branch which is distributed to the skin. 



The course of the blood through the heart cavities is therefore as follows : 

 The venous blood poured by the venae cavse into the sinus venosus passes 

 through the sino-auricular foramen into the right auricle. While the 

 right auricle is being filled from this source, the left auricle is being filled 

 by blood coming through the pulmonary veins. "When the auricles contract, 

 which they do simultaneously, each passes its blood into the corresponding 

 part of the ventricle, which then instantly contracts before the venous and 

 arterial bloods have time to mix. Since the conus arteriosus springs from 



