7 2 4 TEXT-BOOK OF PHYSIOLOGY. 



THE ANATOMY OF THE FROG HEART AND THE VASCULAR 



APPARATUS. 



The heart of the frog can be readily exposed after the animal has been 

 made insensible by destruction of the brain. The sternum is divided longi- 

 tudinally and each half drawn outward by gentle traction of the anterior 

 extremities. The pericardium is then divided and turned aside. 



When viewed from the ventral surface, Fig. 375, the heart shows two 

 auricles, a right and left, a single ventricle and a more or less conical vessel, 

 the conus arteriosus, which arises from the right side of the base of the ven- 

 tricle. When viewed from the dorsal surface, Fig. 376, it presents a tri- 

 angular-shaped vessel, the sinus venosus, formed by the union of the 

 terminations of the two superior and inferior venae cavae. A dissection of 

 the heart shows that the cavity of the sinus venosus communicates with 



FIG. 375- VENTRAL SURFACE OF THE FROG HEART. (After Howes.) ra. Right auricle. 

 la. Left auricle, v. Ventricle, ca. Conus arteriosus. p' '. Pulmo-cutaneous trunk, s'. Sys- 

 temic aortic trunk, c' . Carotid trunk, ac. Left anterior caval vein. 



the cavity of the right auricle by means of a transversely oval foramen, in 

 the posterior wall of the auricle. This opening is provided with two 

 valves, a ventral and dorsal, the free edges of which are directed toward 

 the cavity of the auricle. Two pulmonary veins, a right and left, penetrate 

 the dorsal wall of the left auricle. 



A longitudinal section, Fig. 377, of the heart shows that the auricles, 

 though separated by a septum, communicate below by a common orifice 

 with the cavity of the single ventricle. This orifice, the auriculo-ventric- 

 ular, is provided with two valves the free edges of which are directed toward 

 the cavity of the ventricle. 



