54 



PEACTICAL PHYSIOLOGY 



abdominal vein. By a transverse cut the xiphisternum is divided and 

 the junction of the anterior abdominal vein with the heart preserved. 

 The pectoral girdle is next divided in the middle line. The inner 

 blade of the scissors is kept hard against the sternum to avoid injuring 

 the heart beneath. The divided halves of the pectoral girdle are pulled 

 widely apart. The heart is now seen enclosed in a thin membrane 

 the pericardium. This is picked up with forceps and slit open. A 

 slender band of connective tissue the fraenum connects the posterior 

 surface of the heart with the pericardium. A thread is passed under 

 the fraenum with fine pointed forceps and tied. The fraenum is then 

 divided on the side of the thread remote from the heart. 



FIG. 54.-The frog's heart. A, Anterior view ; B, Posterior view ; C, Longitudinal 

 section. (Mudge.) 



By means of the thread the heart can be lifted up and turned over 

 for examination. In the front aspect of the heart a single blunt pointed 

 ventricle is seen with the bulbus arteriosus and the two auricles the 

 bulbus ascends over the right auricle from right to left. It separates 

 into two aortae. Each aorta is divided by longitudinal septa into three 

 channels which soon separate and become the carotid, the aortic, and the 

 pulmono-cutaneous arches. 1 The auriculo-ventricular groove separates 

 the auricles from the ventricle. On turning the heart over the sinus 

 venosus is seen, and the white crescentic line which marks the 

 1 The frog respires both by skin and lungs. 



