194 ELEMENTARY BIOLOGY. [XIII. 



chloroformed Frog the heart is distended with 

 blood when it ceases to beat. "When all signs 

 of contractility have disappeared, the distended 

 heart should be removed from the body by cutting 

 through the adjacent parts in such a manner as 

 to leave the terminations of the veins and the 

 origins of the aortic trunks intact. The organ 

 should next be transferred to a shallow dissecting 

 dish and covered with weak spirit. The right and 

 left walls of the atrium being now carefully slit 

 and the blood which they contain washed away, 

 the delicate septum of the auricles will become 

 visible. By cautiously removing the ventral face 

 of the ventricle, its cavity will be laid open and 

 the auriculo-ventricular opening will be displayed. 

 If the ventral wall of truncus arteriosus is laid 

 open longitudinally with fine scissors, the valves 

 in its interior will become visible. The pul- 

 monary vein runs along the dorsal aspect of the 

 sinus venosus, between the right and left superior 

 venae cavse, to the left auricle, into which it opens 

 close to the dorsal attachment of the septum. 



The natural relations of the different divisions 

 of the heart should be carefully studied in such 

 a dissection as is described in A. 6. 



ii. The pulsation of the heart. This should be studied 

 in a Frog rendered insensible by chloroform or by 

 being pithed ; though the latter operation causes such 

 dilatation of the vessels that little or no blood may 

 afterwards flow through the heart, yet the organ goes 

 on beating. 



a. Watch the movement carefully ; it is a regularly 

 alternating series of contractions and dilatations. 



