LESSON XL- HEART-BEAT OF FROG. INHIBITION. 

 LIGATURE OF SINO- AURICULAR JUNCTION. 



For the experiments on the heart it is essential to 

 have fine pointed forceps and scissors. 



1. Cut through the skin of a pithed frog in the mid 

 ventral line, make transverse cuts and turn the skin 

 back. Cut transversely through the xiphisternum just 

 above the abdominal vein. Cut through the sternum in 

 the mid line, pull the parts gently asunder, and cut 

 through the muscles attached to their inner surface. Pin 

 out the arms. The heart will be seen beating in the thin 

 membranous pericardium. Note the alternate beats of 

 the auricles and the ventricle; and the synchronous 

 beats of the two auricles. Lay open the pericardium 

 and observe (best with the aid of a lens) 



a. The synchronous contractions of the two auricles, 

 followed almost immediately by 



b. The contraction of the ventricle; note that the 

 ventricle during its contraction or systole becomes paler 

 and more conical, and that its apex is thrown forwards 

 and upwards ; the obviousness of these changes, especially 

 of the latter, depends upon the force of the contraction 

 and on the amount of blood. 



c. The contraction of the bulbus arteriosus imme- 

 diately succeeding the ventricular systole. Note the 

 distension of the bulbus and the rush of blood through 

 it, as the ventricle contracts. 



