97 



CHAPTER IX 



EXAMINATION OF THE FROG S HEART. 

 LIGATURE 



THE FIRST STANNIUS 



To examine the main features of the beat of a frog's heart proceed 

 in the following way : — 



1. Expose the heart. Pith the brain, leaving the spinal cord intact, 

 and lay the frog on its back. Pick up the skin over the sternum and 

 slit it up in the mid line, make transverse incisions on either side and 

 reflect the four flaps of skin thus formed. The sternum should now 

 be completely exposed. Make a transverse cut through the lower 

 cartilaginous piece of the sternum (xiphi sternum), taking care not to 

 wound the anterior abdominal vein, and cut through the sternum by 

 longitudinal incisions a little on either side of the mid line, and thus 

 remove the central piece. Care is to be taken not to injure the peri- 

 cardium which lies beneath. The lateral pieces may now be pulled 

 apart. This exposes the heart still lying within the pericardium. 

 Lift up the pericardium with fine forceps and snip it through from 

 the apex of the heart to the base. 



2. Examine the different parts of the heart. Above lie the two 

 thin-walled auricles, fig. ^^ ^^\ n 



i % 



82, their line of division 



not being clearly seen, 



as it is mainly hidden by 



thebulbus arteriosus, b a, 



which crosses the auricles 



from below upwards and 



from right to left. 



Below is the single ven 



tricle, bluntly pointed, 



and with a well-marked 



groove, the auriculo-ven 



tricular groove, separat- 



ingitfromthetwoauricles. 



If the heart be now lifted up by the apex, the lower half of the sinus 



Fig. 82. — Anterior and Posterior Surfaces or 

 the Frog's Heart, sv, Sims Venosdb. a, Auri- 

 cles, v. Ventricles, ha, Bulbus Arteriosus. 



