LII.] THE frog's heart. 259 



PHYSIOLOGY OF THE CIRCULATION. 



LESSON LII. 



THE FROG'S HEART— BEATING OP THE HEART— 

 EFB^BCT OF HEAT AND COLD— SECTION OF 

 THE HEART. 



1. Heart of the Frog and how to Expose it. 



(a.) Pith a frog, and lay it on its back and pin ont its legs on 

 a frog-plate. Make a median incision through the skin over the 

 sternuin, continue the incision upwards and downwards, and from 

 the middle of the sternum make transverse incisions. 



(b.) Reflect the four flaps of skin, raise the lower end of the 

 episternum with a pair of forceps, and cut through the sternal carti- 

 lage just above its lower end, to avoid wounding the epigastric 

 vein. With a strong pair of scissors cut along the margins of the 

 sternum, and divide it above. Do not injure the heart, wliich is 

 exposed still beating within its pericardium. 



(c.) With a fine pair of forceps carefully lift up the thin trans- 

 parent pericardium, and cut it open, thus exposing the heart. 



2. General Arrangement of the Frog's Heart. 



(a.) Observe its shape, noting the two auricles above (fig. i8o, 

 A^, As), and the conical apex of the single ventricle below (?;), the 

 auricles being mapped ofi' from the ventricle by a groove which 

 runs obUquely across its anterior aspect. The ventricle is con- 

 tinuous anteriorly with the bulbus aortae (B), which projects in 

 front of the right auricle, and divides into two aortae — right and 

 left, the left being the larger. 



(b.) Tilt up the ventricle and observe the sinus venosus (fig. 

 1 8 1, s.v.) continuous with the right auricle, and formed by the 

 junction of the large inferior vena cava {e.i.) and the two (smaller) 

 superior vense cavae {c.s.s, c.s.d). 



3. Note the sequence of contraction of the several parts, viz., 

 sinus venosus, auricles, ventricle, and bulbus arteriosus. 



Tliis sequence of events is difiicult to note, but what can be 

 easily observed is the relative condition of vascularity of the 

 ventricle. The frog's ventricle has no blood-vessels supplying its 

 muscular walls. Note that during systole of the ventricle, i.«., 

 during its contraction, it becomes pale, and immediately thereafter, 



