LIL] THE FROG'S HEART. 259 



PHYSIOLOGY OF THE CIRCULATION. 



LESSON LIT. 



THE FROG'S HEART BEATING OF THE HEART- 

 EFFECT 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 out its legs on 

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

 sternum, 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 witli 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, which is 

 exposed still beating within its pericardium. 



(e.) 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. 180, 

 Ad, As), and the conical apex of the single ventricle below (v), the 

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

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

 tinuous anteriorly with the bulbus aortse (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 (c.i.) and the two (smaller) 

 superior venae 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. 



This sequence of events is difficult 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 \valls. Note that during systole of the ventricle, i.e., 

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



