32 THE FROG 



may be either spherical or irregular in form; the cytoplasm 

 may be either clear or granular. Often the cytoplasm is very 

 granular so that the nucleus cannot be seen. Draw several 

 white corpuscles differing in form or structure. If one is 

 found actively changing in form, sketch it at intervals of 

 about one-half minute, to show changes in form. 



B. The Heart. The heart has the form of a cone with 

 its apex directed posteriorly. The posterior part (ven- 

 tricle) is thick-walled and opaque; the anterior part (auri- 

 cles) is thin- walled and usually filled with clotted blood. 

 Observe the large Y-shaped vessel, the truncus arteriosus, 

 arising from a short tube with thick muscular walls, the 

 bulbus cordis, which joins the right anterior corner of the 

 triangular ventricle ; the truncus arteriosus passes anteriorly 

 and to the left over the ventral surface of the auricles and 

 branches immediately into two diverging trunks. Sketch a 

 ventral view showing ventricle, auricles, bulbus cordis and 

 truncus arteriosus. Make this drawing small (X^) and 

 place it in the center of a clean page so that the arterial sys- 

 tem may be added later. 



Gently lifting the apex of the heart, examine the dorsal 

 surface carefully from behind and from the sides; notice the 

 thin-walled sinus venosus, one of the divisions of the heart, 

 filled with clotted blood, dorsal to the auricles. Three veins 

 enter it : the posterior vena cava from behind, the right 

 and left anterior venae cavae separately from in front. 

 These veins, especially the posterior vena cava, are large, 

 though like all the veins they have thin walls. The small 

 pulmonary veins may also be seen arising from the inner 

 faces of the lungs and uniting to form a single trunk, the 

 short pulmonary vein proper, which enters the left auricle. 



