THE FROG 99 



Bend the ventricle forward and study its dorsal aspect. Identify 

 the sinus venosus, the three caval veins, and the pulmonary veins. 



Exercise 8. Make a drawing of the dorsal aspect of the heart on a 

 scale of 2 or 3. 



The Internal Structure of the Heart. Cut the aortic arches and 

 the caval veins and remove the heart from the body, being careful 

 not to injure any of the neighboring organs. Place the heart in a 

 small dish of water, with the dorsal side uppermost, and remove 

 the pericardium. Identify the stumps of the three caval veins 

 and the pulmonary vein. Cut off the dorsal wall of the sinus 

 venosus and wash the blood from its cavity. Note the opening 

 into the right auricle; if it is not easily seen, use the blowpipe. 



Place the ventral surface of the heart uppermost ; cut away the 

 ventral wall of the two auricles and wash the blood from their 

 cavities. Note the thin septum which separates the right from 

 the left auricle. 



In the right auricle, which is the larger of the two, note the 

 opening into the sinus venosus, near the septum ; also note the mus- 

 cular ridges on its inner surface. In the left auricle note the small 

 opening of the pulmonary vein, also near the septum, and the 

 muscular ridges. 



Remove the ventral wall of the ventricle and expose its cavity. 

 Note the smallness of its cavity, its thick walls, and the longi- 

 tudinal muscular ridges on its inner surface which divide the 

 cavity into compartments. 



The presence of these elongated compartments is important, 

 inasmuch as they prevent the complete mixing of the venous and 

 the arterial blood which would otherwise take place. Since the 

 ventricle consists of a single chamber into which both kinds of 

 blood are poured from the auricles, it would contain an equal mix- 

 ture of these if the blood could flow unimpeded from one part of 

 the ventricle to another. As it is, venous blood from the right 

 auricle goes into the compartments on the right side of the ven- 

 tricle, and the more nearly arterial blood from the left auricle 

 goes into those on the left side, and but a slight mixing of the 

 blood probably takes place. 



