178 VERTEBRATE ZOOLOGY 



The Internal Structure of the Heart. Remove the heart from 

 the body. Identify the roots of the three caval veins which enter 

 the right auricle; the right precaval and the postcaval will be 

 quickly found; the root of the left precaval is attached to the 

 dorsal wall of the auricles and lies across them. Identify the roots 

 of the two pulmonary veins, which appear close together near the 

 middle of the dorsal surface and enter the left auricle. Identify 

 the short pulmonary artery and its right and left branches ; iden- 

 tify the aorta, and the right and left innominate arteries which 

 spring from it. 



Cut across the wall of the right auricle ; open the auricle, and 

 remove the blood that will probably be found caked in it. Note 

 the openings of the three caval veins ; also the muscular fold pro- 

 jecting into the auricle, which forms a valve guarding the open- 

 ing of the postcaval. Note the partition separating the right from 

 the left auricle ; also that separating the right auricle from the 

 right ventricle. In the ventricle is a deep crescentic depression, 

 the auriculo-ventricular opening, through which the blood goes 

 into the ventricle. 



Make a median longitudinal slit in the wall of the right ventricle 

 and note the small extent of this ventricle. Note the auriculo- 

 ventricular opening, and the valve projecting into the ventricle, 

 which prevents the flow of blood back into the auricle. Probe 

 through the pulmonary arteries into the right ventricle; follow 

 the probe with scissors and cut open the arteries. Note the three 

 transversely placed semilunar valves, which are situated at the 

 base of the median pulmonary artery. 



Open the left auricle by a transverse slit through its wall and 

 remove the blood in it. Note that the pulmonary veins communi- 

 cate with a partly separated chamber of the auricle. Observe the 

 septum separating the auricle from the left ventricle, and the round 

 auriculo-ventricular opening in it. Open the left ventricle by a 

 median longitudinal slit ; note the thickness of the walls, and the 

 relatively small cavity with its longitudinal muscular ridges. The 

 auriculo-ventricular or mitral valve will be seen to be formed of 

 two flaps which are joined with the wall of the ventricle by cords, 

 the chordae tendinese. Probe the aorta into the left ventricle and 



