Chap. III.] GENERAL ANATOMY. 25 



system. When the frog breathes the anterior nares are opened, 

 and the floor of the mouth is depressed. Air rushes in. Then 

 the nares are closed and the floor of the mouth is raised. The 

 air is forced into the lungs. When the nares again open the air 

 rushes out of the lungs, which tend to collapse from their own 

 elasticity. 



3. The Heart and Circulatory System. The heart lies in its own 

 special pericardia! cavity (7, pe. ca.), which projects backwards 

 into the pleuro-peritoneal cavity. This cavity is lined with a 

 glistening pigmented membrane, the pericardium, which has a 

 parietal and a visceral layer, the one lining the cavity, the other 

 reflected on to the heart. The heart itself has four parts, (1) 

 sinus venosus, (2) atrium, (3) ventricle, (4) truncus arteriosus. The 

 ventricle is the posterior fleshy part of the heart (F.). The 

 truncus arteriosus (T. A.) passes forwards from the ventricle as a 

 fleshy tube. The atrium is seen in Fig. 7 (At.) on either side of 

 T. A. It is thin-walled. The sinus venosus is also thin-walled 

 and dorsal in position, lying above the atrium. It may be seen 

 by lifting up the ventricle. Into this division of the heart the 

 blood is received from the various parts of the body, and is 

 passed on to the atrium. Thence it flows into the ventricle, and 

 thence into the truncus arteriosus to be distributed throughout 

 the body. In a recently-killed frog the heart may still be beat- 

 ing. Its parts will be seen to contract in the order given. 



After all the other points in the general anatomy of the frog 

 have been made out, the heart may be removed and dissected 

 under water. The atrium will be found to be divided by a 

 septum into two chambers, a right and a left auricle. The ven- 

 tricle has an undivided cavity (Fig. 9, iv. v.). The right auricle 

 receives the blood from the general mass of the body through 

 the sinus venosus (s. v.). The left auricle receives blood from the 

 lungs by the pulmonary vein (p. v.). Blood from both auricles 

 passes into the ventricle, and thence to the truncus arteriosus for 

 distribution. 



The truncus arteriosus soon divides into a right ancl a left 

 branch, each of which gives origin to three vessels (Figs. 7 and 

 10). Of the three arteries into which it splits, the most anterior 



