xiii.] THE FROG. 175 



the innominate veins, and between the posterior lymph- 

 hearts and the iliac veins. 



The heart is connected with the walls of the pericardium, 

 on which spots of pigment may be observed, by the vessels 

 which enter and leave it, and by a slender band which 

 passes from the dorsal face of the base of the ventricle to 

 the posterior and dorsal wall of the pericardial chamber. 



The heart consists of four readily distinguishable seg- 

 ments, (i) the sinus venosus, (2) the atrium, (3) the ven- 

 tricle, and (4) the truncus arteriosus, disposed in such a 

 manner that the sinus venosus, which is the hindermost 

 division, lies in the middle line on the dorsal aspect of the 

 heart: the atrium is also median and on the dorsal side, 

 but is in front of the sinus venosus; the ventricle is me- 

 dian, ventral and posterior; and the truncus passes ob- 

 liquely forwards from the right side of the ventricle and is 

 ventral and anterior. The heart therefore may be com- 

 pared to a tube divided by constriction into four portions 

 and bent somewhat into the shape of an S. 



The sinus venosus receives on each side, in front, a large 

 vein, the vena cava superior; while behind the, usually sin- 

 gle, vena cava inferior opens into it. It opens by a valvu- 

 lar aperture into the atrium. The latter shews no signs of 

 division externally, but, internally, it is divided by a deli- 

 cate partition, the septum of the auricles, into a smaller left 

 auricle and a larger right auricle. The sinus venosus opens 

 into the atrium, to the right of the septum and therefore 

 into the right auricle. Into the left auricle, the common 

 pulmonary vein, a small trunk formed by the junction of the 

 veins from the right and left lungs, opens. 



At its posterior end the atrium opens by the auriculo- 

 ventricular aperture into the ventricle. 



A small valve, prevented from flapping back by fine ten- 



