I.] THE FROG. 17 



at opposite ends of the body, through the agency of two 

 pairs of contractile muscular sacs {lymph- hearts), which pump 

 the lymph contained in the wide lyniphatic vessels and in 

 the pleuroperitoneal cavity into the great veins in their 

 neighhourhood. 



The blood and lymph systems may thus be regarded as 

 subdivisions of a common circulatory system. 



The heart is connected with the walls of the pericardium, 

 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 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 median, ventral and posterior; and the truncus 

 passes obliquely forwards from the right side of the 

 ventricle and is ventral and anterior. The heart there- 

 fore may be compared to a tube divided by constriction 

 into four portions and bent somewhat into the shape of 

 anS. 



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 



M. 2 



