XIII.] THE FROG. 167 



sub-vertebral lyraph-sinus, which lies between the layers of the 

 root of the mesentery and communicates by small pores with 

 the pleuroperitoneal cavity. There are four lymph-hearts. 



The blood consists of a colourless plasma which contains 

 colourless corpuscles, similar to those of the lymph, and in 

 addition a great number of oval nucleated red corpuscles. 

 It is contained in the blood-vessels, which consist of capilla- 

 ries, arteries and veins, the two latter being connected on 

 the one side by the capillaries and, on the other, by the heart 

 into which they open. The lymphatics and the blood-vessels 

 are brought into connexion with one another by communica- 

 tions between the anterior lymph-hearts and 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, (1) the sinus venosus, (2) the atrium, (3) the ventricle, 

 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 therefore may be compared to a tube divided by con- 

 striction 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 single, 

 vena cava inferior opens into it. It opens by a valvular 



