VASCULAR SYSTEM. 591 
“between stomach and duodenum, and its secretion enters 
the distal portion of the bile-duct. The bladder is a ventral 
-outgrowth of the cloaca, has no connection with the ureters, 
and seems to bé homologous with the allantois of Reptiles, 
Birds, and Mammals. , 
Vascular system.—The heart, enclosed in a pericardium, 
is three-chambered, consisting of a muscular conical ven- 
tricle, which drives the blood to the body and the lungs, of 
a thin-walled right auricle receiving impure blood from the 
body, and of a thin-walled left auricle receiving purified 
blood fromthe lungs. From each of the auricles blood 
enters the ventricle. The two superior vene cave which 
bring back blood from the anterior regions of the body, and 
the inferior: vena cava which brings back blood from the 
posterior parts, unite on the. dorsal surface of the heart in a 
thin-walled sinus venosus, which serves as a porch to the 
right auricle. From the ventricle the blood is driven up a 
truncus arteriosus, which is at first single (the py/angium) 
and then multiple (the syzangium). 
Thus we may distinguish five regions in the heart,—the ventricle, 
the right auricle, the left auricle, the sinus venosus, and the truncus 
arteriosus. The sinus venosus is the hindmost, the truncus arteriosus 
the most anterior part. The opening of the pylangium into the 
ventricle is guarded by two semilunar valves ; the cavity of the pylangium 
is incompletely divided by a longitudinal valve; there are also valves 
separating pylangium from synangium, and in the cavity of the latter. 
The complex mechanism is interesting because it determines the course 
of the blood: leaving the ventricle. The truncus arteriosus corresponds, 
in part at least, to the conus arteriosus of many fishes. 
As the heart continues to live after the frog is really dead, its contrac- 
tions can be readily observed. The sinus venosus contracts first, then 
the two auricles simultaneously, and finally the ventricle. Although 
the ventricle receives both impure and pure blood, the structural ar- - 
rangements are such that most of the impure blood jis driven to the 
lungs, the purest blood to the head, and somewhat mixed blood to 
the body. 
The blood contains in its fluid plasma—(a) the oval 
“red” corpuscles, with a definite rind, a distinct nucleus, 
and the pigment hemoglobin; (4) white corpuscles or 
leucocytes, like small amcebe in form and movements ; 
(c) very minute bodies, usually colourless and variable in 
shape. When the blood clots, the plasma becomes a 
colourless serum, traversed by coagulated fibrin filaments, 
