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 be 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 from the lungs. From each of the auricles blood 

 enters the ventricle. The two superior venae cavae 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 pylangium} 

 and then multiple (the synangiuni). 



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 is 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 haemoglobin ; (b) white corpuscles or 

 leucocytes, like small amoebae 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, 



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