VASCULAR SYSTEM. 541 



Vascular System. 



The heart, enclosed in a pericardium, is three-chambered, 

 consisting of a muscular conical ventricle, 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 ventricles. 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 

 soon divides into two branches, each of which divides into 

 three aortic arches. 



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 two auricles are often included in the term 

 atrium, the undivided part of the truncus arteriosus next the ventricle is 

 called the pylangium, the more anterior part from which the arches arise 

 is known as the synangium. The truncus arteriosus corresponds, in 

 greater 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 arrange- 

 ments 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 leuco- 

 cytes, 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, the red 

 corpuscles often arrange themselves in rows, and the white 

 corpuscles are entangled in the coagulated shreds. When 

 the web of a living frog is examined under the microscope, 

 it will be seen that the flow of blood is most rapid in the 

 arteries, more sluggish in the veins, most sluggish in the 

 capillaries or fine branches which connect the arteries and 



