542 AMPHIBIA. 



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 corre- 

 sponds, 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 

 arrangements 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 

 the veins. The red corpuscles are swept along most rapidly, 

 and are often deformed by pressure ; the leucocytes tend to 

 cling to the walls of the capillaries, and may indeed pass 

 through them (diapedesis). 



The arterial system. — Each branch of the truncus 

 arteriosus is triple, and divides into the following on each 

 side : — 



I. The carotid arch, the most anterior, corresponding 

 to the first efferent branchial of the tadpole, gives off — • 



A lingual artery to the tongue ; 



A carotid artery, which bears near the origin of the 

 lingual a spongy swelling (the " carotid gland "), and 

 gives off an external carotid to the mouth and the 

 orbit, and an internal carotid to the brain. 



II. The systemic arch, the median one of the three, 



