The Frog. TJ 



The walls of the ventricle are thick and spongy ; its cavity 

 is in communication with the truncus arteriosus, from which 

 again arise the arteries. The truncus arteriosus is divisible 

 into two regions. That nearest the heart is known as the 

 pylangium ; it is guarded by three semilunar-shaped valvules 

 at its orifice into the ventricle, the effect of which is to prevent 

 the reflux of blood into the ventricle when the former contracts. 

 Along the rest of the pylangium is a free fold attached along 

 one wall, which is believed to represent a series of smaller 

 valves, such as occur in the heart of the lower fishes, fused 

 together. At the end of this, above, three semilunar valves 

 mark off from the pylangium the distal synangium. From this 

 immediately spring the two (right and left) arterial trunks. 

 These latter, although apparently single trunks, are really 

 divided internally into three vessels on each side. 



The arterial system of the frog is illustrated in the accompany- 

 ing diagram (Fig. 34). The anterior of the three vessels arising 

 from the synangium is the carotid ; it divides into two branches, 

 the external and internal carotid. At the origin of the former is 

 a little thickened portion of the arterial trunk which is known as 

 the carotid gland. The term " gland," however, is quite in- 

 accurate ; it is simply a network upon the vessel, the trunk of 

 which divides into^ a skein of vessels to reunite again. The 

 middle of the three trunks is the systemic aorta. It passes 

 round the gullet, and joins its fellow of the opposite side 

 beneath the oesophagus. At the points of junction a stout 

 branch is given off, the coeliac, which supplies the viscera of 

 the abdomen. From each half of the aorta before they join a 

 branch arises which again divides into the brachial (supplying 

 the fore limb) and a vertebral artery. The third of the three 

 arches is the pulmo-cutaneous ; it divides into two trunks, one 

 going to the lungs, the other to the skin. 



The blood is returned to the heart by a system of veins, 

 which is rather more complicated than the arterial system. It 

 is more complicated because there are two subsidiary circulations 

 introduced along the course of the vessels. The blood from 

 the head and from the fore limbs is returned to the right auricle 

 by a series of trunks which are shown in the accompanying 



