XXI] CIRCULATORY ORGANS 535 



seizes with its jaws. Both upper and lower jaws are armed with 

 minute teeth, and there are in addition two longitudinal rows of 

 teeth on the roof of the mouth borne by the conjoined vomer and 

 palatine on each side. The function of these teeth is not so much 

 to crush as to retain a hold of the prey, which is swallowed whole. 

 The tongue is a circular cushion on the floor of the mouth, 

 supported by the second visceral arch. Its hinder edge is partially 

 free. The lungs are long, smooth-walled, tube-like elastic sacs, 

 attached to the liver and other organs at their base, but their tips 

 float freely in the body-cavity. 



The heart lies far forward, between the roots of the lungs, 

 enclosed in the pericardium. Externally all the four 

 divisions of the piscine heart are visible, viz., sinus 

 venosus, atrium, ventricle, conus. The venous system 

 is essentially that of the Dog-fish, only the veins are indicated by 

 names borrowed from human anatomy. Thus the blood from the 

 head is returned by two internal jugular veins, representing the 

 anterior cardinals of the fish. These are joined by external 

 jugulars from the superficial part of the throat and face and by 

 a subclavian vein from each arm. The common trunk formed 

 by the union of all three is, of course, the ductus Cuvieri, but it is 

 called the superior vena cava, and it receives on each side close 

 to the middle line a posterior cardinal vein. As in fishes, this 

 vein in its course breaks up into capillaries through the kidney; and 

 along the outer edge of the kidney, its posterior portion, the renal 

 portal, may be made out. The two renal portals when followed 

 further back are found to coalesce in the caudal vein which 

 returns the blood from the tail: each receives a sciatic vein from 

 the dorsal side of the leg joined by a femoral from the ventral 

 surface of the limb. 



The increased importance of the hind-limb has brought with it 

 this increase in the vessels draining it, which are represented only 

 by the small pelvic vein in fishes. 



There are certain vessels, however, unrepresented in any fishes 

 except the Dipnoi. These are: first, the pulmonary veins, 

 which receive the blood from the lungs and open directly into the 

 left side of the atrium, which is separated from the rest by a septum 

 and constitutes the left auricle; secondly, the inferior vena 

 cava, a large trunk situated in the median dorsal line just beneath 

 the aorta, which receives most of the blood that has traversed the 

 kidneys and conveys it into the sinus venosus just between the 



