738 CIRCULATION IN TADPOLE. 



extreme tenuity that its folds are really the means by which its 

 existence is recognized. Passing along the course of the great 

 vessels to the right and left of the heart, the eye is arrested by a 

 large oval body (d) of a more complicated structure and dazzling 

 appearance. This is the Internal Gill, which, in the Tadpole, is a 

 cavity formed of most delicate transparent tissue, traversed by 

 certain arteries, and lined by a crimson network of blood-vessels, 

 the interlacing of which, with their rapid currents and dancing 

 globules, forms one of the most beautiful and dazzling exhibitions 

 of vitality." Of the three great arterial trunks which arise on 

 each side from the Truncus Arteriosus, b, the first or Cephalic, e, 

 is distributed entirely to the head, running first along the upper 

 edge of the gill, and giving off a branch, /, to the thick fringed lip 

 which surrounds the mouth, after which it suddenly curves upwards 

 and backwards, so as to reach the upper surface of the head, where 

 it dips between the eye and the brain. The second main trunk, h, 

 seems to be chiefly distributed to the gill, although it freely com- 

 municates by a network of vessels both with the first or cephalic 

 and with the third or abdominal trunk. The latter also enters the 

 gill and gives off branches; but it continues its course as a large 

 trunk, bending downwards and curving towards the spine, where it 

 meets its fellow to form the Abdominal Aorta, i, which, after 

 giving-off branches to the abdominal viscera, is continued as the 

 Caudal Artery, Tc, to the extremity of the tail. The blood is 

 returned from the tail by the Caudal Vein, I, which is gradually 

 increased in size by its successive tributaries as it passes towards 

 the abdominal cavity ; here it approaches the Kidney, m, and sends 

 off a branch which encloses that organ on one side, while the main 

 trunk continues its course on the other, receiving tributaries from 

 the kidney as it passes. (This supply of the kidney by venous 

 blood is a peculiarity of the lower Vertebrata). The venous blood 

 returned from the abdominal viscera, on the other hand, is collected 

 into a trunk, p, known as the Portal Vein, which distributes it 

 through the substance of the Liver, o, as in Man; and after 

 traversing that organ it is discharged by numerous fine channels, 

 which converge towards the great abdominal trunk, or Vena Cava,??, 

 as it passes in close proximity to the liver, onwards to the Sinus 

 Venos'us, q, or rudimentary auricle of the heart. This also receives 

 the Jugular Vein, r, from the head, which first, however, passes 

 downwards in front of the gill close to its inner edge, and meets a 

 vein, t, coming up from the abdomen, after which it turns abruptly 

 in the direction of the heart. Two other Abdominal veins, u, meet 

 and pour their blood direct into the Sinus Venosus ; and into this 

 cavity also is poured the aerated blood returned from the gill by the 

 Branchial Vein, v, of which only the one on the right side can be 

 distinguished. — The Lungs may be detected in a rudimentary state, 

 even in the very young Tadpole ; being in that stage a pair of 

 minute tubular sacs, united at their upper extremities, and lying 



