VERTEBRATE!) ANIMALS. 291 



ing-out the course of the abdominal vessels, that the creature should have 

 been kept without food for some days, so that the intestine may empty 

 itself. This starving process reduces the quantity of red corpuscles, and 

 thus renders the blood paler; but this, although it makes the smaller 

 branches less obvious, brings the circulation in the larger trunks into 

 more distinct view. " Placing the Tadpole on his back," says Mr. Whit- 

 ney, " we look, as through a pane of glass, into the chamber of the chest. 

 Before us is the beating heart, a bulbous-looking cavity, formed of the 

 most delicate transparent tissues, through which are seen the globules of 

 the blood, perpetually, but alternately, entering by one orifice and leaving 

 it by another. The heart (Plate xxiv., fig. 3, a) appears to be slung, as 

 it were, between two arms or branches, extending right and left. From 

 these trunks (b) the main arteries arise. The heart is inclosed within an 

 envelope or pericardium (c), which is, perhaps, the most delicate,|and is, 

 certainly, the most elegant beauty in the creature's organism. Its ex- 

 treme fineness makes it often elude the eye under the single Microscope, 

 but under the Binocular its form is distinctly revealed. Then it is seen 

 as a canopy or tent, inclosing the heart, but of such 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 Tad- 

 pole, is a cavity formed of most delicate transparent tissue, traversed by 

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

 terlacing of which, with their rapid currents and dancing globules, forms 

 one of the most beautiful and dazzling exhibitions of vascularity." Of 

 the three arterial trunks which arise on each side from the truncus arte- 

 riosus, b, the first, or cephalic, e, is distributed entirely to the head, run- 

 ning 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 

 communicates 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 ab- 

 dominal viscera, is continued, as the caudal artery, Jc, 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 to- 

 wf'irds the abdominal cavity; here it approaches the kidney, m, and sends 

 off a branch which incloses 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 por- 

 tal 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, n, as it passes in close proximity to the liver, onwards to the sinus 

 venosus, 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 



