GS4 THE MICROSCOPE. 



bonized blood which has already circulated through the 

 body, and flows into the ventricle from the right auricle, 

 with the pure aerated blood returned from the lungs, and 

 which also flows at the same instant into the ventricle 

 from the left auricle. Thus the habitual circulation of 

 this "cold-blooded" mixture is the cause of the low tone 

 of vitality that distinguishes reptiles. 



For the purpose of observation the tadpole must, of 

 course, be selected during the period in which the skin 

 is perfectly transparent. The first examinations reveal 

 plainly enough the appearances already described of the 

 form and situation of the heart, and the three great arte- 

 rial trunks proceeding (right and left) from it. Many ob- 

 servations are required to arrive at the true anatomical 

 arrangement of these vessels. First, they are closely 

 connected with the corresponding gill. The upper one 

 (the cephalic) runs along the upper edge of the gill, and 

 gives off, in its course, a branch which ascends to the 

 mouth, which, with its accompanying vein, is termed the 

 labial artery and vein. The cephalic artery continues its 

 course around the gill, until it suddenly curves upwards 

 and backwards, and reaches the upper surface of the head, 

 where it dips between the eye and the brain, towards 

 which it is evidently travelling. 



It would be a mistake to suppose that you can make 

 this out distinctly, in the average of tadpoles taken, 

 without some preparation. The great obstacle is the large 

 coil of intestines, usually distended with dark-coloured 

 food. This must be got rid of by making your tadpoles 

 live on plain water for some days. Plate VII. No. 152, 

 exhibits the view of the vessels obtained under the in- 

 fluence of low diet, and we are now enabled to trace the 

 course of the three large arteries. The third trunk, 

 traversing the lung, is seen to emerge from the lower 

 edge and descend into the abdomen to form the great 

 abdominal aorta. A small black-looking starved little 

 tadpole shows the heart beating and the blood circulating, 

 but the latter is quite colourless, not a single red globule 

 visible anywhere. The globules chase one another away 

 like globules of water, the heart is* a colourless globe, the 

 gills two transparent ovals, and the bowels a colourless, 



