CIRCULATION IN TADPOLE AND FISH. 619 



examining the circulation of its lungs or mesentery. The tadpole 

 of the Frog, when sufficiently young, furnishes a good display of 

 the circulation in its tail ; and the difficulty of keeping it quiet 

 during the observation may be overcome, by gradually mixing 

 some hot water with that in which it is swimming, until it be- 

 comes motionless ; this usually happens when it has been raised 

 to a temperature between 100 and 110 ; and notwithstanding 

 that the muscles of the body are throw T n into a state of spasmodic 

 rigidity by this treatment, the heart continues to pulsate, and the 

 circulation is maintained. The larva of the Water-Newt, when it 

 can be obtained, furnishes a most beautiful display of the circu- 

 lation, both in its external gills, and in its delicate feet. It may 

 be enclosed in a large aquatic box or in a shallow cell, gentle 

 pressure being made upon its body, so as to impede its move- 

 ments, without stopping the heart's action. The circulation may 

 also be seen in the tails of small fish, such as the Minnow or 

 Stickle-back, by confining these animals in tubes, or in shallow 

 cells, or in a large aquatic box ; but although the extreme trans- 

 parency of these parts adapts them well for this purpose in one 

 respect, yet the comparative scantiness of their bloodvessels pre- 

 vents them from being as suitable as the Frog's web in another 

 not less important particular. One of the most beautiful of all 

 displays of the circulation, however, is that which may be seen 

 upon the yolk-bag of young Fish (such as the trout) soon after 

 they have been hatched ; and as it is their habit to remain almost 

 entirely motionless at this stage of their existence, the observa- 

 tion can be made with the greatest facility by means of the zoo- 

 phyte-trough, provided that the subject of it can be obtained. 

 Now that the artificial breeding of these fish is largely practised 

 for the sake of stocking fish-ponds, there can seldom be much 

 difficulty in procuring specimens at the proper period. The store 

 of yolk which the yolk-bag supplies for the nutrition of the em- 

 bryo, not being exhausted in the Fish (as it is in the bird) pre- 

 viously to the hatching of the egg, this bag hangs down from the 

 belly of the little creature on its emersion ; and continues to do 

 so until its contents have been absorbed into the body, which 

 does not happen for some little time afterwards. And the blood 

 is distributed over it in copious streams, partly that it may draw 

 into itself fresh nutritive material, and partly that it may be sub- 

 jected to the aerating influence of the surrounding water. 



433. Injected Preparations. Next to the circulation of the blood 

 in the living body, the varied distribution of the Capillaries in its* 

 several organs, as shown by means of "injections" of coloring 

 matter thrown into their principal vessels, is one of the most in- 

 teresting subjects of microscopic examination. The art of making 

 successful preparations of this kind, is one in which perfection 

 can usually be attained only by long practice, and by attention 

 to a great number of minute particulars; and better specimens 

 may be obtained, therefore, from those who have made it a busi- 

 ness to prepare them, than are likely to be prepared by amateurs 



