CIRCULATION IN TADPOLE. INJECTIONS. 739 



behind the intestine and close to the spine. They may be best 

 brought into view by immersing the Tadpole for a few days in a 

 weak solution of Chromic acid, which renders the tissues friable, 

 so that the parts that conceal them may be more readily peeled 

 away. Their gradual enlargement may be traced during the period 

 of the Tadpole's transparency ; but they can only be brought into 

 view by dissection, when the metamorphosis has been completed. 

 The following are Mr. "Whitney's directions for displaying the 

 Circulation in these organs: — "Put the young Frog into a wine- 

 glass, and drop on him a single drop of Chloroform. This suffices 

 to extinguish sensibility. Then lay him on the back on a piece of 

 cork, and fix him with small pins passed through the web of each 

 foot. Remove the skin of the abdomen with a fine pair of sharp 

 scissors and forceps. Turn aside the intestines from the left side, 

 and thus expose the left lung, which may now be seen as a glisten- 

 ing transparent sac, containing air-bubbles. With a fine camel- 

 hair pencil the lung may now be turned-out, so as to enable the 

 operator to see a large part of it by transmitted light. Unpin the 

 Frog, and place him on a slip of glass, and then transmit the light 

 through the everted portion of lung. Remember that the lung is 

 very elastic, and is emptied and collapsed by very slight pressure. 

 Therefore, to succeed with this experiment, the lung should be 

 touched as little as possible, and in the lightest manner, with the 

 brush. If the Heart is acting feebly, you will see simply a trans- 

 parent sac, shaped according to the quantity of air-bubbles it may 

 happen to contain, but void of red vascularity and circulation. 

 But should the operator succeed in getting the Lung well placed, 

 full of air, and have the Heart still beating vigorously, he will see 

 before him a brilliant picture of crimson network, alive with the 

 dance and dazzle of blood-globules, in rapid chase of one another 

 through the delicate and living lace-work which lines the chamber 

 of the lung." The position of the Lungs in relation to the 

 Heart and the great Vascular trunks, is shown in Plate xxiv., 

 fig. 6. 



577. 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 colour- 

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

 interesting subjects of Microscopic examination. The art of 

 making successful preparations of this kind is one in which perfec- 

 tion 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 business 

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

 themselves. For this reason, no more than a general account of the 

 process will be here offered ; the minute details which need to be 

 attended-to, in order to attain successful results, being readily 



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