292 THE MICROSCOPE AND ITS REVELATIONS. 



from the abdomen, after which it turns abruptly in the direction of the 

 heart. Two other abdominal veins, w, meet and pour their blood direct 

 into the sinus venosus; and into this cavity is also 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 the upper extremities, and 

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

 brought into view by immersing the tadpole for a few days Jn a weak so- 

 lution of chromic acid, which renders the tissue friable, so that the parts 

 that conceal them may be more readily peeled away. Their gradual en- 

 largement may be traced during the period of the tadpole's transparence; 

 but they can only be brought into view by dissection when the metamor- 

 phosis has been completed. The following are Mr. Whitney's directions 

 for displaying the Circulation in these organs: ''Put the young Frog 

 into a wineglass, 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. 

 Eemove 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 glistening 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 transparent 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 beat- 

 ing vigorously, he will see before him a brilliant picture of crimson net- 

 work, 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 xxiy., fig. 6. 



687. Injected Preparations. Next to the Circulation of the Blood in 

 the living body, the varied distribution of the Capi.lic.ries in its several 

 organs, as shown by means of ' injections ' of coloring matter thrown into 

 their principal vessels, is one of the most interesting subjects of Micro- 

 scopic 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 business to produce 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 accessible elsewhere 

 to such as desire to put it in practice. l Injections may be either opaque 



1 See especially the article 'Injection,' in the " Micrographic Dictionary;" M. 

 Robin's work, "Du Microscope et des Injections;" Prof. H. Frey's Treatise "Das 

 Mikroscop und die Mikroskopische Technik:" Dr. Beale's "How to Work with the 



