40 THE AMERICAN MONTHLY [February, 



them. Till the student is able to carry out this operation with that 

 perfection which comes of practice, it is better that he should limit his 

 labors to the simple and less demanding process of injecting the arterial 

 system. The apex of the heart may be cut oif with scissors, when two 

 openings in it will be noticed ; these are the ventricles. If a probe be 

 carefully inserted into the lower of the two, that nearest to the vertebral 

 column, it may be made to pass through the valves which guard the 

 entrance to the aorta or main artery of the system into the ai'ch of the 

 aorta, which bends over before it gives off its ascending and descending 

 branches. The greatest care must be exercised in performing this 

 operation in a small animal, as the tissues are delicate, and rough 

 manipulation would end in rupture, which would spoil the injection. 



Injections may be either transparent or opaque ; the latter are but 

 seldom seen now, but were formerly held in high esteem, and may oc- 

 casionally be met with in old collections. They were made by a pro- 

 cess of double decomposition of certain chemical salts, with the addi- 

 tion of size or gelatine, to give body to the injection. Mr. Thomas 

 Davies, in his work " On the preparation and mounting of microscopic 

 objects," gives the following formula for the yellow injecting fluid : 

 Acetate of lead, 380 grains ; bichromate of potash, 152 grains ; size, 8 

 ounces. The lead salt is to be dissolved in the warm size, and the 

 bichromate of potash, in powder, is added gradually with a good shak- 

 ing-up between each addition; or acetate of lead, 190 grains; chro- 

 mate of potash, 100 grains ; size, 4 ounces. The first of these pro- 

 duces the deepest color, and is the most generally used. For a white 

 injection the second salt must be varied thus: Acetate of lead, 190 

 grains; carbonate of potash, 83 grains ; size, 4 ounces. In these com- 

 binations a double decomposition takes place, and an insoluble salt is 

 thrown down, a chromate of lead being formed in the case of the yellow 

 and a carbonate of lead or " white lead " formed in the latter. These 

 injections, when made and while still warm, must be strained through 

 fine flannel to reduce the particles to such a degree of minuteness as to 

 enter the capillary system without blocking it, and causing extravasa- 

 tion. Preparations injected in this manner can only be examined under 

 the microscope, as are other opaque objects. 



By far the most interesting and instructive are transparent injections ; 

 at the same time they are more easily accomplished. Dr. L. S. Beale 

 in his book " How to Work with the Microscope," in giving directions 

 for this operation, recommends the following formula for a blue trans- 

 parent injection : Price's glycerine, 2 ounces by measure ; tincture of 

 sesquichloride of iron, 10 drops ; ferrocyanide of potassium, 3 gi-ains ; 

 strong hydrochloric acid, 3 drops ; water, one ounce. Mix the tincture 

 of iron with one ounce of the glycerine and the ferrocyanide of potas- 

 sium, first dissolved in a little water, with the other ounce. These so- 

 lutions are to be mixed together very gradually in a little bottle. The 

 iron solution must be added to the ferrocyanide of potassium. 

 Lastly, the water and the hydrochloric acid are to be added ; sometimes 

 a little alcohol, about 2 drachms, may be added to the mixture. About 

 six ounces should be made for a rat or small guinea-pig. If properly 

 made this injecting fluid should be entirely free from particles or even 

 granules, and look more like a blue stain than the result of a double de- 

 composition. The author makes it a little more acid" than does Dr. 



