tions of the vessels. Since the time of De Graaf, however, various 

 other substances have been used, and amongst the chief of these may 

 be mentioned, injections of wax, tallow, varnish and size, with some 

 colouring agent diffused through them; these are injected whilst hot, 

 and when allowed to cool become so firm, that, with the exception of 

 those of size, the individual vessels may be dissected away from the 

 various tissues and organs to which they are distributed, and when 

 perfectly dried and well varnished such preparations will keep for 

 many years. From the circumstance of wax, tallow, and varnish re- 

 quiring a high temperature to keep them fluid, unless the tempera- 

 ture of the part injected be also very high, at least above the melting 

 point of the materials used, the fluid injected will not run into the 

 most minute vessels, hence none but those easily discernible by the 

 naked eye will be filled ; this has been found inconvenient, and other 

 fluids, such as size and isinglass, which melt at a very low tempera- 

 rature, and get solid on cooling, have been substituted; these will 

 fill other vessels so minute that the microscope is required to disco- 

 ver their wonderful ramifications ; and of all the important discove- 

 ries made by means of the microscope, perhaps the nature of the 

 capillaries, and the capillary circulation, are the most highly to be 

 esteemed, for it is to this instrument that we owe the proof of the 

 correctness of the doctrine taught by the immortal Harvey, that the 

 blood really circulates in the arteries and veins ; though Harvey 

 himself never witnessed the circulation, he argued, from the structure 

 and arrangement of the valves in the heart and in the veins, that the 

 blood must flow from the heart through the arteries to the veins, and 

 from the veins back to the heart again, yet he knew nothing of the 

 connecting vessels or capillaries. We are indebted for all that is 

 now known of them to the microscope, and to the art of injection. 

 It was left for Malpighi to make the discovery of the capillary circu- 

 lation, and he, we are told, first noticed it in 1G61, by the microscopic 

 examination of the distended urinary, or allantoid bladder of a frog. 

 Since then, the discovery of Malpighi has been confirmed by nume- 

 rous other observers. Anatomists have been led to divide injections 

 into the coarse and the fine, the coarse being used when the mode of 

 distribution of the large arteries and veins is required for examina- 

 tion, the fine when an intermediate system of vessels between or 

 connecting the arteries and veins, termed capillaries, is to be in- 

 vestigated ; the coarse injections are sufiicient for the unassisted 

 sight, the fine for microscopic investigation. Lieberkuhn appears to 

 have been the first to apply the microscope to injections. Many of 



