ON INJECTION. xxiii 



Successive slices cut from an animal, or part of an animal, 

 with a razor are often exceedingly instructive ; this is especially 

 the case with the mussel and snail, and with the brains of 

 the rabbit and pigeon. The specimens must be previously 

 hardened with spirit or other reagent, and the slices should 

 be examined in water or spirit. 



V. ON INJECTION. 



The injection of coloured fluids into the blood-vessels or 

 ducts of an animal renders them much easier to see, and to 

 follow to their distribution. 



The colouring matter used must not be soluble in, or 

 affected by, any of the fluids in which the specimen is after- 

 wards to be dissected, hardened, or preserved. The most 

 convenient are French blue, Prussian blue, chrome yellow, 

 vermilion, and carmine. 



In the case of the larger animals, freshly prepared plaster 

 of Paris forms, if coloured, a convenient substance for injec- 

 tion : it solidifies in the vessels, and so does not escape if a 

 vessel is cut during the dissection. 



For smaller animals, thick gum-water or white of egg 

 may be injected cold, or a jelly made of gelatin and water 

 injected warm : the animal should afterwards be put into 

 alcohol to harden the injection. If the animal is not to be 

 dissected after injection, water coloured with any of the above 

 pigments may be used with advantage, and this is particu- 

 larly useful for the alimentary and excretory systems of the 

 liver-fluke. 



For injecting small animals, a suitable syringe consists of 

 a glass tube with an india-rubber cap fitted on one end, the 

 other end being drawn out to a point sufficiently fine to 

 enter the vessel to be injected. After the tube is drawn out 

 in the flame and cut off, its sharp edges must be shgbtly 

 rounded off by holding for a moment in the flame. Several 

 such cannulas of various sizes- should be kept ready. 



For injecting larger animals, such as a rabbit, with plaster 

 of Paris, the blood must first be washed out of the vessels by 



