26 HISTOLOGICS TECHNIC 



acetic acid solution be then added as directed above, the precipitated 

 granules are so fine, even with an acid reaction, that they readily pass 

 through the capillaries. The precipitation of the carmine in the shape 

 of coarser granules is of advantage when it is desired to have an in- 

 jection mass which will fill the arteries or veins only, without passing 

 over' into the capillaries. 



The injecting apparatus consists of a vessel which contains the 

 injection mass, and some means of keeping the latter under a con- 

 stant but easily varied pressure. With the vessel is connected a tube 

 ending in a cannula, through which the injection is made. 



A very simple apparatus consists of a shelf which can be raised 

 and lowered, and upon which the vessel stands. The tube connecting 

 with the cannula may be attached to a faucet in the vessel or to 

 a bent glass tube which passes into the top of the vessel and acts on 

 the principle of a siphon. 



In a somewhat more elaborate apparatus the injection mass is 

 placed in a closed vessel, and this is connected with a second vessel 

 containing air compressed by means of an air pump. 



Accurate regulation of the pressure may be obtained by connect- 

 ing the injection vessel with a manometer. 



If the injection is to occupy considerable time, a hot-water bath 

 in which the gelatin may be kept at an even temperature is also 

 necessary. 



Whole animals or separate organs may be injected. For injecting 

 a whole animal, the animal, which is usually a small one such as a 

 guinea-pig, rat, mouse, or frog, is chloroformed, the tip of the heart 

 is cut away and a cannula is inserted through the heart into the aorta. 

 This is first connected with a tube leading to a bottle containing 

 warm normal saline solution. Pressure is obtained in the same 

 manner as above described for the injection mass. By this means 

 the entire arterial and venous systems are thoroughly washed out 

 until the return flow from the vena cava is perfectly clear. The 

 cannula is next connected with the tube from the vessel containing 

 the injection mass, the pressure being only sufiicient to keep the 

 liquid flowing. When the injection mass flows easily and freely from 

 the vena cava, the vessel is tied and the pressure is increased slightly 

 and continued until the color of the injection mass shows clearly 

 in the superficial capillaries. The aorta is now tied and the animal 

 immersed in cold water to solidify the gelatin. After the gelatin 

 becomes hard; the desired organs are removed and fixed and hardened 



