PRACTICAL EXERCISES 205 



tube connected with the cannula must be tightened, the other 

 opened, and the syringe refilled. Great care must be taken never to 

 open the two clamps at the same time, as in that case blood may 

 regurgitate through the jugular and fill the syringe, or methylene 

 blue may be sucked into the circulation. As many observations as 

 possible should be taken, and the mean determined. The circula- 

 tion-time observed is approximately that of the lesser circulation, 

 the time taken by the blood to pass from the left ventricle to the 

 carotid being negligible for the purposes of the student. 



The specific gravity of the blood may also be tested at the 

 beginning and end of the experiment by Hammerschlag's method 

 (p. 54). If a large number of injections have been made in quick 

 succession, the specific gravity will be Isss than normal ; but if a 

 considerable interval has been allowed to elapse after the last 

 injection, little or no difference may be found, as the surplus liquid 

 readily passes out of the bloodvessels. 



Autopsy. Observe particularly the state of the lungs, whether the 

 bladder is distended or not, and whether any of the serous cavities 

 or the intestines contain much liquid ; so as to determine, if possible, 

 by what channel the water injected into the blood may have been 

 eliminated. Study the distribution of the methylene blue in such 

 organs as the kidneys and the muscles immediately after death, and 

 notice that the blue colour becomes more pronounced after exposure 

 for a time to the air. Make a longitudinal section through a kidney, 

 and observe that the pigment is found especially in the cortex and 

 around the pelvis at the apices of the pyramids, or it may be only in 

 the cortex. The urine is greenish. If some methylene blue has been 

 injected after the heart ceased to beat, the bloodvessels, particularly 

 in the mesentery, may be beautifully mapped out by the pigment. 

 This is not the case if the last injection took place before death, 

 since the methylene blue is rapidly reduced by living tissues to a 

 colourless substance, leuco-methylene blue. 



