PRACTICAL EXERCISES 193 



of vein between the upper ligature and the bulldog is now dis- 

 tended with blood. With fine-pointed scissors make a small slit in 

 the vein, taking great care not to divide it completely, insert the 

 cannula, and tie the loose ligature firmly over its neck. Fill the 

 cannula and the small piece of rubber tubing attached to it with 

 normal saline by means of a pipette with a long point. Expose the 

 carotid on the other side, isolate it for f inch, clear it carefully from 

 its sheath, slip under it a strip of thin sheet indiarubber, and between 

 this and the artery a little piece of white glazed paper. Connect 

 the cannula in the jugular with the T-piece attached to the syringe. 

 Care must be taken that no air remains in the cannula or its con- 

 necting-tube, as an animal not unfrequently dies instantaneously when 

 a bubble of air is injected into the right heart. 



Now take off the bulldog from the vein, and make a series of 

 observations on the pulmonary circulation-time. The animal must 

 be so placed that a good light falls on the carotid. If necessary, the 

 light of a gas-flame may be concentrated on it by a lens. The 

 student holds the stop-watch in one hand, and injects a measured 

 quantity of the methylene-blue solution with the other. Uniformity 

 in the quantity injected is secured by fastening on the piston of the 

 syringe a screw-clamp, which stops the piston at the desired point. 

 The observation consists in setting off the watch at the moment when 

 injection begins and stopping it when the blue appears in the carotid. 

 After each injection the screw-clamp or pinchcock on the tube con- 

 nected 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 circulation-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. 

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

 and end of the experiment by Hammerschlag's method (p. 57). 



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. Notice 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 blue is rapidly reduced by living tissues. 



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