PRACTICAL EXERCISES 217 



way below the cricoid cartilage. Reflect the skin and isolate the 

 external jugular vein, which is quite superficial. Carefully separate 

 about f- inch of the vein from the surrounding tissue, and pass two 

 ligatures under it, but do not tie them. Compress the vein with a pair 

 of bulldog forceps between the heart and the ligatures. Now tie the 

 uppermost of the two ligatures (that next the head), but only put a 

 single loose loop on the other. The piece of vein between the upper 

 ligature and the bulldog is now distended 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 o'g per cent, salt solution by means of a pipette 

 with a long point. Expose the carotid on the other side, isolate it for 

 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 connecting-tube, as a rabbit not unfrequently dies 

 instantaneously when a bubble of air is injected into the right heart, 

 although a considerable quantity of air can generally be injected into 

 the jugular of a dog without killing it. 



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

 tions 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 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 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 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. 62). If 

 a large number of injections have been made in quick succession, the 

 specific gravity will be less than normal ; but if a considerable interval 

 has been allowed to elapse after the last injection, little or no differ- 

 ence may be found, as the surplus liquid readily passes out of the 

 bloodvessels. 



Necropsy. 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 elimin- 

 ated. 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 



