204 A MANUAL OF PHYSIOLOGY 



and repeat the observations. The ' circulation-time ' will be found 

 to be diminished. This corresponds to an increase of blood-pressure 

 due to increased activity of the heart, without change in the calibre 

 of the bloodvessels. Next, leaving the bottle in its present position, 

 diminish the outflow by tightening the clamp ; the circulation-time 

 will be increased. This corresponds to an increase of blood-pressure 

 due to diminution in the calibre of the small arteries. 



(b) Fill the syringe* with methylene-blue solution (0*2 per cent, in 

 0-9 per cent, salt solution), as in (a). Keep the solution warmed to 

 40 C. by immersing the small beaker containing it in a water-bath, 

 or heating it over a bunsen with a small flame. Weigh a rabbit or 

 cat. In the case of the rabbit, inject \ grm. chloral hydrate into 

 the rectum, and later on give ether if necessary. If a cat, give 

 ether alone. Fasten it on a holder, back downwards (Fig. 51, 



LI24). Cover it with a towel to keep it warm. Clip off the 

 ir on the front of the neck, and make an incision i^ inches 

 long in the middle line, beginning a little 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 0^9 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 

 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 



* A burette, sloped so as to make a small angle with the horizontal, 

 may be substituted for the syringe. The burette is supported on a stand 

 at such a height that the methylene-blue solution runs without great force 

 into the jugular (say 10-15 cm - above the level of the cannula). The 

 danger of producing an abnormal result by suddenly raising the pressure 

 in the right side of the heart is thus avoided. 



