192 A MANUAL OF PHYSIOLOGY 



the trachea and another cannula in the central end of the carotid 

 artery. Isolate the depressor nerve. Put double silk ligatures on it, 

 and divide between them. Connect the cannula in the carotid with 

 the manometer and take a blood-pressure tracing. Stimulate the 

 central (upper) end of the depressor. A marked fall of blood- 

 pressure, accompanied with a slowing of the heart, will be obtained 

 (Fig. 55). Stimulate the peripheral (lower) end ; no effect. Divide 

 both vagi, and again stimulate the central end of the nerve. The 

 blood-pressure again falls, but there is no alteration in the rate of the 

 heart (p. 160). Close the tracheal cannula, and obtain another 

 tracing, showing the effect of asphyxia (Fig. 56, p. 163). 



Autopsy. Dissect the nerve that has been stimulated, up to the 

 origin of the superior laryngeal branch of the vagus, to make sure 

 that it is the depressor (Fig. 54, p. 161). 



26. Determination of the Circulation- time. (a) Begin with an 

 artificial scheme (Fig. 75). Fill the syringe with a 0*2 per cent, 

 solution of methylene blue. Allow the water to flow from the bottle 

 by loosening the clamp. Inject a definite quantity of the methylene- 

 blue solution, and with a stop-watch observe how long it takes to 

 pass from the point of injection to the end of the glass tube filled 

 with beads. Make ten readings of this kind and take the mean. 

 Then raise the bottle so as to increase the rate of flow of the water, 

 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 

 normal saline), 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, and inject 

 j gramme chloral into the rectum. Fasten it on a holder, back 

 downwards (Fig. 43, p. 125). Cover it with a towel to keep it warm. 

 Clip off the hair on the front of the neck, give ether if necessary, 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 



* A burette, sloped so as 10 .make a small angle with tae 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 cannulu). The 

 danger of producing an abnormal result by suddenly raising the pressure 

 in the right side of the heart is thus avoided. 



