202 EXPERIMENTAL PHYSIOLOGY 



one side is next isolated and a thread passed under it, so that it may readily 

 be lifted up when required. The nerve is easily found as a branch of the 

 vagus running off at right angles to the trunk towards the rnid-line at the 

 level of the larynx. It usually passes beneath the carotid artery. The glosso- 

 pharyngeal of one side is separated in a similar manner. It is found by 

 tracing the vagus up to the base of the skull. It is found running from the 

 vagus deep down opposite the angle of the jaw, and there lies to the outer 

 side of the carotid. It runs forwards to disappear under the posterior edge 

 of the mylo-hyoid. 



A median longitudinal incision about 2 inches in length is next made 

 with the xiphoid cartilage as its central point. All the tissues are then cut 

 through down to the sterniun and cartilage, and any bleeding is stopped. 

 The abdominal cavity is opened at the tip of the ensiform cartilage, and the 

 two muscular strips of the diaphragm isolated on either side. The cartilage 

 is then cut across, care being taken not to injure the attachment of slips to 

 its under surface nor the blood vessels to the slips, which leave the lower 

 surface of the sternum at about its junction with the cartilage. A sharp hook 

 made of a bent pin is then passed through the edge of the cartilage, and 

 attached by a thread to one of the levers of fig. 110. The magnification of 

 the movement need not be greater than two-fold, and the loading should be 

 effected isotonically and varied until the best excursion of the lever is 

 obtained. 



The first few records of figs. 160 and 161 show the form of the trac- 

 ing. A rise of the lever is caused by a contraction of the diaphragm 

 slip and therefore represents an inspiration, a fall, expiration. It is seen 

 that relaxation of the diaphragm is carried out very rapidly, much 

 more so than the corresponding contraction. The rate of breathing is 

 rapid, at times as fast as two per second, at other times much slower. 

 If the animal be deeply under the anaesthetic the rate is usually 

 somewhat slowed. To show the influence of nervous stimuli upon 

 the rate and depth of breathing, the following nerves should be 

 stimulated : — 



1. The superior laryngeal. — We may stimulate this nerve in two 

 ways. We may imitate the normal method by introducing into the 

 larynx a curved probe whose end has been wrapped in cotton wool. 

 In this instance, and indeed for all these nerve stimulations, the rabbit 

 must not be too deeply under the anaesthetic. The result of this 

 stimulation is shown in i, fig. 160. In n, fig. 160, is seen the effect of 

 weak tetanising shocks applied directly to the nerve. In both cases 

 we see that there is slowing of the respirations, pauses occurring in 

 expiration. By contrasting these with the well-known result of the 

 presence of a foreign body in the larynx we see quite clearly that the 

 results are very different. The effect of the anaesthetic has been to 

 very greatly diminish the effect upon the respiratory centre, and in the 

 case of the electrical excitation of the nerve there seems to be no 

 doubt that the impulses thus originated are of very different character 

 from those normally passing along the nerve after it is stimulated. 



