322 RESPIRATION. 



gists are "inhibitory" i.e., tend to increase the resistance 

 above referred to. In the fresh state of the nerve, the influ- 

 ence of these fibres is completely overbalanced by that of the 

 others. In the exhausted state, this relation is reversed, so 

 that the two sets of afferent fibres are as much distinguished 

 from each other by their difference of endurance as by their 

 differences of function. Recent experiments (Burkhart) make 

 it probable that the " inhibitory" fibres .come mostly from the 

 recurrents. 



104. Excitation of the Superior Laryngeal Nerve. 

 The experimental investigation of the superior laryngeal is 

 much more difficult than that of the trunk of the vagus, partly 

 because the nerve is difficult to reach and runs a short course, 

 partly because it is very slender. To expose it in the rabbit, 

 an incision should be made extending from the side of the tra- 

 chea, at the level of its first and second rings, to the hollow 

 between the angle of the jaw and the larynx. After severing 

 the skin in the usual way, the fascia which extends forwards 

 from the edge of the sterno-mastoid muscle must be carefully 

 broken through with the aid of two pairs of dissecting forceps, 

 so as to expose the parts seen in Fig. 22T. The space is di- 

 vided into two by the artery, the direction of which coincides 

 exactly with that of the original incision. lS T ear its lower end 

 the artery gives off its thyroid branch. At the top the space 

 is limited by the tendon of the stylohyoid muscle, and the pos- 

 terior cornu of the hyoid bone. Immediately below the muscle 

 is the trunk of the ninth nerve, which arches forwards towards 

 the tongue. The descending branch of that nerve passes down- 

 wards and forwards to reach the muscles which cover the front 

 of the trachea, giving communicating branches to the cervical 

 plexus, and a branch which arches forwards over the artery to 

 gain the muscles which draw the larynx upwards. Before pro- 

 ceeding to expose the deeper nerves, it is well, in order to avoid 

 confusion, to remove the descendens noni; the next step is to 

 draw the larynx well to the side opposite to that chosen for 

 the incision, so as to widen the space between it and the caro- 

 tid artery. This done, the exposure oi the superior laryngeal 

 becomes easy. Its exact position is indicated in the figure ; its 

 course is much twisted, so as to allow of the up-and-down 

 movements of the larynx. In preparing it, no cutting instru- 

 ments must be used. It must be freed from the surrounding 

 structures with the aid of two pairs of forceps, any veins in 

 the way having been divided between two ligatures. Care 

 must be taken, however, to leave a certain quantitj^ of cellular 

 tissue about it to serve as a kind of protective sheath, and 

 make it somewhat less liable to get dry. The nerve having 

 been prepared, a ligature must be tied round it as near as pos- 

 sible to the thyrohyoid membrane, after which it must be di- 



