170 



EXPERIMENTAL PHYSIOLOGY 



hyoid, which brings into view the carotid artery o, fig. 133, accompanied 

 by a small vein or veins and in relation to several nerves. Superficially lies 

 the descendens noni. Immediately behind the artery lies the vagus, w, 

 behind and to the inner side the sympathetic, sy, and depressor nerve, d. 

 The vagus is first of all separated high up opposite the larynx, and its trans- 

 verse branch, the superior laryngeal, sl, fig. 126, is isolated. A small nerve, 

 arising either from the superior larjmgeal or by two roots, one from this and 



Fig. 133. 



-Dissection of the Nerves of 



Rabbit's Neck. 



the other from the vagus, is next sought for. This nerve is the depressor 

 nerve, d. It is isolated for about two inches of its course and a fine silk 

 thread passed under it. The ends of the thread are knotted, so that a loop is 

 formed round the nerve, by which it can be easily lifted from the wound at 

 any time. A double silk thread of another colour is then passed round the 

 vagus and its ends tied together. The vagus of the opposite side is next 

 exposed and ligatures passed round it. The sciatic nerve is then exposed by 

 an incision down the middle of the external surface of the thigh, cutting 

 through the vastus externus, and a ligature is passed round it. 



A cannula is now inserted into one of the carotids. The most convenient 

 cannula is of the form shown in fig. 134, b. It consists of a glass bulb with 

 ._, three tubes leading from it. One is of fine bore 



for insertion into the artery. It has a constricted 

 neck and its lower end is cut off obliquely and 

 has rounded margins. The tube opposite this has 

 two constrictions on it, so that rubber tubing does 

 not easily slip off. A short piece of thick-walled 

 pressure tubing is fixed to this and a screw clip 

 fitted over it, so that it can be completely closed. 

 The lateral tube is for attachment to the mercury 

 manometer. To insert the cannula a long piece 

 of the carotid is isolated and a double thread 

 passed under it. One is then tied at the upper 

 end of the isolated piece so as to stop the flow of 

 blood. On the lower end a pair of bull-dog forceps 

 is placed in such a way as to compress the vessel 

 and stop the blood-flow. "With a pair of sharp- 

 pointed scissors the artery is now cut into by an 

 incision which cuts through about one-third of 

 the circumference, and is directed obliquely down- 

 wards. By holding up the flap of artery wall thus made the cannula can 

 easily be inserted into the vessel and tied in by the second thread already 

 placed there, the thread lying in the constriction on the wall of the cannula. 



Fig. 



134. — Two Forms of 

 Cannula. 



