18 ELEMENTARY PHYSIOLOGY. [l. 



each side the external jugular vein arising an- 

 teriorly from two branches : avoid puncturing it. 



17. Cut through in the middle line the thin super- 

 ficial muscle (platysnia) ; draw it to one side, 

 clearing away the connective tissue. Lying on 

 either side of the muscles immediately surround- 

 ing the trachea will be seen the sterno-mastoid 

 muscle (cp. 28) diverging from the lower part 

 of the neck. Cut through the connective tissue 

 on the inner side of one sterno-mastoid and draw 

 the muscle outwards; there will be seen the 

 common carotid artery, and, running along 

 the outer side of this, the pneumogastric nerve. 

 Free in one place the carotid, and lift it up with 

 a hook. In the underlying connective tissue 

 will be seen two nerves more or less closely 

 united by tissue ; the larger is the sympathetic, 

 the smaller the superior cardiac (depressor). 



18. Clear away the connective tissue from the artery. 

 Draw the larynx from the carotid by means of a 

 hook to which is tied a string having a weight 

 at the end. Passing over the carotid at the 

 level of the larynx will be seen the descendens 

 noni, a branch of the 12th nerve. Cut this 

 through and remove it entirely. Passing under- 

 neath the carotid nearly at the same level is the 

 superior laryngeal branch of the pneumogastric. 

 Trace this with especial care; soon after it leaves 

 the pneumogastric it will be seen to give off a small 

 nerve, the depressor. Follow this down the neck, 

 separating it from the sympathetic. Sometimes 



