20 ELEMENTARY PHYSIOLOGY. [l. 



23. Trace out the arch of the aorta by clearing away 

 the tissue from its upper surface. Take care 

 not to injure the pneumogastric nerves (see 

 next section). Observe on the right side the 

 innominate artery, which gives off first the 

 left common carotid, and then divides into 

 the right subclavian and right common 

 carotid ; on the left side the left subclavian. 

 Note the vertebral artery on either side pro- 

 ceeding from the subclavian. On a level with 

 the anterior part of the larynx, note the division 

 of the common carotid into external carotid 

 and internal carotid. The former curls round 

 the angle of the jaw, the latter enters the skull 

 a little in front and to the median side of the 

 tympanic bulla. 



24. Trace both pneumogastric nerves downwards, 

 observing the recurrent laryngeal branches 

 passing on the right side round the subclavian 

 artery, and on the left round the aorta. Place 

 a double ligature round the innominate artery 

 and divide between the ligatures. Trace the 

 recurrent laryngeal nerves along the back of the 

 trachea to the larynx. Pursue the main pneu- 

 mogastric trunks on the oesophagus to the point 

 where they were seen in A, § 10. 



25. Trace the sympathetic nerve dovmwards to the 

 inferior cervical ganglion lying a little above 

 the subclavian artery, and close to the vertebral 

 artery; follow it thence to the first thoracic 

 ganglion. 



