xi PRACTICAL DIRECTIONS 551 



2. a, The pulmonary artery and its division into left and 

 right trunks/ b, the pulmonary veins (the course of which will 

 be better seen at a later stage) ; c, the two precaval veins, 

 each formed by the union of a subdavian, an external jugular, 

 and a smaller internal jugular ; d t the thoracic portion of the 

 postcaval vein. 



3. a, The 7i//<?2 (cervical and thoracic portions) ; b, the 

 phrenic nerves (p. 532) and the thoracic portions of the vagi 

 (Fig. 135) ; c, the arcA o/ the aorta, continuous with the 

 dorsal aorta, and the short ligament (vestige of the foetal 

 ductus arteriosus) connecting the former with the pulmonary 

 artery ; d, the thoracic portions of the sympathetic nerves 

 and their ganglia, lying on the heads of the ribs ; e t the 

 azygos vein (p. 525), best seen by turning the heart and 

 lungs over to the left side of the animal. -(The thoracic 

 duct (p. 528) cannot easily be made out at this stage.) . 



4. a, The innominate artery, giving off the left and right 

 common carotids^ and the right subdavian; b, the left sub- 

 davian artery ; "c, the division of each common carotid, at 

 about the level of the anterior and of the larynx, into an 

 external and an internal carotid. 



Sketch the heart and origins of the main vessels. 



III. Now dissect out certain of the nerves and other 

 structures in the neck, as follows : 



1. The vagus (Fig. 135), running to the outer side of each 

 common carotid : with the seeker carefully separate it from 

 the carotid, and trace it forwards to the angle between the 

 head and neck at which point it enlarges to form a ganglion 

 and backwards to the thorax. Branches are given off to 

 the larynx (anterior and posterior or recurrent laryngeal 

 nerves] and to the heart (depressor) nerves, but the dissection 

 of these may be omitted by the beginner. 



2. The sympathetic (Fig. 135), also running alongside 

 the carotid artery. It is most easily distinguished by seizing 

 the carotid with the small forceps just at the junction of the 

 head and neck and putting it slightly on the stretch : the 

 vagus ganglion will then be seen on the outer side of the 

 artery, and the more elongated anterior cervical ganglion of 

 the sympathetic on its inner side : trace the sympathetic 

 nerve from this ganglion backwards to the thorax, at the 

 anterior end of which (close to the subdavian artery) it 

 enlarges to form the posterior cervical ganglion, and then 

 becomes continuous with the thoracic portion of the cord. 



3. The hypoglossal nerve will be seen at about the level of 

 the vagus ganglion, passing forwards to the tongue. 



