THE HEAD AND NECK 301 



the middle line by a thin median portion, the isthmus 

 (cf. p. 133). 



(w) The common carotid artery (a. carotis communis) passes 

 forward from the superior thoracic aperture along the side 

 of the trachea. Its branches on the neck include the 

 superior thyreoid artery (a. thyreoidea superior), to the 

 thyreoid gland, the oesophagus, the larynx, and the 

 cricothyreoid muscle of the larynx (inferior laryngeal 

 artery). The (superior) laryngeal artery rises from the 

 upper end of the common carotid or the beginning of the 

 external carotid artery near the level of the anterior edge 

 of the thyreoid plate and accompanies the superior laryngeal 

 nerve through the thyreoid foramen to the interior of the 

 larynx after sending branches to the oesophagus, the hyo- 

 thyreoid membrane, and the hyothyreoid, sternothyreoid, 

 and sternohyoid muscles. 



(n) The internal jugular vein (v. jugularis interna) lies to the 

 lateral side of the common carotid artery, traversing the 

 neck from the jugular foramen of the skull to the superior 

 thoracic aperture. 



(o) The tenth cranial or vagus nerve (n. vagus) is the largest 

 of four nerves accompanying the carotid artery. It lies to 

 the lateral side of the common carotid, between the latter 

 and the internal jugular vein. It gives off the n. laryngeus 

 superior to the larynx, this nerve crossing the dorsal side 

 of the common carotid artery. 



The superior laryngeal nerve passes through the thyreoid foramen 

 (p. 313) into the larynx and supplies sensory fibres to the mucous 

 membrane of the larynx and motor fibres to the cricothyreoid muscle. 

 The vagus is a mixed nerve, containing both afferent and efferent 

 fibres. Its action on the heart is inhibitory, that on the stomach is 

 excitatory. Section of the nerves increases heart beat. 



(p) The ramus descendens of the twelfth cranial or hypo- 

 glossal nerve (p. 309) crosses the root of the vagus from a 

 lateral to a medial position. It passes backward on the 

 ventral surface of the common carotid artery, and is dis- 

 tinguishable chiefly by its branches to the sternohyoideus 

 and related muscles. 



