7O The Cranial Nerves 



leave by the superior laryngeal nerve. This important branch runs 

 downwards and forwards beneath the internal carotid, and then divides 

 into an external and an internal laryngeal branch ; the former supplies 

 the crico-thyroid and the inferior constrictor, whilst the internal branch 

 passes though the thyro-hyoid membrane to supply the arytenoideus 

 and the mucous membrane of the larynx. 



Irritation of the superior laryngeal nerve in weakly children sets up 

 reflex spasm of the muscles of the glottis, and is thus concerned in 

 laryngeal asthma or laryngisimts stridulus. 



The recurrent laryngreal nerve winds round the first part of the 

 subclavian artery on the right side, and the transverse aorta on the 

 left, and, ascending behind the common carotid and into the groove 

 between the trachea and oesophagus, enters the lower and back part 

 of the larynx. It gives off cardiac, cesophageal, and tracheal branches, 

 and supplies all the muscles of the larynx except the crico-thyroid. 



Pressure upon this nerve by an aortic, innominate, subclavian, or 

 carotid aneurysm causes spasmodic contraction of the laryngeal 

 muscles, and sets up a dry and suggestive cough. Sometimes the 

 nerve is ' caught ' in epithelioma of the oesophagus. 



When one recurrent laryngeal nerve is paralysed the vocal cord of 

 that side does not move ; the voice is ' uncertain/ or it may be entirely 

 lost, but the tidal air flows as usual. The abductor of the vocal 

 cord is especially affected, and in due course ' contracture ' occurs in 

 the unopposed adductor, the cord being held in a useful position for 

 phonation. Even when both nerves are paralysed there is no shortness 

 of breath, but the voice is then completely lost. 



In their course through the neck and through the thorax the vagi 

 give off cardiac branches, which join with twigs of the left recurrent 

 laryngeal to end in the aortic and cardiac plexuses. 



No. ii. The spinal accessory consists of an ' accessory ' part 

 (which arises like the vagus and eventually blends with that nerve) and 

 of a ' spinal ' part. The latter arises from the anterior grey cornu of 

 the cord by several filaments, and, emerging from the lateral tract, 

 ascends between the ligamentum denticulatum and the posterior roots 

 of the five upper cervical nerves. It enters the skull by the foramen 

 magnum, and leaves again with the vagus. It then passes downwards 

 and backwards through the sterno-mastoid, which, with the help of 

 the second and third cervical nerves, it supplies, and it ends in the 

 trapezius. These two muscles derive a considerable supply from the 

 cervical nerves. But probably their chief motor influence comes from 

 the spinal part of the spinal accessory nerve, in order that there may 

 be harmonious working between two important muscles of elevation 

 and fixation of the sternum, clavicle, and scapula, and those of vocalisa- 

 tion. 



That part of the nerve which is ' accessory ' to the vagus conveys 

 to it the motor filaments for the muscles of vocalisation : so actually 



