ioo THE NERVOUS SYSTEM 



Within the thorax, the right vagus descends on the right 

 side of the trachea and then passes behind the root of the 

 right lung. In this part of its course, it is exposed to 

 pressure from mediastinal tumours or enlarged mediastinal 

 lymph glands. The left vagus crosses the left or anterior 

 aspect of the arch of the aorta and then passes behind the 

 root of the left lung. Both vagi assist in the formation of 

 the pulmonary plexuses, in which they unite with branches 

 from the sympathetic. The pulmonary branches of the vagi 

 are said to supply the circular muscular coats of the bronchi, 

 and they are believed by some authorities to be at fault in 

 the condition of spasmodic asthma. 



On leaving the roots of the lungs, the two vagi pass 

 downwards on the oesophagus and form the oesophageal 

 plexus. They leave the thorax in company with the oeso- 

 phagus and enter the abdomen, where their terminal branches 

 are distributed to the stomach and, probably, also to the 

 liver and the small intestine. 



Summary. The vagus nerve, therefore, is responsible for 

 the motor supply of (i) The soft palate; (2) the pharyngeal 

 muscles; (3) the laryngeal muscles; (4) the heart inhibitory: 

 (5) the oesophagus, stomach, etc. ; and (6) the bronchial 

 muscles (?). It conveys afferent impulses from (i) The 

 stomach and oesophagus; (2) the heart; (3) the larynx, 

 bronchi and lungs ; and (4) the external acoustic meatus 

 (p. 199). 



All these structures may be affected in lesions of the nerve 

 or its nuclei (p. 94) ; and any one of them may be affected 

 reflexly in pathological conditions of one of the others 

 (cf. pp. 190 and 195). 



SUPRA-NUCLEAR LESIONS produce no noticeable effects, 

 unless they are bilateral. Any paresis that may exist is 

 completely masked, owing to bilateral representation of the 

 muscles in the cerebral cortex (p. 34). 



NUCLEAR LESIONS are of fairly common occurrence. They 

 are usually bilateral and constitute a part of a slowly progressive 



