THE CRANIAL NERVES 901 



of the crico-thyroid muscle. The inferior or recurrent laryngeal sup- 

 plies the rest of the laryngeal muscles, and the sensory fibres for the 

 mucous membrane of the trachea and the larynx below the glottis. 

 The superior laryngeal contains afferent fibres, stimulation of which 

 gives rise to coughing, slows respiration, or stops it in expiration. 

 Reflex movements of deglutition are also caused. The vagus supplies 

 the lungs both with motor and sensory filaments through the pulmonary 

 plexus. The motor fibres when stimulated cause constriction of the 

 bronchi; excitation of the afferent fibres causes reflex changes in the 

 rate or depth of respiration. The cardiac branches contain inhibitory 

 fibres probably derived from the spinal accessory, and depressor fibres 

 which pass up in the vagus trunk (dog), or as a separate nerve to join 

 the vagus or its superior laryngeal branch or both (rabbit). The gastric 

 and intestinal branches contain both motor and sensory nerves for the 

 stomach and intestines. The sensory are probably large medullated 

 fibres (7 /* to 9 ft). The afferent vagus fibres from the stomach carry 

 up impulses which excite the action of vomiting. Lesions of the vagus, 

 its nuclei of origin, or its branches, are associated with many interest- 

 ing forms of paralysis and other symptoms. Paralysis of the pharynx 

 is generally caused by disease of the nucleus in the medulla. From its 

 anatomical relation to the nuclei of the glosso-pharyngeal and hypo- 

 glossal, it will be easily understood that these nerves are often involved 

 in localized central les'ions along with the vagus. But the fact that in 

 progressive bulbar palsy (glosso-labio-laryngeal paralysis) a condition 

 characterized by progressive paralysis and atrophy of the muscles of 

 the tongue, lips, larynx, and pharynx the orbicularis oris and other 

 muscles of the mouth and chin are paralyzed, while the rest of the 

 muscles supplied by the facial remain intact, might seem to indicate 

 that in system diseases it is not so much anatomical groups of nerve- 

 cells which are liable to simultaneous degeneration and failure, as 

 physiological groups normally associated in particular functions. Such 

 functional groups of cells, occupied with the same kinds of labour at 

 the same times and under the same conditions, might be supposed to 

 take on a similar bias or tendency to degeneration a tendency not 

 indicated, it may be, by any structural peculiarity, but traced deep in 

 the molecular activity of the cells. There is no foundation for the 

 view that the lips are involved in progressive bulbar palsy because the 

 fibres of the facial which supply them arise from the hypoglossal 

 nucleus, any more than for the idea that the upper part of the face 

 escapes because its motor fibres, while reaching it in the seventh nerve, 

 really arise from the oculo-motor nucleus (Bruce). Difficulty in swal- 

 lowing is the chief symptom of pharyngeal paralysis. The symptoms 

 of laryngeal paralysis have been already described under ' Voice ' 

 (p. 310). Tachycardia, or a permanent increase in the rate of the 

 heart, has been stated to occur in certain cases of paralysis of the 

 vagus, caused by disease or accidental interference; and a persistent 

 slowing of the respiration has been occasionally attributed to the same 

 cause. But it is difficult to reconcile many of these cases with experi- 

 mental results, for in most of them the lesion only involved one vagus ; 

 and in animals section of one vagus has no permanent effect on the 

 rate of the heart or of the respiratory movements. 



Destruction of the nerve near its origin has been sometimes found 

 associated with disappearance of the food-appetites, hunger and thirst, 

 and it has been assumed that this was due to loss of afferent impulses 

 from the stomach. But clinical testimony is by no means unanimous 

 on this point, and experiments on animals show that other factors are 

 involved in these sensations (see Chapter XVIII.). 



