126 INNERVATION. [CHAP. XX. 



fever often go together, because the marsh poison which gives rise 

 to the one, may likewise excite the other. Asthma, which has oc- 

 curred once, is easily reproduced by irregularities of diet, and con- 

 sequent disturbance of digestion ; and the frequent recurrence of 

 the asthmatic paroxysm causes the dilated state of the air-passages 

 and air-cells of the lungs, the dilatation of the right cavities of the 

 heart, and the general displacement of that organ, which are inva- 

 riably present in persons who have long been subject to this disease. 

 Vomiting may be excited by irritation of the central or the peri- 

 pheral extremity of this nerve. In disease at the base of the brain, 

 vomiting is frequently an early symptom, being caused by irritation 

 of the central extremity of the vagus. In sea-sickness the cerebral 

 extremity of the nerve is irritated by the disturbance of the circu- 

 lation in the cranium. By introducing emetic substances into the 

 stomach, the vomiting is produced by the irritation of the peri- 

 pheral extremity. 



Many of the actions in which the vagus nerve is concerned are 

 of the physical kind. Of these, cesophageal deglutition is the most 

 marked. The closure of the glottis upon the application of any 

 stimulus to its mucous membrane is another example of the same 

 nature. But in both these instances and in all the movements 

 with which this nerve has to do, sensation accompanies the act. 

 In cesophageal deglutition, there is less sensation than in the other 

 movements, but it is, nevertheless, present, particularly in case of any 

 impediment being offered to the passage of the food. With reference 

 to the movements of the glottis, it is interesting to remark, that whilst 

 the will exercises a minuteness of control over them which is only 

 surpassed by the power which it has over those of the fingers, it is 

 only closure of the glottis, which is caused by a physical stimulus. 

 The obvious explanation of this is derived from the great prepon- 

 derance of the constrictor over the dilator muscles of the glottis. 



On what grounds Dr. M. Hall asserts that " the vagus nerve is 

 certainly the least sentient of any in the class vertebrata," * we are 

 at a loss to discover. We do not hesitate to affirm that every act of an 

 excitor kind, in which it is concerned, is accompanied by sensation, 

 which in some is exquisite, in others feeble. Nor can we derive, either 

 from the anatomy or physiology of this nerve, any confirmation to 

 his hypothesis of a special series of excitor and motor nerves. It is well 

 known that continued stimulation of the pharyngeal portion of the 

 glosso-pharyngeal nerve upon the fauces will produce the feeling of 



* The whole passage is " This nerve (the vagus) is certainly the least sen- 

 tient and the most purely excitor, of any in the class vertebrata." New 

 Memoir on the Nervous System, Adv. p. ix. 



