484 THE NERVOUS SYSTEM. 



get, the pneumogastric at its origin is exclusively sensitive. Irritation 

 of the nerve roots, separated from the medulla, is without effect ; but if 

 applied to the trunk of the nerve at a lower level, it excites muscular 

 contraction. At this situation the nerve contains motor fibres derived 

 from the spinal accessory, the facial, the hypoglossal, and the two upper 

 cervical nerves. It is, accordingly, a mixed nerve, and is capable of 

 providing both for movement and sensibility in the organs to which it 

 is distributed. 



Its sensibility, however, to mechanical irritation is but slightly 

 marked, as shown by the experience of all observers. It may fre- 

 quently be divided, in the unetherized animal, without causing signs 

 of pain ; and this want of reaction is at times so complete as to indi- 

 cate entire absence of ordinary sensibility. In other instances, accord- 

 ing to Bernard, it appears sensitive ; but the conditions on which this 

 difference depends are unknown. It is certain that, as a rule, the pneu- 

 mogastric is deficient in that kind of sensibility which produces pain ; 

 and the organs to which it is distributed have little or no appreciation 

 of tactile impressions. Nevertheless, it evidently possesses a sensibility 

 of peculiar kind, and of the highest importance for the vital functions. 



Connection with Respiration. The most important endowment of 

 the pneumogastric nerve is that connected with the movements of res- 

 piration. Its influence in this respect is evident from the results fol- 

 lowing its division in the neck. 



When the nerves have been simultaneously divided on both sides in 

 the dog, and the slight disturbance which immediately follows their sec- 

 tion has subsided, the most striking change produced in the animal's 

 condition is a diminished frequency of respiration. The respiratory 

 movements sometimes fall at once to ten or fifteen per minute, becom- 

 ing, in an hour or two, still more infrequent. They are performed 

 easily and regularly ; and the animal, if undisturbed, usually remains 

 quiescent, without any special sign of discomfort. By the second or 

 third day the respirations are often reduced to five, four, or even three 

 per minute ; the general condition of the animal being also exceedingly 

 sluggish. The movement of inspiration is slow, easy, and silent, occu- 

 pying several seconds in its duration ; while that of expiration is sud- 

 den and audible, and is accompanied by a well-marked effort, which has, 

 to some extent, a convulsive character. The intercostal spaces sink in- 

 ward during the lifting of the ribs ; and the whole movement of respi- 

 ration has an appearance of insufficiency, as if the lungs were not thor- 

 oughly filled with air. 



Death takes place from one to six days after the operation ; the only 

 marked symptoms during this time being steady failure of the respira- 

 tion, with increasing general sluggishness. After death the lungs are 

 found in a peculiar state of solidification ; of a dark purple color, leath- 

 ery and resisting to the touch, destitute of crepitation, and infiltrated 

 with blood. Pieces of the pulmonary tissue cut out sink in water. 

 The pleural surfaces, however, are natural in appearance, and there is 



