470 THE CRANIAL NERVES. 



corresponding with the different degrees of alteration exhibited by 

 different parts. 



The explanation usually adopted of the above consequences fol- 

 lowing division of the pneumogastrics is as follows : The nerves 

 being divided, the impression which originates in the lungs from 

 the accumulation of carbonic acid, and which is destined to excite 

 the respiratory movements by reflex action, can no longer be trans- 

 mitted to the medulla oblongata. The natural stimulus to respira- 

 tion being wanting, it is, accordingly, less perfectly performed. The 

 respiratory movements diminish in frequency, and, growing con- 

 tinually slower and slower, finally cease altogether, and death is 

 the result. 



The above explanation, however, is not altogether sufficient. It 

 accounts very well for the diminished frequency of respiration, but 

 not for its partial continuance. For if the pneumogastric nerves 

 be really the channel through which the stimulus to respiration is 

 conveyed to the medulla, the difficulty is not to understand why 

 respiration should be retarded after division of these nerves, but 

 why it should continue at all. In point of fact, the respiratory 

 movements, though diminished in frequency, continue often for 

 some days after this operation. This cannot be owing to force of 

 habit, or to any remains of nervous influence, as has been some- 

 times suggested, since, when the medulla itself is destroyed, respira- 

 tion, as we know, stops instantaneously, and no attempt at move- 

 ment is made after the action of the nervous centre is suspended. 



It is evident, therefore, that the pneumogastric nerve, though the 

 chief agent by which the respiratory stimulus is conveyed to the 

 medulla, is not the only one. The lungs are undoubtedly the 

 organs which are most sensitive to an accumulation of carbonic 

 acid, and an imperfect arterialization of the blood ; and the sensa- 

 tion which results from such an accumulation is accordingly first 

 felt in them. There is reason to believe, however, that all the vas- 

 cular organs are more or less capable of originating this impression, 

 and that all the sensitive nerves are capable, to some extent, of trans- 

 mitting it. Although the first disagreeable sensation, on holding 

 the breath, makes itself felt in the lungs, yet, if we persist in sus- 

 pending the respiration, we soon become conscious that the feeling 

 of discomfort spreads to other parts : and at last, when the accu- 

 mulation of carbonic acid and the impurity of the blood have 

 become excessive, all parts of the body suffer alike, and are per- 

 vaded by a general feeling of derangement and distress. It is easy ; 



