PNEUMOGASTRIC NERVE. 467 



ments of the right vocal chord instantly cease, owing to the para- 

 lysis of the posterior crico-arytenoid muscle on that side. If the 

 inferior laryngeal nerve on the left side be also divided, the para- 

 lysis of the glottis is then complete, and its respiratory movements 

 cease altogether. A serious difficulty in respiration is the imme- 

 diate consequence of this operation. For the vocal chords, being 

 no longer stretched and separated from each other at the moment 

 of inspiration, but remaining lax and flexible, act as a double valve, 

 and are pressed inward by the column of inspired air ; thus par- 

 tially blocking up the passage and impeding the access of air to 

 the lungs. If the pneumogastrics be divided in the middle of the 

 neck, the larynx is of course paralyzed precisely as after section 

 of the inferior laryngeal nerves, since these nerves are given off 

 only after the main trunks have entered the cavity of the chest. 

 The immediate effect of either of these operations is to produce 

 a difficulty of inspiration, accompanied by a peculiar wheezing or 

 sucking noise, evidently produced in the larynx and dependent on 

 the falling together of the vocal chords. In very young animals, 

 as, for example, in pups a few days old, in whom the glottis is 

 smaller and the larynx less rigid than in adult dogs, this difficulty 

 is much more strongly marked. Legallois 1 has even seen a pup 

 two days old almost instantly suffocated after section of the two 

 inferior laryngeal nerves. We have found that, in pups two 

 weeks old, division of the inferior laryngeals is followed by death 

 at the end of from thirty to forty hours, evidently from impeded 

 respiration. 



The importance, therefore, of these movements of the glottis in 

 respiration becomes very evident. They are, in fact, part and 

 parcel of the general respiratory movements, and are necessary to 

 a due performance of the function. It has been found, moreover, 

 that the motor filaments concerned in this action are not derived, 

 like those of the voice, from a single source. While the vocal 

 movements of the larynx are arrested, as mentioned above, by 

 division of the spinal accessory alone, those of respiration still go 

 on ; and in order to put a stop to the latter, either the pneumo- 

 gastrics themselves must be divided, or all five of the motor nerves 

 from which their accessory filaments are derived. This fact has 

 been noticed by Longet as showing that nature multiplies the safe- 

 guards of a function in proportion to its importance ; for while the 



1 In Longet's Traite de Phvsiologie, vol. ii. p. 324. 



