142 FUNCTIONS OF THE NERVOUS SYSTEM. 



tated by movements which are almost of a convulsive nature ; and no effort 

 of the will can then prevent the ingress of air.* It is easy to understand why, 

 in the higher animals at least, and more especially in Man, the respiratory 

 actions should thus be placed under the control of the will ; since they are 

 subservient to the production of those sounds by which individuals commu- 

 nicate their feelings and desires to each other ; and which, when articulate, 

 are capable of so completely expressing what is passing in the mind of the 

 speaker. If the respiratory muscles of Man were ho more under his control 

 than they appear to be in the Insect or Molluscous animal, he might be pro- 

 vided with the most perfect apparatus of speech, and yet he would not be 

 able to employ it to any advantage. 



187. The motor power of the respiratory nerves is exercised, however, not 

 only on the muscles which perform the inspiratory and expiratory movements, 

 but on those which guard the entrance to the wind-pipe, and also on certain 

 other parts. The movements of the internal respiratory apparatus are chiefly, 

 if not entirely, effected through the medium of the motor fibres, which the 

 Par Vagum contains. These motor fibres exist in very different amount in its 

 different branches. For example, the pharyngeal and ossophageal branches, 

 by which (as will hereafter appear) the muscles of deglutition are excited 

 to contraction, possess a much larger proportion of them, and exhibit much 

 less sensibility when irritated, than do other divisions of the trunk. Between 

 the superior and inferior laryngeal nerves, again, there is an important differ- 

 ence, which anatomical and experimental research have now very clearly 

 demonstrated. It has long been known, that section of the Par Vagum in the 

 neck, above the inferior laryngeals, is frequently followed by suffocation, 

 resulting from closure of the glottis ; and hence it has been inferred, that the 

 office of the inferior laryngeals was to call into action the dilater% of the 

 larynx, whilst the superior laryngeals, were supposed to stimulate the con- 

 strictors. This view, however, is incorrect. It is inconsistent with the results, 

 just stated, of anatomical examination into the respective distribution of these 

 two trunks ; and it has been completely overthrown by the very careful and 

 satisfactory observations and experiments of Dr. J. Reid, which have esta- 

 blished that, whilst the inferior laryngeal is the motor nerve of nearly all the 

 laryngeal muscles, the superior laryngeal is the excitor or afferent nerve, 

 conveying to the medulla oblongata the impressions by which muscular move- 

 ments are excited. Its motor endowments are limited to the crico-thyroid 

 muscle, to which alone of all the muscles its filaments can be traced, the 

 remainder being distributed beneath the mucous surface of the larynx ; and 

 its sensibility is very evident, when it is pinched or irritated during experi- 

 ments upon it. On the other hand, the motor character of the inferior laryn- 

 geal branch is shown by its very slight sensibility to injury, its nearly exclu- 

 sive distribution to muscles, and its influence in exciting contraction of these 

 when its separated trunk is stimulated. 



188. It has been ascertained by Dr. Reid that, if the inferior laryngeal 

 branches be divided, or the trunk of the par vagum be cut above their origin 

 from it, there is no constriction of the glottis, but a paralyzed state of its mus- 



* It is asserted by M. Bourdon (Recherches sur le Mecanisme de la Respiration, p. 

 81,) that no person ever committed suicide, though many have attempted to do so, by 

 simply holding the breath ; the control of the will over the respiratory muscles not being 

 sufficiently great to antagonize the stimulus of the "besoin de respirer," when this has 

 become aggravated by the temporary cessation of the action. But such persons have 

 succeeded better by holding the face beneath the surface of water; because here an- 

 other set of muscles is called into action, which are much more under the control of the 

 will than are those of respiration; and a strong volition applied to these can prevent all 

 access of air to the lungs, however violent may be the inspiratory efforts. 



