REFLEX ACTIONS. RESPIRATORY MOVEMENTS. 295 



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

 should be thus placed under the control of the will : since they are subser- 

 vient to the production of those sounds, by which individuals communicate 

 their feelings and desires to each other; and which, when articulate, are capa- 

 ble of so completely expressing what is passing in the mind of the speaker. 

 If the respiratory mucles of Man were no more under his control, than they 

 appear to be in the Insect or Molluscous animal, he might be provided with 

 the most perfect apparatus of speech, and yet he would not be able to employ 

 it to any advantage. 



378. The motor power of the Respiratory nerves is exercised, however, 

 not only on the muscles which perform the inspiratory and expiratory move- 

 ments, but on those which guard the entrance to the windpipe, 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 cesopha- 

 geal 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 difference, which anatomical and experimental research has now 

 very clearly demonstrated. It has long been known, that section of the Par 

 Vagum in the neck, above the inferior laryn'geals, is frequently followed by 

 suffocation, resulting from closure of the glottis ; and hence it has been in- 

 ferred, that the office of the inferior laryngeals was to call into action the 

 dilators of the larynx, whilst the superior laryngeals were supposed to stimu- 

 late the constrictors. 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 established 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 movements 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 experiments upon it. On the other hand, the motor character of the 

 inferior laryngeal branch is shown by its very slight sensibility to injury, its 

 nearly exclusive distribution to muscles, and its influence in exciting contrac- 

 tion of these when its separated trunk is stimulated. 



379. 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- 

 cles. After the first paroxysm occasioned by the operation, a period of qui- 

 escence and freedom from dyspnoea often supervenes, the respirations being 

 performed with ease, so long as the animal remains at rest; but an unusual 

 respiratory movement, such as takes place at the commencement of a struggle, 

 induces immediate symptoms of suffocation, the current of air carrying in- 

 wards the aryterund cartilages, which are rendered passive by the paralyzed 



vated by the temporary cessation of the action. But such persons have succeeded better, by 

 holding the face beneath the surface of water; because here another 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. 



