OF THE RESPIRATORY MOVEMENTS. 517 



552. 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 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 

 Pneumogastric contains. These motor fibres exist in very different proportions 

 in its different branches. For example, the pharyngeal and oesophageal branches, 

 by which the muscles of deglutition are excited to contraction ( 427, 428), 

 possess a much larger amount of them, and exhibit much less sensibility when 

 irritated, than do other divisions of the trunk. Between tne superior and in- 

 ferior 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 Pneumogastrics 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 dilators of the larynx, whilst the superior laryngeals 

 were supposed to stimulate the constrictors. This view, however, is incorrect. 

 It is inconsistent with the results 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, 1 

 which have established that, whilst the inferior laryngeal is the motor nerve of 

 nearly all the larycigeal 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 re- 

 mainder 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 dis- 

 tribution to muscles, and its influence in exciting contraction of these when its 

 separated trunk is stimulated. 



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

 branches be divided, or the trunk of the pneumogastric 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 quies- 

 cence and freedom from dyspnosa 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 inwards 

 the arytenoid cartilages, which are rendered passive by the paralyzed state of 

 their muscles j and these, falling upon the opening of the glottis, like valves, 

 obstruct the entrance of air into the lungs. The more effort is made, the greater 

 will be the obstruction : and accordingly, it is generally necessary to counteract 

 the tendency to suffocation, when it is desired to prolong the life of the animal 

 after this operation, by making an opening into the trachea. Dr. Reid further 

 ascertained, that the application of a stimulus to the inferior laryngeal nerves, 

 when separated from the trunk, would occasion distinct muscular contractions 

 in the larynx j whilst a corresponding stimulus applied to the superior laryngeal 

 occasioned no muscular movement, except in the crico-thyroid muscle. But 

 when the superior laryngeals were entire, irritation of the mucous surface of the 

 larynx, or of the trunks themselves, produced contraction of the glottis and 

 efforts to cough ; effects which were at once prevented by dividing those nerves, 



1 "Edinb. Med. and Surg. Journ." Jan. 1838; and " Anat., Pliysiol. and Tathol. Res.," 

 chap. iv. 



