380 OF RESPIRATION. 



and no effort of the will can then prevent the ingress of air. 1 It is easy to 

 understand why, in the higher animals at least, and more especially in Man, 

 the respiratory actions should be thus placed under the direction of the will: 

 since they are subservient to the production of those Sounds, by which indi- 

 viduals communicate 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. 



301. 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 

 some other parts. Between the superior and inferior Laryngeal nerves there 

 is an important difference, which anatomical and experimental researches 

 have now very clearly demonstrated. It appears from the very careful and 

 satisfactory observations and experiments of Dr. J. Reid, 2 that whilst the 

 inferior laryngeal is the motor nerve of nearly all the laryngeal muscles, the 

 superior laryngeal is an afferent nerve of extreme sensibility, conveying to 

 the Medulla Oblongata the impressions made on its peripheral extremities. 

 The motor endowments of the latter are limited to the crico-thyroid muscle, 

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

 being distributed to the mucous membrane 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, by its nearly exclusive distri- 

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

 its separated trunk is stimulated. Burkart 3 has, however, shown that cer- 

 tain sensory or centripetal fibres course in this nerve, on irritation of which 

 an inhibitory influence can be exerted on the respiratory acts ; slight irrita- 

 tion producing a prolongation of the interval between expiration and inspi- 

 ration, and strong irritation completely stopping the respiratory movements. 



302. It was also ascertained by Dr. J. Reid, 4 that, if the inferior laryngeal 

 branches be divided, or the trunk of the pneumogastric be cut above their 

 origin from it. no constriction of the glottis follows, but a paralyzed state of 

 its muscles. After the first paroxysm occasioned by the operation, a period 

 of quiescence and freedom from dyspnoea often supervenes, the respirations 

 being performed with ease so long as the animal remains at rest ; but an un- 

 usual 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 thei/muscles ; and these, falling upon the opening of the 

 glottis like valves, obstruct the entrance of air into the lungs. Tlu> 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 de- 



1 It is asserted by M. Bourdon (Keohorches sur le Meeanisme do la Respiration, p. 

 21), that no person over committed suicide, though many have attempted to do so, 

 by simply holding the breath ; the control of the will over the respiratory mii-des 

 not being sufficiently great to antagonize the stimulus of the " besoin df respiivr,'' 

 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 heir 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 inspi- 

 ratory efforts. 



2 Edin. Med. and Surg. Journ., Jan. 1838; and Anat., Physio]., and Pathol. Res., 

 chap, iv 



3 I'lhiger's Archiv f. gesammtc Physiol., Md. i, p. 107, 18C8. 



4 Op. cit. See also abstract of Levcns's Researches, in Lancet, vol. i, 1870, p. 357. 



