492 THE NERVOUS SYSTEM. 



the pneumogastric trunks themselves, had been divided. The total 

 result in the two cases, however, is very different. Section of the 

 pneumogastrics, or of their inferior laryngeal branches, paralyzes all 

 the movements of the glottis, those of respiration as well as those of 

 phonation; since these nerves contain all the motor fibres distributed 

 to the larynx, except those for the crico-thyroid muscles. On the other 

 hand, evulsion of the spinal accessory nerves paralyzes the movements 

 of phonation alone, namely, those in which the vocal chords are approxi- 

 mated and the rima glottidis narrowed ; leaving untouched the move- 

 ments of respiration, in which the vocal chords are separated and the 

 glottis opened. 



The larynx, accordingly, performs two distinct functions, and is sup- 

 plied with motor nerves from two diiferent sources. Those which 

 preside over the production of sound originate from the spinal acces- 

 sory ; those for respiration are derived from other motor nerves (facial, 

 hypoglossal, cervical) which also communicate with the pneumogastrics. 



The function of the external or muscular branch of the spinal acces- 

 sory nerve is not so fully understood. The sternomastoid and tra- 

 pezius muscles, to which it is distributed, also receive filaments from 

 the cervical spinal nerves ; and they still retain the power of motion 

 after evulsion of the spinal accessory on both sides. The sterno- 

 mastoid and trapezius muscles have no such peculiar action as that 

 of the larynx in vocalization ; and it is not easy to distinguish what 

 movements of these muscles are paralyzed by division of the spinal 

 accessory, and what remain unaffected. The most plausible conclusions 

 are those derived by Bernard from the continued observation of animals 

 after division of these nerves. 



According to this view, the external branch of the spinal accessory, 

 like the internal branch, performs a function antagonistic to respiration. 

 Respiration is naturally suspended during strenuous and prolonged 

 muscular eifort. In the acts of straining, lifting, pushing, and the 

 like, respiration ceases, the spinal column is made rigid, and the head 

 and neck are fixed in position largely by means of the sternomastoid 

 and trapezius muscles. Such efforts cannot be made with success if 

 these muscles be paralyzed. According to Bernard, they also take part 

 in the production of a cry, or prolonged vocal sound. After destruc- 

 tion of the entire spinal accessory the voice is completely abolished by 

 paralysis of the laryngeal muscles. If its external branch alone be 

 divided, the animal can still produce a vocal sound; but this sound 

 cannot be prolonged into a cry, and the voice is confined in duration 

 to the ordinary length of an expiratory movement. Although the 

 animals, furthermore, are apparently not otherwise inconvenienced by 

 this operation so long as they remain quiet, any increased exertion, as 

 in running or leaping, causes a want of harmony between respiration 

 and muscular action, which results in shortness of breath. 



The sternomastoid and trapezius muscles, like those of the larynx, 

 are therefore animated by two sets of motor fibres. Those coming 



