574: THE CRANIAL NERVES. 



muscles. Such efforts cannot be made with success if the muscles in 

 question are paralyzed. In the lower animals, according to the obser- 

 vations of Bernard, they also take part in the production of a cry, or 

 prolonged vocal sound. If the entire spinal accessory be destroyed, as 

 already shown, the voice is completely abolished by loss of power in 

 the laryngeal muscles. If the external branch alone be divided, the 

 animal can still produce a sound in the larynx ; 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 not apparently inconvenienced by this operation so 

 long as they remain quiet, any increased exertion, as in running or 

 leaping, causes a want of harmony between the movements of respira- 

 tion and those of the limbs, which results in unusual shortness of 

 breath. 



The sterno-mastoid and trapezius muscles, like those of the larynx, 

 are therefore animated by two sets of motor nerve fibres. One set, 

 coming from the cervical spinal nerves, provides for all the movements 

 connected with ordinary changes of attitude and locomotion ; the 

 others, derived from the spinal accessory, supply the requisite stimulus 

 for continuous muscular exertion, or for a prolonged vocal sound. 



Twelfth Pair. The Hypoglossal. 



The hypoglossal nerve, the motor nerve of the tongue, emerges from 

 the anterior part of the medulla oblongata by a linear series of ten or 

 twelve slender filaments in the furrow between the outer edge of the 

 anterior pyramids and the rounded 'projection of the olivary bodies 

 (Fig. 179,5). The vertical line along which these filaments make their 

 appearance corresponds exactly with the line of origin of the anterior 

 roots of the cervical spinal nerves below ; and the whole external aspect 

 of their anatomical relations resembles that of a motor nerve root. 



The central origin of the hypoglossal root fibres, according to 

 Clarke, Dean, Kolliker, Henle, and Meynert, is a nucleus of gray matter 

 situated at the posterior part of the medulla oblongata next the median 

 line, at the inferior extremity of the fourth ventricle. This collection of 

 gray matter has an elongated, irregularly cylindrical form, extending 

 longitudinally from about the level of the divergence of the posterior 

 columns upward and forward to that of the auditory nucleus. It is, 

 therefore, parallel in its position with the spinal accessory and pneumo- 

 gastric nuclei, but situated between them and the median line. In 

 transverse sections of the medulla, made successively from below up- 

 ward, this nucleus is first seen (Fig. 180) to be placed immediately 

 about the central canal, which is already approaching the posterior sur- 

 face of the medulla ; and the roots of the nerve run in a curvilinear 

 course downward and outward to their point of emergence. 



Above this point, after the central canal has opened into the cavity 

 of the fourth ventricle (Fig. 181), the hypologlossal nucleus has itself 

 receded quite to the posterior surface of the medulla, where it occupies 



