THE CRANIAL NERVES. 493 



from the cervical spinal nerves provide for the ordinary movements of 

 locomotion ; those derived from the spinal accessory supply the stimu- 

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



Twelfth Pair. The Hypoglossal. 



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

 from the anterior part of the medulla oblongata by ten or twelve 

 slender filaments between the anterior pyramids and the olivary bodies 

 (Fig. 127, 5 ), on a line with the anterior roots of the cervical spinal 

 nerves. 



The central origin of these fibres, according to Clarke, Dean, Kolliker, 

 Henle, and Meynert, is a nucleus of gray substance in the posterior 

 part of the medulla oblongata next the median line, at the inferior ex- 

 tremity of the fourth ventricle. It has an elongated form, extending 

 from the divergence of the posterior columns upward and forward to 

 the level of the auditory nucleus. It is parallel in position with the 

 spinal accessory and pneumogastric nuclei,, but situated between them 

 and the median line. 



During the passage of the hypoglossal nerve roots through the 

 medulla oblongata, they reach the inner surface of the olivary nucleus, 

 and pass in great measure between the folds or through the substance 

 of its convoluted wall. It is shown by Dean* that although a direct 

 continuity between the root fibres of the nerve and the cells of the 

 olivary nucleus cannot be demonstrated, yet prolongations of these 

 cells can sometimes be traced upward and inward, in company with 

 the nerve roots, toward the hypoglossal nucleus ; and in the sheep, 

 the tracts of fibres connecting the two nuclei are very evident. Accord- 

 ing to Henle, in some transverse sections through the olivary body 

 fibres from the hypoglossal nerve roots may be seen bending round 

 the inner border of the nucleus into its interior ; while others emerge 

 in a corresponding manner from the opposite border and continue on- 

 ward, with the main root-bundles, to the hypoglossal nucleus. Although 

 the minute anatomical structure of these parts is not fully made out, it 

 is evident that a close relation exists between the gray substance of 

 the olivary bodies and the hypoglossal nucleus and roots. 



Kolliker regards the roots of the hypoglossal nerves as undergoing 

 complete decussation through the raphe, at the level of the nuclei. 

 According to Clarke and Dean, a portion of the fibres of each root ter- 

 minate in the corresponding nucleus, while another portion decussate 

 with those of the opposite side. It is certain that the hypoglossal, like 

 other cranial nerves, has, in some way, a connection with the opposite 

 side of the brain ; since cases of facial paralysis from cerebral hemor- 

 rhage are often accompanied by paralysis of the tongue on the same 

 side with that of the face, and opposite to the lesion. One of the genio- 



* Gray Substance of the Medulla Oblongata and Trapezium. Washington, 1864, 

 p. 36. 



