THE HYPOGLOSSAL NERVE. 1275 



Variations. — Considerable deviation from the normal has been described with regard to 

 the spinal portion. The lower limit of its origin has been observed as high as the third cervical 

 nerve and from that level as far down as the first thoracic. In one instance the nerve left 

 the subdural space below the first cervical nerve and re-entered at a higher level. Quite fre- 

 quently it fails to pierce the sterno-mastoid muscle. In one reported case the nerve ended in 

 the sterno-mastoid, the trapezius being supplied only by the third and fourth cervical nerves. 

 Two similar cases have been observed in the dissecting room of the University of Pennsylvania. 

 Rarely it gives off a filament which joins the n. descendens cervicalis. 



Practical Considerations. — The spinal accessory nerve supplies the sterno- 

 cleido-mastoid and trapezius muscles. A few fibres of the second and third cervical 

 nerves enter into the supply of the sterno-mastoid, but the muscle is almost com- 

 pletely under the control of the spinal accessory. The cervical nerves take a greater 

 part in the supply of the trapezius, so that paralysis of the spinal accessory does not 

 alw^ays paralyze this muscle. 



Spasm of the trapezius will draw the head backward and toward the afTected 

 side and will pull the scapula toward the spine. In spasm of the sterno-mastoid, as 

 in "wry neck," the chin will be turned to the opposite side and elevated, while the 

 ear will look forward. If both sterno-mastoids are in contraction the chin will be in 

 the median line and will be drawn toward the sternum. Paralysis of one muscle will 

 produce a condition somewhat similar to that produced by a spasm of the opposite 

 one. 



The spinal accessory nerve enters the under surface of the sterno-mastoid muscle 

 near the junction of its upper and middle thirds, where it may be reached by an 

 incision along the anterior border of the muscle. The nerve emerges from the 

 muscle near the middle of its posterior border. 



THE HYPOGLOSSAL NERVE. 



The twelfth or hypoglossal nerve (n. hypoglossus) is a purely motor nerve and 

 supplies the musculature of the tongue, intrinsic as well as extrinsic, with the excep- 

 tion of the palato-glossus. 



Central and Cortical Connections. — The hypoglossal nerve takes its deep origin from 

 several associated groups of neurones called the hypoglossal nucleus (nucleus n. hypoglossi) 

 (Fig. 949), which underlies the floor of the fourth ventricle. This nucleus is a narrow 

 elongated collection of large multipolar cells, measuring about 18 mm. in length by 2 mm. 

 in width, that partly corresponds in position to the trigonuni hypoglossi in the floor of the 

 fourth ventricle. The entire nucleus, however, is more extensive than the trigonum and 

 extends from the level of the striae acusticae above into the closed part of the medulla as far 

 down as the decussation of the pyramids (Fig. 927). It lies ventral and very slightly lateral 

 to the central canal of the medulla and the median groove in the floor of the fourth ventricle, 

 close to the mid-line and its fellow of the opposite side. The large size and branched form 

 of the nerve-cells composing the nucleus, as well as their ventral position in relation to 

 the central canal, emphasize the close correspondence of these elements with the cells of 

 the motor roots of the spinal nerves. Indeed, as noted later (page 1380), the gray matter 

 enclosing the hypoglossal nucleus is the morphological equivalent of the bases of the anterior 

 cornua. Immediately after arising and before leaving the nucleus, the axones converge 

 into a number of fasciculi which, emerging from the ventral aspect of the nucleus, take a 

 ventro-lateral course and traverse the interval between the gray and white reticular formations. 

 From this situation the hypoglossal fibres continue their course to the anterior surface of the 

 medulla by passing, for the most part, between the nucleus of the inferior olive and the mesial 

 accessory olivary nucleus, although quite a number of the strands penetrate the ventral portion 

 of the olivary nucleus (Fig. 927). 



The central connections of the hypoglossal nucleus include: (a) crossed fibres from the 

 nucleus of the opposite side ; {b) fibres from, and probably also to, the posterior longitudinal 

 fasciculus, by means of which the nucleus of the twelfth is brought into relation witii the nuclei 

 of other motor nerves; and (^) fibres which join the dorsal bundle of Schiitz, a system of 

 longitudinal fibres underlying the floor of the fourth ventricle and traceable upward beneath the 

 Sylvian aqueduct, by which the nuclei of the sensory cranial nerves are connected. 



The cortical centre of the hypoglossal nerve probably lies within the lower or opercular 

 extremity of the precentral convolution. The fibres arising as the axones of the cells within 

 this area pass over the upper border of the lenticular nucleus and through the internal capsule 

 and descend in the brain-stem within the median part of the pyramidal tract as far as the 



