1 28 INNERVATION. [CHAP. XX. 



to the posterior roots of the lower cervical nerves. By the union 

 of all the fascicles the nerve is formed, and it enters the cranial 

 cavity from that of the spine through the foramen magnum. 



Sometimes some of the upper roots of the spinal accessory nerve 

 coalesce with the posterior roots of the sub-occipital arid the second 

 and third cervical nerves. This appears to be nothing more than a 

 junction of the fibres of two nerves which emerge from the nervous 

 centre in close proximity to each other. 



Very shortly after the escape of the spinal accessory nerve from 

 the foramen lacerum, it divides into an internal and an external 

 branch. The former coalesces with the vagus,^where its second 

 ganglion is formed, and, according to some physiologists, supplies the 

 motor branches of that nerve ; the latter passes outwards and down- 

 wards, through the deeper fibres of the sterno-mastoid muscle, to 

 which it gives some branches, and anastomoses with branches of the 

 second and third cervical nerves ; and having crossed the triangular 

 space in the neck between the sterno-mastoid and trapezius, it pene- 

 trates the latter muscle at its deep surface, and is distributed in it, 

 anastomosing with other branches of the cervical plexus. 



We learn from the anatomy of this nerve that it supplies two 

 great muscles, which play an important part in effecting certain 

 movements of the head and shoulder, and, in a secondary manner, 

 contribute to the actions of respiration, especially to those of a 

 forced or extraordinary kind ; and likewise that it forms a junction 

 with the vagus nerve by a branch which consists of a considerable 

 number of fibres. 



There can be no doubt, from anatomy, that those fibres of the 

 nerve which pass to the trapezius and sterno-mastoid muscles are 

 principally motor, for their main distribution is to these muscles ; 

 and all experimenters agree in stating that, whenever stimulated, 

 they excite these muscles to contract. Anatomy, however, equally 

 indicates that these muscles derive motor power from branches of 

 the cervical plexus likewise. 



The office of the internal branch, which incorporates itself with 

 the vagus nerve, is not so easily determined. Scarpa, Arnold, 

 Bischoff, Bendz, and others, viewed it as contributing the motor 

 fibres to that nerve, bearing to it the same relation as the anterior 

 to the posterior root of a spinal nerve. 



An objection to this view, although not an insuperable one, is sug- 

 gested by the origin of the nerve, which seems more in accordance 

 with that of the posterior roots of the spinal nerves than with their 

 anterior roots, and this is especially the case with the lower fascicles 



