842 THE NERVES. 



Here it slightly ascends, crosses that region below the inner face of the cervical 

 trapezius, and is distributed to the dorsal trapezius. 



On its passage it emits : 1. One or two thick filaments to the superior cervical 

 ganglion, proceeding from the spinal accessory by a small plexiform network, 

 where the nerve separates from the pneumogastric. 2. Near the maxillary gland, 

 a thick branch to the steruo-maxillaris muscle. 8. Slightly beyond this, another 

 branch destined to the anterior portion of the mastoido-humeralis. 4. A series 

 of ramuscules to the cervical trapezius. 



In its course, the spinal accessory traverses the anterior part of the supei"ficial 

 cervical plexus, and receives additional ramuscules from the first, second, third, 

 fourth, fifth, and sometimes even from the sixth cervical nerves. 



Reduced to its medullary root, as has been dsscribed, the spinal accessory 

 stimulates the muscles to which it is distributed, and through them has a share 

 in expiration. By the contraction of these muscles, the ribs may be maintained 

 raised for a certain period, and cause the air to be slowly expelled ; also allowing 

 the sounds or voice produced by its passage through the larynx to be modulated. 

 Crowing is no longer possible in Birds when the medullary root of the spinal 

 accessory has been divided, as has been demonstrated by Bernard. 



12. Twelfth Pair, or Hypoglossal Nerves (Figs. 171, 22 ; 456, 5 ; 459, 10). 



The hypoglossal nerve is exclusively motor, and stimulates the muscles of the 

 tongue. 



Origin. — It apparently arises from the lower face of the medulla oblongata, 

 on the prolongation of the line of origin of the inferior spinal nerves, by a dozen 

 converging filaments. Traced into the medulla, these filaments are found to pass 

 into a principle nucleus situated near the point where the central canal of the 

 spinal cord enlarges to form the floor of the fourth ventricle, inside the pneumo- 

 spinal nucleus, and in the small masses of grey substance placed below and out- 

 side the preceding, named the accessory nucleus. 



The roots of the hypoglossal nerve pass through the dura mater in two or 

 three bundles, which enter the condyloid foramen of the occipital bone, where 

 they unite to form a single cord. The hypoglossal has also a ganglionic root 

 which Toussaint constantly found in the Ass, Mule, Ox, and Dog, and which 

 had been previously seen by Meyer and Vulpian. The ganglion of the hypo- 

 glossal in the Horse is fusiform, and the size of a small lentil (Fig. 456, 5'). It 

 is sometimes absent. * 



Distribution. — The hypoglossal nerve, thus constituted, immediately after its 

 departure from the condyloid foramen, communicates with the first cervical pair 

 by means of a transverse ramuscule ; it then passes between the spinal accessory 

 and pneumogastric nerves, descends on the external face of the guttural pouch, 

 where it is connected with the superior cervical ganglion of the sympathetic by 

 numerous filaments, which in great part form the p'cxiform network called the 

 " guttural plexus." The nerve afterwards crosses to the outside of the external 

 carotid artery, in proceeding forward and downward on the side of the pharynx 

 and larynx, receiving at that point a slender ramuscule from the first cervical ; 

 it then passes within the inferior extremity of the stylo-hyoid muscle and the 

 submaxillary artery, which it crosses very obliquely, is prolonged between the 

 mylo-hyoid and great hyo-glossus muscles, sends numerous small filaments to 

 the latter and a ramuscule to the genio-hyoideus, and finally terminates in a 



