198 THE ANATOMY OF THE HORSE. 



muscles ; and it here crosses to the inner side of the external carotid 

 artery. Reaching the pharynx, it continues to descend either close 

 behind the posterior edge of the great cornu or under cover of it ; 

 and here it is crossed by the pharyngeal artery, which may pass 

 either over or under it. It next passes within the articulation of 

 the great and small cornua to reach the root of the tongue, where 

 its terminal branches will be seen at a later stage. It gives off the 

 following branches, the first of which will not now be seen : — 



1. The Nerve of Jacobson, given off from Andersch's ganglion — a 

 minute ganglion placed on the nerve where it issues from the cranium. 

 Jacobson's nerve penetrates between the petrous and temporal parts of 

 the temporal bone and enters the tympanum, to the mucous membrane 

 of which it is distributed. 



2. Branches of communication with the superior cervical ganglion of 

 the sympathetic. 



3. A Branch to the Carotid Plexus. 



4. A Pharyngeal Branch, which is given off at or near the point 

 where the nerve crosses the external carotid. It is as large as the 

 glossal continuation of the trunk, and it passes on to the wall of 

 the pharynx, behind the stylo-pharyngeus muscle. 



5. A Branch to the Stylo-pharyngeus muscle. — This may be detached 

 either before or after the preceding branch. It enters the outer side 

 of the muscle. 



The 12th Cranial Nerve, called also the Hypoglossal (Plate 32), 

 leaves the cranium by the condyloid foramen. It passes through the 

 angle of separation of the 10th and 11th nerves, and descends on the 

 guttural pouch, crossing to the outer side of the external carotid at or 

 near the origin of the submaxillary artery. It next crosses the pharynx 

 in company with the submaxillary artery, and passes under the angle 

 formed by that vessel and its lingual branch. It is continued to the 

 muscles of the tongue. Where the nerve lies on the guttural pouch 

 it is covered by the stylo-maxillaris, digastric, and stylo-hyoid muscles, 

 and below that point it is covered by the internal pterygoid muscle. 



On the pharynx the hypoglossal is joined by a considerable twig from 

 the inferior primary branch of the 1st spinal nerve. On the guttural 

 pouch it constantly communicates with the superior cervical ganglion of 

 the sympathetic. It has no other branches until it reaches the tongue. 



The 10th Cranial Nerve, also termed the Vagus or Pneumogastric 

 (Plate 32), issues from the cranium by the extreme posterior part of 

 the foramen lacerum basis cranii. For about one inch and a half of 

 its course it forms a common cord with the 11th nerve, which issues 

 at the same point. The two nerves then separate (the 12th nerve 

 passing through the angle), and the vagus passes downwards and 

 backwards on the guttural pouch. It passes over the internal carotid 



