262 THE ANATOMY OF THE HORSE. 



grey matter at the floor of the 4th ventricle, and possibly also from the 

 cerebellum. The 8th nerve enters the internal auditory meatus, and 

 penetrates to the internal ear. 



The Glosso-pharyngeal or 9th nerve springs out of the side of the 

 medulla, a little behind the outer extremity of the trapezium. It is 

 here compounded of two or three bundles, the outermost being in line 

 with the roots of the next two nerves. The fibres emanate from nerve 

 cells of the grey matter at the floor of the 4th ventricle. The nerve 

 leaves the cranium by the posterior part of the foramen lacerum basis 

 cranii, and at that point it shows a minute ganglion — the petrous 

 ganglion or the ganglion of Andersch, from which the nerve of Jacobson 

 arises (pages 198 and 277). 



The Pnedmogastric, Vagus, or 10th nerve is formed by a number of 

 rootlets which spring from the side of the medulla, behind and in line 

 with the outermost fibres of the 9th nerve. Its fibres arise from nerve 

 cells of the medulla. The nerve passes out of the cranium by the 

 posterior part of the foramen lacerum basis cranii, and is joined by the 

 inner division of the 11th nerve. As the nerve passes through the 

 foramen it presents an enlargement — the upper ganglion, or ganglion of 

 the root. From this ganglion arises the auricular branch of the vagus, 

 which penetrates to the aqueduct of Fallopius, where it anastomoses 

 with the 7th nerve; afterwards emerging from the bone in company 

 with that nerve, it passes through a minute foramen in the conchal 

 cartilage, and is distributed to the lining membrane of the external 

 ear. 



The Spinal Accessory or 11th nerve comprises two sets of roots — a 

 spinal and a medullary. The spinal roots appear along the lateral 

 column of the cervical part of the spinal cord, in which they arise from 

 a group of nerve cells towards the middle of the grey crescent. By the 

 union of these roots there is formed a cord which travels upwards 

 between the superior and the inferior roots of the cervical spinal nerves, 

 becoming thicker as it ascends. This cord enters the cranial cavity by 

 the foramen magnum, and is then joined by the medullary roots. The 

 medullary roots spring out of the side of the medulla oblongata, behind 

 and in line with the roots of the 10th nerve, the fibres arising from 

 nerve cells at the floor of the 4th ventricle. These roots join the spinal 

 part of the nerve, which then leaves the cranium by the foramen lacerum 

 basis cranii, along with the 10th nerve. In the foramen of exit the 

 trunk of the nerve resolves itself into two portions — an internal and an 

 external. The internal portion joins the 10th nerve; the external portion 

 is that which has already been seen in the dissection of the neck (page 

 155). 



The Hypoglossal or 12th nerve is formed by the fusion of rootlets 

 that spring from the lower face of the medulla, along the line that indi- 



