256 THE ANATOMY OP THE HORSE. 



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 meduUary roots. The 

 medkdlary 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 tlie 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 

 151). 



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- 

 cates the outer limit of the inferior pyramid. These roots are in series 

 with the inferior roots of the spinal nerves ; and sometimes there is also 

 present a superior root, in series with the superior roots of the same 

 nerves, and provided with a minute ganglion. These roots arise from 

 nerve cells of the medulla. The roots of the nerve perforate the dura 

 mater, and unite in emerging from the cranium by the condyloid 

 foramen. 



