182 DISSECTION OF THE BRAIN. 



Deep origin. The glosso-pharyngeal penetrates as far as the vagus nucleus, where 

 it ends in fibres which enter that deposit. 



The vagus nerve arises in a special nucleus (p. 217) ; but some fibres pass through 

 the hypoglossal nucleus. 



The accessory part of the spinal accessory nerve is transmitted to a special nucleus 

 below that of the vagus ; and some fibres decussate across the middle line with 

 their fellows. 



The roots of the spinal part of the nerve, piercing the lateral column of the cord, 

 pass through a collection of cells to the anterior cornu of the crescent (Clarke) . 



The NINTH or HYPOGLOSSAL NKRVE of Willis (twelfth nerve, Sommer- 

 ring) ( 9 ) is placed on the front of the medulla oblongata, and arises by a 

 series of filaments from the sulcus between the pyramidal and olivary 

 bodies, in a line with the anterior roots of the spinal nerves. 



The filaments of origin unite into two bundles, which pierce separately 

 the dura mater, and do not blend together till they are outside the cranial 

 cavity. 



Deep origin. The filaments of the nerve can be traced through the corpus olivare 

 to a nucleus below the level of the fourth ventricle, and in front of the canal of 

 the cord ; but some bend inwards to decussate through the rapht with the nerve of 

 the opposite side (Clarke). 



SECTION III. 



MEDULLA OBLONGATA AND PONS VAROLII. 



THE medulla oblongata and the pons are interposed between the spinal 

 cord and the brain proper. 



Directions. On a single brain the student may ascertain nearly all the 

 anatomy of the parts composing the medulla and the pons ; but if he can 

 procure one hardened specimen of the medulla and the pons united, and 

 another of a vertical median section through those bodies, he will compre- 

 hend much more readily the following description. 



Position. The brain is to remain in the same position as for the exami- 

 nation of the nerves and vessels. 



The MEDULLA OBLONGATA is the dilated upper part of the spinal cord 

 which is contained in the cranium (fig. 54). Its limit is the lower border 

 of the pons in one direction, and the upper margin of the atlas in the 

 other. This part of the cord is pyramidal in form, and measures about 

 one inch and a quarter in length ; half an inch in breadth below, and about 

 an inch at its widest part. 



Its base joins the pons, the transverse fibres of the latter marking its 

 limit ; and its apex is blended with the spinal cord. The anterior surface 

 is irregularly convex, and is in contact with the hollowed basilar process 

 of the occipital bone. The opposite surface is somewhat excavated supe- 

 riorly, where it forms the floor of th ; fourth ventricle ; and it rests in the 

 fissure between the halves of the cerebellum. On the posterior aspect there 

 are not any cross fibres of the pons, as in front, to mark the extent of the 

 medulla. 



The medulla oblongata is divided into halves by a median fissure in 

 front and behind. The fissures are in a line with those along the cord : 

 the anterior ceases at the pons in a dilated part (foramen caecum), but 



