THE MEDULLA OBLOXGATA. 



143 



Fig. 98. 



fio 



rentnculi quart i, Reissner; velum medull are poster ins). The 

 blood-vessds of the plexus will be described together with the 

 other vessels of the brain (p. 162). They 

 are supported by a connective-tissue matrix, 

 and the whole covered with flattened epi- 

 thelium, which in the fourth ventricle is 

 ciliated and often pigmented. 



The ventral surface of the medulla ob- 

 longata (Fig. 1 02 Mo] has a median ventral 

 longitudinal fissure, a direct continuation of 

 that of the cord ; in the anterior part of the 

 ' :>Ua oUongat^a there is also to either side 

 of this a lateral fissure, continued on to the 



>i cerel ri ; these fissures correspond to 

 the positions of the two rami posterior** of 

 the internal carotid arteries ; they are always 

 well seen in microscopical sections. The 



''I/a ollongata is so intimately connected 

 with the pars commissv. rails (pp. 149, 150) 

 that the minute anatomy of the two is best 

 described at the same time. 



ft. Internal structure. Examined by 

 means of serial sections, the medulla M on- 

 got a is seen to have, in comparison with the Ad ^^ ^ 

 c >rd, an increased amount of grey matter ; c _ 

 this is especially the case in its anterior part. 

 The floor and inner parts of the walls of G 

 the ventricle are formed of grey matter, in 

 which the largest-sized cells have disap- 

 peared, to be replaced by medium-sized * Li SSSToS^wntrieie. 

 cells. Traced from behind, the ventral LM Olf ^7 lobe - 



L.op Optic lobe. 



horns of the cord are seen to increase in x Medulla obiongata. 



1 . :" -11 JT ,1-iS Longitudinal fissure of the 



size and to be more widely separated until f ourt h ventricle, 



thev form two isolated masses, while the If ^ u , rth Tentr j cl , e - 



J Tho Thalamencephalon, 



dorsal horns gradually diminish ; at the 



same time they are forced outwards and upwards, until they lie under 



the floor of the ventricle, and so extend to the jjars peduncularls. 



Qrey toattej. The sulstantla retleularls is not present, but the 

 septum medium extends forwards as far as the par* peduncularis. 

 The central canal extends upwards at the expense of the tissue 

 above it. and is here pear-shaped ; at the same time the dorsal 



Dorsal view of brain of Rann 



xus. 



Cerebellum. 



Groove between cerebral 

 hemispheres and Ifaetory 



Opening in the roof of the 



third ventricle. 

 Gp Pineal body. 



Cerebral hemispheres. 



