THE MEDULLA OBLOXGATA. 



143 



Fig. 98. 



roideiis veniricnli qvaHi, Reissner; velum meclullare jjosterhis). The 

 blood-vessels 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 pig-mented. 



The ventral surface of the medulla ob- 

 longata (Fig-. ]02 Mo) has a median ventral 

 longitudinal fissure, a direct continuation of 

 that of the cord ; in the anterior part of the 

 merhilla oblongata there is also to either side 

 of this a lateral fissure, continued on to the 

 crwa cerebri ; these fissures correspond to 

 the positions of the two ra?ni poderiores of 

 the internal carotid arteries ; they are always 

 well seen in microscopical sections. The 

 wednlla oblongata is so intimately connected 

 with the jijar* commissttralis (pp. 149, 150) 

 that the minute anatomy of the two is best 

 desci'ibed at the same time. 



yS. Internal structure. Examined by 

 means of serial sections, the medulla otjlou- 

 gata is seen to have, in comparison with the 

 cord, an increased amount of grey matter ; 

 this is especially the case in its anterior part. 

 The floor and inner parts of the walls of 

 the ventricle are formed of grey matter, in 

 which the largest-sized cells have disap- 

 peared, to be replaced by medium-sized 

 cells. Traced from behind, the ventral 

 horns of the cord are seen to increase in 

 size and to be more widely separated until 

 they form two isolated masses, while the 

 dorsal horns gradually diminish ; at the 

 same time they are forced outwards and upw^ards, until they lie under 

 the floor of the ventricle, and so extend to t}xe pars peduncnlaris. 



Grey matter. The substantia reticularis is not present, but the 

 septum, medium extends forwards as far as the pars peduncularis. 

 The central canal extends upwards at the expense of the tissue 

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



Dorsal view of brain of Rami 

 escitlenta. 



Ad 

 C 



f 



Choroid plexus. 

 Cerebelhim. 



Groove between cerebral 

 hemispheres and olfactory 

 lobes. 

 Opening in the roof of the 



tliird ventricle. 

 Pineal l»dy. 

 Cerebral liemispheres. 

 Olfactory nerve. 

 Wall of fourtli ventricle. 

 L.ol Olfactory lobe. 

 L.op Optic lobe. 

 Mo Mednlla oblongata. 

 S Longitudinal fissure of the 



fourtli ventricle. 

 S.r Fourth ventricle. 

 Tho Thalamenceplialon. 



