540 



THE BRAIN 



demonstrated in a specimen obtained in the dissecting-room. They lie 

 nearer the median plane. 



Dissection. A series of transverse sections should now be made 

 through the pons and the medulla oblongata, in order that something of 

 their internal structure may be learned. 



As a matter of fact little of the structure of the medulla oblongata can be 

 learnt from specimens obtained in the dissecting-room ; but in sections 

 properly prepared and stained a number of important points can be 

 seen. 



Structure of the Medulla Oblongata. When transverse sections are 

 made through the medulla oblongata at different levels, a faint line, called 

 the median raphe and occupying the median plane, is seen to divide it 



Gracile nucleus 

 Cuneate nucleus 



Cuneate nucleus 



Tractus spinalis of fifth 



nerve 



Substantia gelatinosa 



Rolandi 



Cerebello-spinal 



fasciculus 



Lateral cerebro-spinal 



fasciculus 



Detached anterior 

 column of grey matter 



Decussation of pyramids 



Fasciculus anterior proprius 



FIG. 226. Section through the lower part of the 

 Medulla Oblongata of the Orang. 



into two exactly similar lateral portions. The raphe is formed by the 

 close intersection of fibres running in different directions. 



Each half of the medulla oblongata is composed of (a) strands of white 

 matter ; (b] grey matter, which is present both in the form of direct con- 

 tinuations into the medulla oblongata of portions of the grey matter of 

 the spinal medulla, and in the form of isolated clumps, which are not 

 represented in the spinal medulla ; and (c) the formatio reticularis, a 

 substance which is composed of grey matter coarsely broken up by fibres 

 which traverse it in different directions. The white matter, as in the 

 spinal medulla, is disposed for the most part on the surface and the grey 

 matter in the interior, but in the open part of the medulla oblongata the 

 grey matter comes to the surface on its dorsal aspect, and forms the 

 obex (p. 538). 



When the grey matter of the spinal medulla is traced up into the medulla 

 oblongata, many striking changes in its arrangement become apparent. 

 Owing to the increase in size of the large wedge-shaped gracile and cuneate 

 funiculi, the posterior columns of grey matter become pressed laterally, so 

 that they soon assume a position at right angles to the median plane, and 

 lie very nearly in the same transverse line. At the same time, the cuneate 



