MACROMYELON OF MAMMALIA. 



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Macromyelon, posterior or dorsal aspect, with section of 

 cerebellum. Infant, nat. size. 



seated tracts become superficial at a greater distance from the 

 skull than on the ventral surface, and do not decussate ; they ex- 

 pand as they enter the macromyelon, and form the ' post-pyra- 

 midal bodies,' fig. 49, y. 

 The posterior myelonal 

 columns which they 

 push aside, diverge as 

 they are continued into 

 the macromyelon, and 

 combine with the con- 

 tiguous lateral columns 

 to form the post-resti- 

 form tracts, x. In ad- 

 vance of the post- 

 pyramids, still deeper 

 columns of the myelon 

 come into view, as the 

 J teretial tracts,' ib. A, f, bounding the sides of the fissure, called 

 * calamus scriptorius,' at the floor of the expanded macro - 

 myelonal canal called ( fourth ventricle.' This is over-arched 

 by the cerebellum, here bisected, and one half reflected at R ; the 

 peduncle or f crus ' of the opposite half being shown at u. The 

 thin layer roofing the ventricle anterior to the crus is called 

 ■ valve of Vieussens,' b. 



Sections of the macromyelon, as at fig. 50, show the form of 

 the grey matter, called ' corpus dentatum,' of the 

 olives, o o, and the relative position of the en- 

 larging columns. Those on each side the fissure 

 a, are the prepyramids ; those on each side the 

 fissure P, are the post-pyramids ; the lateral 

 or restiform tracts intervene between them and 

 the olivary tracts, o. 



In the Monotremes the macromyelon is large 

 in proportion to the rest of the brain, but the 

 I pons ' bears relation to the cerebellum in its 

 smallness. The prepyramids, figs. 51 and 52, 

 a, are long, narrow, flat, and contract as they 

 approach the pons, especially in the Ornitho- 

 rhynchus; the olives, fig. 51, «, fig. 52, b, are 

 also long and flat, but expand as they approach the pons, and 

 are crossed, before reaching it, by the ' trapezoid ' homologues 

 of the ' arciform ' fibres in Man. The distinction between the 

 olivary and pre-restiform tracts is less marked. The grey matter 



G 2 





Transverse sections of 

 the macromyelon, at the 

 parts marked x and Y 

 fig. 49. Man, nat. size 



