ANATOMY OF VERTEBRATES. 



reinforced by masses of grey neurine, and the transverse commis- 

 sural fibres are so intermixed with the longitudinal ones as to 

 compel their being combined in description as in delineation, 

 figs. 48, 56. But, before quitting the Mammalian class, the 

 reduction of the ( pons,' concomitantly with that of the side-lobes 

 of the cerebellum, as in figs. 51 and 53, is such as significantly 

 to testify against its title to be regarded as a primary division of 

 the brain ; and in birds a ( tuber annulare ' or ' pons varolii,' 

 ceases to appear upon the under surface of the myelencephalous 

 tract above defined. From this tract the cerebral nerves, from 

 the fifth to the hypoglossal or ninth inclusive, arise. 



In advancing to the formation of the macromyelon growing 



central tracts of the myelonal 

 columns come to the peri- 

 phery, and push aside the medial 

 tracts on both the ventral and 

 dorsal surfaces. On the former, 

 fig. 48, they decussate, as they 

 appear, at d, and, with a con- 

 tiguous portion of the anterior 

 myelonal columns, b, expand 

 to form the ' prepyramidal 

 bodies,' p. The rest of the 

 anterior columns, b, with the 

 contiguous antero-lateral co- 

 lumn, in their course along the 

 macromyelon, are associated 

 with a mass of grey matter oc- 

 casioning a swelling out of the 

 surface, called the f olivary 

 bodies,' ib. o. A thin layer of 

 superficial fibres which, in lower Mammals with non-prominent 

 ( olives ' pass outward, as a ' trapezoid layer,' in Man curve round 

 the exterior of the olivary prominences, and constitute the ' arci- 

 form fibres,' ib. A. 



The transverse fibres defining anteriorly the ( prepyramids ' 

 and ' olives ' increase in mass, from the lowest Mammals ( Orni- 

 thorhynchus, fig. 51, c, Didelphys, fig. 53, b\ to Man, fig. 48, a. 

 As they arch over the fore part of those macromyelonal tracts 

 they have been called ' pons ;' but their true position is that of an 

 inverted or suspended bridge : their developement is in the ratio 

 of that of the side-lobes of the cerebellum. 



On the posterior or dorsal surface of the myelon the deep- 



Macromyelon, anterior or ventral aspect. 

 Man, nat. size. 





