144 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
longata, which also bends in the same direction. The pontal flexure, 
beneath the cerebellum, bends in the opposite direction and thus 
tends to counteract the other two. Nuchal and pontal flexures are at 
best but weakly developed in the ichthyopsida and all are practically 
obliterated in the adult, but in the amniotes they are increasingly 
developed and persist through life (fig. 148). 
The brain, like the spinal cord, is composed of nerve cells (gray 
matter) and fibres (white matter), but their arrangement is exceedingly 
complicated and but the slightest outline of their distribution can be 
attempted here, in connection with the general account of the regions 
of the brain. 
Fic. 149.—Cross-section of medulla of Acanthias embryo, 60 mm. long, showing the 
greatly broadened roof plate and, below, a bit of the meninx of the nervous system. c, 
cartilage of basal plate; e, ependyma; mp, meninx primitiva; pc, perichondrium (endo- 
rhachis); 7, roof plate. 
The myelencephalon is most nearly like the spinal cord of any part 
of the brain. It is triangular in outline, viewed from above, and is 
widest anteriorly, due in part to the separation of the side walls by the 
great development of the roof plate over the fourth ventricle. Blood- 
vessels press against the roof, carrying parts of it before them into the 
ventricle, thus forming the chorioid plexus of the fourth ventricle, a 
means of introducing nourishment into the brain. (Usually in dis- 
sections this roof is torn away, leaving a triangular or rhomboid opening 
into the fourth ventricle—fossa rhomboidea). The floor plate in 
this region is obliterated by the development of numerous nerve centres 
—‘nuclei’ or ganglia—in the walls, some closely connected with the 
