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. 



FIG. 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); r, roof plate. 



The myelencephalon is most nearly like the spinal cord of any part 

 of the brain. It is triangular hi 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 rhomb oidea). 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 



