422 HISTOGENESIS OF THE BRAIN. 



The inftmdibulum or perhaps rather the point of origin 

 of the optic nerves is to be regarded as the anterior termination 

 of the axis of the base of the brain. 



The cranial flexure is least marked in Cyclostomata (fig. 253), Teleostei, 

 Ganoidei, and Amphibia, while it is very pronounced in Elasmobranchii, 

 Reptilia, Aves, and Mammalia. In Teleostei, and still more in Cyclostomata, 

 it permanently remains slight, owing to the small development of the 

 cerebral hemispheres. 



In addition to the cranial flexures, two other flexures make their 

 appearance in the base of the brain. A posterior at the junction of the 

 brain and spinal cord, and an anterior at the boundary between the 

 cerebellum and medulla oblongata, just at the point where the pons Varolii 

 is formed in Mammalia. The anterior of these is the most marked and 

 constant ; it is shewn in fig. 250. It arises considerably later than the main 

 cranial flexure, and since it is turned the opposite way it assists to a con- 

 siderable extent in causing the apparent straightening of the cranial axis. 



Histogenetic changes 1 . The walls of the brain are at first 

 very thin and, like those of the spinal cord, are formed of a 

 number of ranges of spindle-shaped cells. The processes of 

 each of these cells are stated to be continued through the whole 

 thickness of the wall. In the floor of the hind- and mid-brain a 

 superficial layer of delicate nerve-fibres is formed at an early 

 period. This layer appears in the first instance on the floor and 

 sides of the hind-brain, and very slightly, if at all, later on the 

 floor and the sides of the mid-brain. The cells internal to the 

 nerve-fibres become differentiated into an innermost epithelial 

 layer lining the cavities of the ventricles, and an outer layer of 

 grey matter. 



The similarity of the primitive arrangement and histological 

 character of the parts of the brain behind the cerebral hemi- 

 spheres to that of the spinal cord is very conclusively shewn by 

 the examination of any good series of sections. In both brain 

 and spinal cord the white matter forms a cap on the ventral and 

 lateral parts considerably before it extends to the dorsal surface. 

 In the medulla the white matter does not eventually extend to 

 the roof owing to the peculiar degeneration which that part 

 undergoes. 



1 It is not within the scope of this work to give an account of the histogenesis of 

 the brain; in the statement in the text only a few points, of some morphological 

 importance, are touched on. 



