446 



THE INTERNAL STRUCTURE 



are composed for the most part of an extremely delicate 

 reticulum of fibres (probably most akin 



*^"~~ W*" J "^j 9*" ^ V ^M 



^^7 ] ^ to the ' neuroglia'), that composing the 

 * j / ^ outermost layer being in direct con- 



tinuity with the thin and very vascular 

 membrane (the ' pia mater ') which 

 covers the whole surface of the Brain 

 and dips down between the sulci. 



The fibres of the central white stem 

 itself are crossed transversely and ob- 

 liquely by a variable number of other 

 fibres, generally most numerous near 

 its base, where, according to Lockhart 

 Clarke, they cross one another in all 

 directions. These, he thinks, probably 

 consist, for the most part, of ' com- 

 missural fibres,' such as will be de- 

 scribed in the next section. 



Other investigators have since exa- 

 mined the structure of the Grey Matter 

 in several Convolutions situated in dif- 

 ferent parts of the Hemisphere. Al- 

 though differences of detail exist, there 

 is nevertheless considerable uniformity 

 in the type of structure met with. Over 

 much of the Frontal and Parietal Lobes, 

 Meynert describes the Grey Matter as 

 divisible, not so much by ordinary sight 

 as by the microscopic characters of its 

 constituents, into five layers or ' la- 

 minae.' He gives a figure of the arrange- 



FIG. 159. Section through one of the Folds of the Third Frontal Convolution of 

 Man. Magnified 65 diameters. (Ferrier, after Meynert.) 1. Layer of small scattered 

 corpuscles, principally belonging to the ' neuroglia ' ; 2, layer of close-set small 

 pyramidal cells ; 3, layer of large pyramidal cells ; 4, layer of small close-set irregu- 

 larly shaped corpuscles (this lamina in si me regions is occupied by ' giant ' cells) ; 

 5, layer of spindle-shaped corpuscles ; m white or medullary lamina. 



