THE BRAIN AND ITS MEMBRANES. 



787 



there are some exceptions. According to Meynert, these exception* are to be 

 found (1) in the posterior portion of the occip- 

 ital lobe; (2) in the gray cortex of the hippo- 

 campus major; (3) in the wall of the fissure of 

 Sylvius ; and (4) in the olfactory bulb. 



The five layers in the common type (from 

 parietal lobe) are as follows : (1) The first (super- 

 ficial or molecular) layer is principally composed 

 of a matrix of neuroglia, through which a few 

 small ganglion-cells are irregularly distributed, 

 and a nerve-fibre network of both non-medullated 

 and medullated fibres, the latter constituting a 

 delicate white lamina almost in contact with the 

 pia mater. Of the former, the majority come 

 from the processes of the pyramidal cells in the 

 next layer, the remainder being made up of both 

 dendrites (protoplasmic processes) and axis-cyl- 

 inder processes of the ganglion-cells in this layer. 



(2) The second layer consists of numerous 

 small pryamidal cells, which have their long axes 

 vertical to the surface of the convolutions, and 

 are closely aggregated together so as to com- 

 pletely fill the layer. The dendrites of each of 

 these cells extend into the preceding layer, while 

 the axis-cylinder process, starting from the base 

 of the cell, gives off a few collaterals and extends 

 through the white centre of the convolution, and 

 thence to the corpus striatum, as a projection-fibre. 



(3) The third layer is made up of cells, which 

 are the same kind as those in the formation of 

 the cornu Ammonis. These cells are large 

 pyramidal cells, arranged vertically to the sur- 

 face, as was found in the preceding layer, but 

 they are of very much larger size, and increase 

 progressively toward the deeper parts of the 

 layer, and they are much more widely separated 

 from each other, thus forming groups between 

 which are radiating nerve-fibres. This layer is 

 the principal and broadest one of the series, and 

 is at least twice as deep as the preceding layer. 

 The axis-cylinder processes of these cells pass 

 into the white substance, and there become med- 

 ullated. Previously each gives off a number of collaterals, which also become 

 medullated and form ramifications in the layer. 



(4) The fourth layer is termed the layer of polymorphous cells, and consists 

 of numerous, small, irregular cells, each of which has numerous dendrites, but 

 only one axis-cylinder process. This last, from most of the cells, passes into 

 the white centre, but from some it goes peripherally to the first layer and becomes 

 continuous with one of its fibres. 



(5) The fifth layer (layer of fusiform cells) consists of a very large proportion 

 of spindle-shaped or fusiform cells, which are the peculiar elements of this layer. 

 They are especially numerous in the inner half, and are arranged horizontally, 

 extending parallel to the surface. The claustrum is made up almost entirely of 

 an accumulation of cells of the same kind. 



The white centre lies just beneath the fifth layer, which gradually blends with 

 it. As its fibres radiate into the cortex they become finer, and most of them are 

 continuous, as stated above, with the axis-cylinder processes of the large pyr- 



FIG. 474. Gray matter of the cerebral 

 cortex. (Meynert.) 



