1026 HISTOLOGY OF CORTEX. [BOOK in. 



lymphatic and vascular arrangements of the brain and spinal 

 cord. 



The Cerebral Cortex. 



649. While the superficial grey matter of the cerebellum 

 does not differ strikingly as to its histological features in different 

 regions, very considerable differences are observed in different 

 regions of the cerebral cortex. A general plan of structure may 

 perhaps be recognized, but as we pass from one part of the cerebral 

 surface to another we find modifications continually taking place. 

 We must content ourselves here with attempting a description of 

 the general plan followed by an indication of the more striking 

 characteristics of certain regions. 



The cortical grey matter, having an average thickness of about 

 3 mm., but varying considerably in different regions from 1*8 mm. 

 in some parts of the occipital lobe to 4'2 at the dorsal summit 

 of the precentral convolution, is, like other grey matter, composed 

 of nerve cells, and of nerve fibres and fibrils supported by neu- 

 roglia. The nerve cells, at least the conspicuous and easily 

 recognized nerve cells, are scattered, and appear, in sections, 

 to be imbedded in, and separated from each other by a not in- 

 considerable but variable quantity of somewhat peculiar ground 

 substance, not unlike that which forms so large a part of the 

 molecular layer of the cerebellum. Part of this ground substance, 

 which apparently is not confined to any particular layer, but 

 stretches throughout the thickness of the cortex is undoubtedly 

 neuroglial in nature, but part, and probably the greater part, is 

 nervous in nature ; it is largely composed of fine fibrils traversing 

 it in various directions, the transverse sections of these fibrils giving 

 it a characteristic dotted or ' molecular ' appearance ; and the ma- 

 jority of these fine fibrils are probably the continuations of 

 branching nerve cells or dividing nerve fibres, the remainder being 

 neuroglial fibrils. In this respect it resembles the molecular layer 

 of the cerebellum, but it is, to a much greater extent than is that 

 layer, traversed by medullated nerve fibres, especially by fine me- 

 dullated fibres like those seen in the grey matter of the spinal 

 cord, 563. 



The nerve cells imbedded in this ground substance in more or 

 less distinct layers are of various kinds. The most conspicuous, 

 abundant and characteristic nerve cells found in the cortex of 

 all regions of the cerebrum, are those which from their shape 

 are called pyramidal cells. These vary much in size and have 

 been distinguished as 'small pyramidal' cells averaging 12 p in 

 length by 8 p in breadth, and ' large pyramidal ' cells, sometimes 

 called ' ganglionic cells/ of which the medium size is about 40 //, 

 in length by 20 p in breadth. Some of the latter, occurring in 



