THE BRAIN. 419 



fine diverging processes, whose termination and connections are un- 

 known. 



As the bundles of nerve fibres penetrate the gray substance, they 

 rapidly diminish in size, their fibres diverging laterally to pursue a more 

 or less horizontal course ; and in the external portions of the gray sub- 

 stance there are only isolated fibres running in various directions. 

 During this dispersion, the nerve fibres become reduced to their smallest 

 dimensions, measuring, according to Kolliker, from 1 to 2 mmm. in diam- 

 eter. Most of them spread out at various levels in the gray substance, 

 while others reach quite to its superficial portions. 



Structure of the Gray Substance in Special Parts of the Hemi- 

 spheres. The gray substance of the convolutions presents certain 

 peculiarities in particular regions, the most important of which are 

 those described by Betz.* These observations, which were based on 

 the examination of more than one thousand sections, show that there 

 are differences of structure in the gray substance characteristic of ex- 

 tensive portions of the hemispheres. The cerebral surface is divided, 

 in this respect, by the fissure of Rolando into two main departments, 

 an anterior and a posterior. The anterior department, including the 

 convexity of the frontal lobe, its under surface resting on the orbital 

 plate, and its median surface at the great longitudinal fissure, is distin- 

 guished by the preponderance, in its gray substance, of the layer of 

 pyramidal cells. In the posterior department, on the other hand, in- 

 cluding both the occipital and temporal lobes, the nuclear layer pre- 

 dominates, the pyramidal cells being less abundant. 



Furthermore, at the posterior border of each of these two depart- 

 ments there is a special region, characterized by cells of a particular 

 variety. In front is the region of the so-called giant pyramidal cells. 

 It occupies the whole of the anterior central convolution and the upper 

 end of the posterior central convolution, and extends into the "para- 

 central lobule," which is a continuation of these two convolutions on 

 the median surface of the hemisphere. The pyramidal cells in this 

 region, as their name implies, are the largest in the brain, approxi- 

 mating and often equalling in size those of the anterior horns of gray 

 substance in the spinal cord. They are from 40 to 60 mmm. in width, 

 and from 50 to 120 mmm. in length. They all have a number of 

 radiating- processes, among which are two principal ones. One of them, 

 given off from the point of the pyramidal cell, runs in a tapering and 

 branching form toward the external surface of the convolution. The 

 other, which is given off from the base of the cell and runs inward 

 toward the white substance, is slender at its commencement, but soon 

 grows thicker and acquires a medullary layer, assuming the appearance 

 of a nerve fibre. 



The posterior special region occupies the extremity of the occipital 

 lobe. Its characteristic cells are of rather large size, but have few 



* Centralblatt fur die medicinische Wissenschaften. Berlin, 1874. Xos. 37 and 38. 



