466 



THE TISSUES, 



or fasciculi, but all run in parallel and generally straight lines, and 

 certainly proceed from the ganglia of the cerebrum and the corpus 

 callosum to the gray substance of the central convolutions. There 

 are also other fibres crossing the former at right angles (commis- 

 sural fibres), of whose origin nothing satisfactory is yet known. 



The gray matter of the cerebrum is principally situated externally, 

 covering the convolutions, and being J to \ of an inch thick. It 

 contains in its whole thickness both nerve-cells and nerve-fibres; 

 besides a large amount of granular homogeneous substance pre- 

 cisely like that of the cerebellum. It is, however, conveniently 

 divided into three layers ; 1, an internal yellowish-red; 2, a middle, 

 pure gray; and 3, an external, white. The first mentioned, however, 



Fig. 310. 



Nerve-cells from the internal portions of the gray layer of the convolutions of the human cere- 

 brum, a. Larger, b. Smaller, c. Nerve-fibre with axis-cylinder. Magnified 350 diameters. (Ki'l- 

 liker.) 



constituting almost one-half the entire thickness of the gray mat- 

 ter, may itself be divided into four layers ; 1, a yellowish-red layer 

 (inner part) ; 2, the inner white streak ; 3, yellowish-red layer (outer 

 part) ; 4, outer white streak. Then come the two remaining layers 

 above ; 5, the pure gray, and 6, the white. The nerve-cells through- 

 out the gray matter have from 1 to 6 processes giving off numerous 

 branches, ultimately becoming very fine pale fibrils of 333^ of an 



