STRUCTURE OF THE CEREBRUM. 677 



STRUCTURE OF THE CEREBRUM. 



The cerebrum, like the other parts of the great nerve centre, is composed of 

 gray and white matter. In order to give some general idea of its construction, at 

 all events in part, it may be compared, for the sake of illustration, to a tree, the 

 trunk of which divides into two main divisions, and these break up into smaller 

 branches, which finally end in twigs, to which are attached the leaves, forming an 

 investment to the branches and covering the whole tree. The trunk is represented 

 by the medulla oblongata as it passes through the foramen magnum ; the two main 

 divisions by the crura cerebri, which break up into smaller branches ; these diverge 

 from each other, dividing and subdividing, until they reach the surface of the 

 hemispheres, where they terminate in single nerve-fibres, which are continuous with 

 the basal axial cylinder processes of the nerve-cells, the representatives of the 

 leaves. These cells are arranged on the surface, resembling a cap, covering the 

 hemispheres, and constitute the cerebral cortex. But here the analogy ends, for 

 in the cerebrum there are, in addition to this cortex, other masses of gray matter 

 situated in the middle of the brain ; and other white fibres besides the diverging 

 ones that have been mentioned, and which serve either to connect the two cerebral 

 hemispheres, or to unite different structures in the same hemisphere. 



The white matter of the cerebrum consists of medullated fibres, varying in size 

 and arranged in bundles, separated by neuroglia. They may be divided into three 

 distinct systems, according to the course they take. 1. Projection or peduncular 

 fibres, which connect the hemisphere with the medulla oblongata and cord. 2. 

 Transverse or commissural fibres, which unite together the two hemispheres. 3. 

 Association fibres, which connect different structures in the same hemisphere. 

 These are, in many instances, collateral branches of the projection fibres, but 

 others are the axons of independent cells. 



1. The projection or peduncular fibres consist of fibres which pass either to or 

 from the cord. They form the longitudinal fibres of the pons, and at its upper 

 border divide into two main groups, which, diverging from each other, constitute 

 the crura cerebri or cerebral peduncles. In the crura cerebri, as has been before 

 described, the diverging fibres are arranged in two strata, which are separated by 

 the substantia nigra ; the ventral or superficial stratum forming the crusta of these 

 bodies, and the dorsal or deeper stratum, the tegmentum. The fibres derived 

 from these two sources take a different course, and will have to be separately 

 considered. 



The fibres of the crusta are derived from the pyramid of the medulla, and are 

 continued upward through the pons ; they are reinforced in their passage through 

 the crus by accessory fibres, derived from the central gray nucleus around the 

 Sylvian aqueduct and from the substantia nigra. When they emerge from the 

 crus, most of the fibres pass through the internal capsule, and when they leave it, 

 spread out forward, upward, and backward, forming a series of radiating fibres, 

 the corona radiata, which proceed to the cortex. As the fibres pass through the 

 internal capsule they give off branches to the optic thalamus and to the caudate 

 and lenticular nuclei of the striate body, and other fibres, derived especially from 

 the first of these ganglia, form a part of the corona radiata, and pass to the cortex 

 of the cerebral hemispheres. The fibres of the tegmentum are continuous with 

 those longitudinal fibres of the pons which are derived from the nucleus gracilis 

 and nucleus cuneatus, and from the formatio reticularis of the medulla. They are 

 reinforced by fibres from the corpora quadrigemina and the corpora geniculata, and 

 from the superior peduncle of the cerebellum. Some of the fibres are continued 

 directly to the cerebral cortex, but the majority pass to the subthalamic region, 

 and either end there or in the substance of the optic thalamus the connection 

 with the cortex being effected by means of fibres which arise in the optic thalamus. 

 They spread out to form part of the corona radiata, and are distributed especially 

 to the cortex of the temporal and occipital lobes. 



2. The transverse or commissural fibres connect the two hemispheres. They 



