506 THE NERVOUS SYSTEM. 



cells of the various layers without being anatomically in connection 

 with them. If the axis-cylinder processes of tbe pyramidal cells 

 are traced, it will be found that they take various courses. Some, 

 commissural fibers, pass either directly or by collaterals through the 

 corpus callosum from one hemisphere to the other ; some join 

 association fibers, and pass into the gray matter of other parts of 

 the same hemisphere as that in which they originated ; while 

 others, projection fibers and this is the course particularly of those 

 having their origin in the largest pyramidal cells pass downward 

 through the corona radiata, internal capsule, and pyramidal tract. 

 The number of pyramidal fibers has been estimated at 158,222 

 by Blocq and Ozanoif. 



The white matter of the cerebrum, consisting of medullated 

 nerve-fibers, may then be divided into three groups : 



1. Those fibers that connect the cerebrum with the medulla 

 oblongata, pons Varolii, and spinal cord. These are the crura 

 cerebri or cerebral peduncles; hence the group is described as 

 peduncular or projection fibers. It will be remembered that the 

 crura cerebri consist of the crusta and tegmentum. The fibers 

 which come from the pyramids of the medulla and are continued 

 through the pons aid in forming the crusta. To these fibers are 

 added others which originate in the gray matter of the aqueduct 

 of Sylvius and in the locus niger. 



After forming the crura cerebri the fibers pass upward : some 

 of them go directly to the gray matter of the cortex : these form 

 the corona radiata ; others go to the internal capsule, and thence 

 to the corpora striata, where they terminate ; while some of the 

 others continue on, receiving fibers from these bodies, and together 

 they assist in forming the corona radiata. More fibers come from 

 the corpora striata than end there, so that the number of those 

 which emerge is greater than the number of those which enter. 



The tegmentum of the crus is made up of fibers from the 

 anterior and lateral columns of the cord, the olivary body, funiculus 

 cuneatus, funiculus gracilis, corpora quadrigemina, corpora genicu- 

 lata, and the superior peduncles of the cerebellum. These 

 fibers pass into the optic thalami, some terminating there, while 

 others pass through. To these latter are added fibers having their 

 origin in the optic thalami, and together they assist in forming 

 the corona radiata, being traced to the cells in the cortical sub- 

 stance of the temporosphenoidal and occipital lobes. 



2. The second group of fibers in the cerebrum consists of those 

 which connect the hemispheres and the basal ganglia, and are the 

 transverse or commissural fibers. They compose the corpus callo- 

 sum and the anterior and posterior commissures. The fibers of 

 the corpus callosum connect the hemispheres, being traced into 

 the convolutions and intersecting those of the corona radiata. The 

 anterior commissure connects the corpora striata, and then passes 

 through these bodies into the temporosphenoidal lobe. Some of 



