THE CEREBRUM 589 



ferent levels and thence pass to the cortex; and (2) efferent fibers which 

 have their origin in the cortex and thence pass to the lower nerve-centers, 

 terminating at different levels. The former are also termed the cortico- 

 afferent or cortico-petal; the latter, cortico-efferent or cortico-fugal. 

 The afferent fibers, the so-called sensor tract, which transmit nerve im- 

 pulses coming from the general periphery and the sense-organs, pass through 

 the tegmentum as the mesial and lateral fillets, and thence to the cortex 

 directly by way of the internal capsule, or indirectly through the intermedia- 

 tion of the thalamic and subthalamic nuclei. (See Fig. 242, page 562.) The 

 distribution of these fibers to the various areas of the cortex will be stated in 

 subsequent paragraphs. 



The efferent fibers of the so-called motor tract which transmit motor or 

 volitional nerve impulses from the cortex to the pons, medulla, and spinal 

 cord, emerge from the layer of pyramidal cells of the gray matter of the an- 

 terior or the pre-central convolution, the para-central lobule, and immedi- 

 ately 'adjacent areas. From this origin the axons descend through the 

 white matter of the corona radiata, converging toward the internal cap- 

 sule, into and through which they pass, occupying the anterior two-thirds 

 of the posterior limb or segment. Beyond the capsule they continue to 

 descend, occupying the middle three-fifths of the pes or crusta of the crus 

 cerebri, the ventral portion of the pons, and eventually the anterior 

 pyramid of the medulla oblongata. At this point the tract divides into 

 two portions, viz. : 



1. A large portion, containing from eighty-five to ninety per cent, of 



the fibers, which decussates at the lower border of the medulla and 

 passes down the lateral column of the cord, constituting the crossed 

 pyramidal tract. 



2. A small portion, containing from fifteen to ten per cent, of the fibers, 



which does not decussate at the medulla, but passes down the inner side 

 of the anterior column of the same side, constituting the direct pyramidal 

 tract or column of Tiirck. 



After passing through the internal capsule, and as it descends through 

 the crus, pons, and medulla, the cortico-efferent tract gives off a number of 

 fibers which cross the median line and arborize around the nerve-cells of 

 the gray matter beneath the aqueduct of Sylvius (the nuclei of origin of the 

 third and fourth cranial nerves), and around the nerve-cells in the gray 

 matter beneath the floor of the fourth ventricle (the nuclei of origin of the 

 remainder of the motor cranial nerves). The remaining fibers go to form 

 the crossed and direct pyramidal tracts and arborize around the cells in the 

 anterior horn of the gray matter of the opposite side of the cord at successive 

 levels. By this means the cortex is brought into anatomic and physiologic 

 relation with the general musculature of the body through the various cranial 

 and spinal motor nerves. (See Fig. 231, page 561.) 



The fronto-cerebellar and the occipito-temporo-cerebellar tracts are also 

 efferent tracts and parts of the projection system. The fronto-cerebellar, 

 originating in the nerve-cells of the cortex of the frontal lobe, passes down to 

 and through the internal capsule, occupying the anterior one-third of the 

 anterior segment. It then descends along the inner side of the crus cerebri 

 to the pons, where its fibers arborize around the cells of the nucleus pontis. 

 Through the intermediation of these cells this tract is brought into relation 



