THE CEREBRUM. 543 



3. The projection system. The fibers composing this system unite certain 

 areas of the cortex of the cerebrum with the basal ganglia, the pons, 

 medulla oblongata, and spinal cord. They may be divided into: (i) 

 afferent fibers which have their origin in the lower nerve-centers at dif- 

 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 corticopetal; the latter, corticoefferent or corticofugal. 

 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. 245, page 530. The 

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

 following 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 paracentral lobule, and immediately 

 adjacent areas. From this origin the axons descend through the white 

 matter of the corona radiata, converging toward the internal capsule, into 

 and through which they pass, occupying the anterior two-thirds of the poste- 

 rior limb or segment. Beyond the capsule they continue to descend, occupy- 

 ing 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 ninety-one to ninety-seven 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 three to nine 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. 237, page 517.) 



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

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



