576 TEXT-BOOK OF PHYSIOLOGY 



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

 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 througl 

 the tegmen'tum 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.) Th< 

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

 subsequent paragraphs. 



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

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

 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 th( 

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

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

 of the posterior limb or segment. Beyond the capsule they continue t< 

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

 cerebri, the ventral portion of the pons, and eventually the anterio: 

 pyramid of the medulla oblongata. At this point the tract divides int< 

 two portions, viz. : 



1. A large portion, containing from ninety-one to ninety-seven per cent, o 



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

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

 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 sidi 

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

 tract or column of Tiirck. 



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

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

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

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

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

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

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

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

 anterior horn of the gray matter of the opposite side of the cord at successiv< 

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

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

 and spinal motor nerves. (See Fig. 234, page 549.) 



The fronto-cerebellar and the occipito-temporo-cerebellar tracts are als< 

 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 t< 

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

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

 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 relatioi 



