Chapter 12 Central Motor Pathways 343 Figure 12.4 The somatotopic organization of the motor cortex. The points indicate the sites from which musc1e contraction in a particular body part was produced by weak electrical stimulation. Based on Foerster (1936) Electrical stimulation and localized lesions of the precentral gyros (also in humans) have clarified the somatotopic localization within MI (Fig. 12.4). It corresponds closely to the localization in SI (Fig. 7.21). As mentioned above, the pyramidal tract passes downward through the internal capsule, where it occupies a posterior position (Fig. 12.5). It was formerly assumed that the pyramidal tract fibers were spread over a large part of the "posterior leg" of the internal capsule (as seen in cross section in Figs. 3.30 and 12.5). Observations af ter small lesions (Fig. 12.9) and stimulation during brain surgery of the internal capsule in humans point to the more restricted region shown in Figure 12.5. Within this re- gion, fibers governing the muscles of the face are believed to lie most anteriorly. Experimental studies in monkeys show that descending fibers from the PMA and supplementary motor area (SMA) are found in the "knee" region (genu) and the "anterior leg" of the internai capsule, respectively. The internal capsule contains many other fibers than those of the pyramidal tract; the latter constitute only a minority. There are, for example, => page 361    start page