CONNECTIONS OF CORTEX WITH LOWER CENTRES. 777 



(motor) areas of the cortex are connected with the mesencephalic, the 

 pontine, the bulbar, and the spinal motor centres by means of the fibres 

 of the pyramidal tract, which constitute the projection fibres of this 

 portion of the cortex. These fibres take origin in some of the cells of the 

 motor cortex perhaps in the "giant cells," which were described there by 

 Betz and by Bevan Lewis and pass in the corona radiata directly to the 

 middle two-thirds of the internal capsule. This middle two-thirds is the 

 only part of the capsule which is directly excitable by artificial means; 

 the anterior and posterior sixths being apparently inexcitable. The anterior 

 sixth is largely composed of fibres which come from the prefrontal region 

 of the hemisphere, while the posterior sixth appears to be chiefly composed 

 of afferent (sensory) fibres, which probably have their origin for the most 

 part in the thalamus. As the fibres of the pyramidal tract course down- 

 wards through the internal capsule they show a tendency to be arranged in 

 such a way that the fibres which come from the region for the head and eyes 

 and from that subserving movements of the tongue, are chiefly accumulated 

 in the anterior half of the excitable region ; those from the face, from the 

 upper limb, from the trunk, and from the lower limb in the posterior half. 1 

 There is, however, as the study of the detailed results obtained by Beevor 

 and Horsley shows, 2 a considerable overlapping of .the representation of the 

 several parts above enumerated, and this overlapping and intermingling 

 tends to increase as the fibres are traced downwards, and becomes even 

 more marked in the crusta. Here such arrangement as can be detected by 

 localised stimulation shows that the order of fibres from within out is also 

 face, upper limb, trunk, lower limb. 3 The intermingling which obtains is 

 strikingly exemplified by the results of degeneration after localised lesions of 

 the cortex, since after lesions which are strictly limited to one of the excitable 

 areas of the Kolandic cortex, degenerated fibres are found scattered over 

 almost the whole of the excitable part of the capsule (although more numerous 

 in some parts than in others), and over the whole of the excitable part of the 

 crusta. 4 Diagrams, therefore, representing the excitable (motor) part of the 

 internal capsule, as divided up into segments, each of which contains fibres 

 subserving only the movement of a particular part of the body, convey an 

 erroneous impression of the actual distribution of the pyramidal fibres in the 

 capsule. All that can be definitely stated is, that there is a greater accumu- 

 lation of the fibres subserving the movement of this or that part in one section 

 than in the others. 



The pyramidal fibres, after arising from the cortical cells of the Rolandic 

 area, first give off collaterals which pass into the adjacent grey matter, and 

 perhaps others which pass into the corpus callosum, and are then continued 

 into the internal capsule, the middle part of the crusta, and the pyramid 

 bundles of the pons, to be collected in the medulla oblongata into the distinct 

 masses which are known as the pyramids. Many fibres belonging to the 

 pyramidal system end in the substantia nigra and in the grey matter of the 

 pons and medulla oblongata. Arrived at the lower part of the medulla 

 oblongata, the fibres of the pyramids enter the spinal cord in three tracts. By 

 far the larger number pass directly at the decussation of the pyramids into the 

 opposite lateral column of the spinal cord, forming the crossed lateral 

 pyramidal fibres. A certain number pass into the lateral tract of the same 

 side of the cord, 5 and become intermingled with the fibres of the crossed 



1 Franpois-Franck and Pitres, Compt. rend. Soc. de biol., Paris, 1877 ; Beevor and 

 Horsley, Phil. Trans., London, 1890. 



2 See especially Figs. A to R, plates 5 to 9, Phil. Trans., London, 1890, B. 



3 Beevor and Horsley, " Proc. Physiol. Cong.," Journ. Physiol., Cambridge and 

 London, 1898, vol. xxiii. 



4 Cf. Mellns, Proc. Roy. Soc. London, 1895, vol. Iviii. p. 206. 



5 Francois-Franck and Pitres, Proyres, med., Paris, 1880; Schafer, Journ. Physiol., 

 Cambridge and London, 1883, vol. iv. ; Muratotf, Arch. f. Anat. u. Entu-cklngsgcsch. , 



