CHAP, ii.] THE BRAIN. 991 



cortex. From the nucleus caudatus fibres pass down to the crus, 

 and take up a position in the pes dorsal to the tracts just men- 

 tioned, occupying a lens-shaped area immediately ventral to the 

 substantia nigra, and probably passing into the substantia nigra 

 itself. These cannot be traced farther down than the pons, where 

 they appear to end, though possibly some terminate higher up 

 in the substantia nigra. This tract has a descending degene- 

 ration, and may be regarded as a tract analogous to the front and 

 hind cortical tracts, though it begins not in the cortex but in 

 the nucleus caudatus ; it is not however a very pure tract, many 

 fibres of the pyramidal tract passing into it in the pes. 



These are the main tracts of the pedal system. For, though 

 the nucleus lenticularis gives off fibres to the internal capsule, 

 our knowledge of the further course of these is at present 

 imperfect, and though there seem to be longitudinal fibres 

 connecting the bulb, the pons, and the pes at various levels, 

 these are not numerous, and at all events do not form con- 

 spicuous strands. 



Longitudinal Fibres of the Tegmental System. 



633. Cortical Fibres. Although the fibres of the pedal 

 system form, as we have seen, the greater part of, they do not 

 form the whole of, the internal capsule. Fibres coming from all 

 or nearly all parts of the cortex though they help to form the 

 internal capsule, do not go on to form the pes, but pass to the optic 

 thalamus (Fig. 116, LI.) and appear to end in the grey matter of 

 that body. In their passage through the capsule the fibres of 

 this nature from the frontal and parietal regions of the cortex, 

 occupy the extreme front end of the front limb in front of the 

 frontal strand of the fibres of the pedal system (Fig. 121, Th.). 

 The fibres from the occipital and temporal regions, those from 

 the occipital regions being the most numerous and indeed being 

 very conspicuous, occupy the extreme hind end of the hind limb 

 of the capsule, behind the temporo-occipital division of the pedal 

 system (Fig. 121, Op.). Since, as we shall see, we have reason 

 to associate the occipital region of the cortex with vision, the 

 fibres thus radiating to (or from) the thalamus through the 

 extreme hind limb of the capsule from (or to) the occipital 

 cortex have been called the optic radiation. 



All the above tracts of fibres, though joining the thalamus 

 and not passing on to the pes, take part in the formation of the 

 internal capsule. But a considerable number of fibres coming 

 from the temporo-occipital region and especially from the temporal 

 region pass to the thalamus without joining the capsule ; they 

 pass ventral to and behind the pes as this plunges into the 

 hemisphere to become the capsule, and so reach the thalamus. 



F. 63 



