LONGITUDINAL FIBRES OF THE TEGMENTAL SYSTEM. 631 



spinal cord, the other two cease at the pons, and probably through the gray 

 matter of the pons make connections with the cerebellum. Further, while 

 the pyramidal tract coming from the middle region of the cortex occupies a 

 middle position in the capsule and a middle position in the crus, the system 

 from the front part of the cortex occupies a front position in the capsule 

 and an inner or median position in the crus, and the system from the hind 

 part of the cortex, a hind position in the capsule and an outer or lateral 

 position in the crus. As the three systems pass from the cortex through the 

 capsule to form the pes of the crus, their positions in relation to each other 

 are shifted from one plane into another. As the fibres spread out from the 

 pes through the capsule to all parts of the cortex, or, put in another way, as 

 they converge from the cortex through the capsule to the pes, they form a 

 f&n, the corona radiata, which is not only curved, but the constituent parts 

 of which cross each other. 



Besides these three systems all passing from various regions of the cortex 

 to the crus, there is yet a fourth strand contributed to the pes by the cere- 

 bral hemisphere, though not starting in the cortex. From the nucleus 

 caudatus fibres pass down to the crus, and take up a position in the pes 

 dorsal to the tract just mentioned, occupying a lens-shaped area immediately 

 ventral to the substantia nigra, and probably passing into the substantia 

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

 they appear to end, though possibly some terminate higher up in the sub- 

 stantia nigra. This tract has a descending degeneration, and may be re- 

 garded 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 conspicuous 

 strands. 



LONGITUDINAL FIBRES OF THE TEGMENTAL SYSTEM. 



546. Cortical fibres. Although the fibres of the pedal system form, as 

 we have seen, the greater part, 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. 139, /. I.) and appear to end in the gray 

 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 ex- 

 treme end of the anterior limb in front of the strand of the fibres of the 

 pedal system (Fig. 144, 2Vi.). The fibres from the occipital and tem- 

 poral 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. 144, 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 pass- 

 ing 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, 



