992 FIBRES OF THE TEGMENTAL SYSTEM. [BOOK in. 



We may here perhaps diverge for a moment to point out the 

 contrast between the optic thalamus and the corpus striatum, 

 or at least the nucleus candatus. The former does not contribute 

 to the pedal system, the latter supplies a marked contribution. 

 The former receives fibres from all parts of the cortex ; there are 

 no such special contributions from the cortex to the latter. And 

 this difference accords with the experience that when parts of 

 the cortex are removed, or are congenitally absent, no degenera- 

 tion or want of development is observed in the corpus striatum, 

 while degeneration or want of development is observed in the 

 optic thalamus as well as in parts of the pedal and tegmental 

 systems. Hence, while we may regard the optic thalamus as an 

 intermediate mass of grey matter receiving fibres from the cortex, 

 and connecting the cortex with lower parts of the tegmental 

 region, the corpus striatum, appears rather to be analogous to the 

 cortex itself, to be a special modification of the cortex, sending 

 fibres down into the pedal system, but itself receiving no special 

 tracts of fibres from the cortex. Indeed we may probably regard 

 the corpus striatum as the oldest part of the superficial grey 

 matter of the hemisphere, the more ordinary cortex being a later 

 development. 



The tegmentum proper, lying ventral to the hind end of, and 

 behind the thalamus, in which region as we have seen the con- 

 spicuous red nucleus is situated, is thus, by reason of its connection 

 with the thalamus, indirectly connected with the cortex. But 

 besides this, it has direct connections of its own with the cortex. 

 Some of the fibres of the optic radiation, as well as fibres from the 

 temporal and occipital regions described above as sweeping round 

 the base of the internal capsule, are said to pass not to the thala- 

 mus, but to the tegmentum. Other fibres from the frontal and 

 parietal regions traversing the lenticular nucleus in the sheets 

 of white matter splitting the nucleus into parts, are also said 

 to reach the tegmentum either by piercing through or by 

 sweeping round the internal capsule. On their path these fibres 

 are, according to some observers, joined by fibres coming from the 

 lenticular nucleus itself, and possibly from the caudate nucleus, 

 which fibres, on the view that these nuclei are modified cortex, 

 may also be considered as cortical. Thus the forepart of the 

 tegmental region is brought into ample connection with the 

 cerebral hemisphere partly by fibres joining the thalamus, partly 

 by fibres passing directly to the tegmentum proper. 



The mode of degeneration of these cortical fibres of the 



) tegmental system is at present a matter of dispute. Nor is the 



general nature of the fibres conclusively determined, though it is 



v generally supposed that they carry impulses from the thalamus 



' and parts of the tegmentum to the cortex. 



634. In the tegmentum from the subthalamic region to 

 the bulb the reticular formation is, as we have seen, more or less 



