632 THE BRAIN. 



and especially from the temporal region, pass to the thalamus without join- 

 ing 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. 



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 

 caudatus. 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 degeneration 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 gray 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 super- 

 ficial gray 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 conspicuous 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 thalamus, 

 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 len- 

 ticular 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 fore part of the tegmental region is brought into ample 

 connection with the cerebral hemisphere partly by fibres joining the thala- 

 mus, 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 generally supposed that they carry 

 impulses from the thalamus and parts of the tegmentum to the cortex. 



547. In the tegmentum from the subthalamic region to the bulb the 

 reticular formation is, as we have seen, more or less abundant ; this, and the 

 occurrence of various bundles of fibres, gives the region great complexity ; 

 and we must confine ourselves here to touching on one or two important 

 longitudinal strands which traverse it. 



The superior peduncle of the cerebellum is one of the most important of 

 these. This, on each side consists of a bundle of fibres which, taking origin 

 chiefly from the gray matter of the nucleus dentatus and the smaller neighbor- 

 ing collection of gray matter, and also in part from the superficial gray mat- 

 ter, leaves the cerebellum in front of and to the median side of the restiform 

 body and passes forward toward the corpora quadrigemina to converge with 

 its fellow. At first the two peduncles are superficial and dorsal in position 

 (Figs. 134, 135, S. P.) and the space between them is bridged over by the 



