Chapter IV 



THALMIC TERMINATION OF AFFERENT SOMATO-SENSORY 

 FIBER TRACTS. INTRATHALAMIC FIBER SYSTEMS 



Four of the experimental injuries reported in the present mono- 

 ^aph involved the thalamus or the adjacent internal capsule causing 

 various intrathalamac fiber system to degenerate (for the description 

 of lesions see Chapter V). These fiber systems were : 



1. Ascending fiber tracts from the lower centers of the neuraxis. 



2. Fibers of thalamic origin which enter the basal nuclei and the 

 cerebral cortex. 



3. Fibers originating from the cerebral cortex and, entering the 

 thalamus ; these either terminate here or merely traverse it on their 

 way to the midbrain. 



4. Fibers which originate and terminate in the thalamus. In a 

 restricted sense, only the latter fibers are validly intrathalamic. 



On the thalamic termination of fiber tracts ascending from lower 

 levels of the neuraxis the present experiments, natural, can give only 

 general information. Nevertheless, they throw new light upon the 

 subject (Experiment II especially). From the thalamic injury which 

 occupies approximately the position of the ventral portion of the 

 external medullary lamina (figs. 50 and 51) numerous degenerated 

 fibers in dense fascicles ascend dorsalward into the lateral nucleus, 

 crossing at right angles the emerging horizontal thalamo-cortical fibers 

 (figs. 48, 50, 51). These fibers, of different size though mostly of large 

 caliber, evidently belong to the incoming fiber tracts from the lower 

 segmental regions, that is, to the medial fillet (figs. 50, 51), to the 

 cerebello-thalamic tract (fig. 48), and perhaps also to the spino-bulbo- 

 thalamic tracts. These ascending intrathalamic fibers gradually 

 decrease in number and in size toward the dorso-lateral nucleus, and 

 usually disappear before attaining the upper half of the dorso-lateral 

 nucleus. It would, accordingly, appear as though only the ventro- 

 lateral nucleus and the lower half of the dorso-lateral nucleus receive 

 these afferent fibers. Yet it is to be remembered that these fibers lose 

 their myelin sheaths before entering their terminal ramifications. For 



