24 University of California Puhlications in Anatomy [^^oh. 2 



this reason and for the reason that in many sections deg^enerated fillet 

 fi.bers have been traced to the most dorsal comer of the dorso-lateral 

 nucleus, it is much more probable that the aiferent sensory tracts 

 attain all parts of the lateral nucleus including' its dorsal portion close 

 to the zonal stratum. Still another feature, however, seems important 

 in understanding: the mode of distribution of afferent sensory fibers 

 within the thalamus. Figures 50 and 51 suggest the fairly regular 

 course of the bundles of incoming fibers, arranged in semicircles 

 parallel to the slightly bulging' lateral contour of the lateral nucleus. 

 They mix little with one another and chang-e their course little. 

 Besides, the stream of afferent fibers occupies almost the entire width 

 of the lateral nucleus between the external and the internal medullary 

 lamina and is most dense midway between them. This observation 

 sug'g'ests that the afferent thalamo-petal tracts occupy not merely a 

 narrow zone beside the external medullary lamina, as still believed by 

 some investig'ators (Ingvar, 1925), but are really distributed to the 

 entire lateral nucleus of the thalamus. Moreover, the gradual dis- 

 appearance of all afferent fibers within the thalamus and their course, 

 different from that of the thalamo-cortical fibers, malces it also appear 

 fairly evident that there is no basis for accepting the existence of a 

 fillet system continuous to the cortex ; for, all afferent sensory fibers 

 from lower reg'ions end within the thalamus. With respect to the 

 spino-thalamic tract, this was further substantiated by an experiment 

 with Macacus not further reported here. 



The present experiments show numerous other degenerated fibers 

 merely traversing the thalamus in their course from the hemisphere 

 to lower reg^ions of the neuraxis. Such fibers are particularly con- 

 spicuous in the ventral and ventro-lateral portions of the pulvinar 

 (figs. 34-36, 96). They traverse this thalamic region in more or 

 less horizontal, thin bundles containing fine as well as medium sized 

 fibers, and a few coarser ones, all forming a fairly dense meshwork 

 condensed in a few spots to thicker bundles. Eventually they all con- 

 verg^e toward the inner border of the pulvinar where they form a 

 single trunk connecting the pulvinar with the anterior quadrigeminal 

 body, called the brachium or arm of the superior colliculus (fi^. 36, 

 96). In Experiments I and XV (the latter not reported here in 

 detail), where the occipital lobe or the angular convolution was 

 destroyed these fibers deg^enerated partly or entirely, demonstrating 

 their cortical orig-in and their cortico-fugal character. Within the 

 superior colliculus, these efferent fibers form mainly the superficial 



