1932] Poliak:. Afferent Fiber Systems, Primate Cerebral Cortex 41 



probable, therefore, that the entire thalamo-eortieal radiation consists 

 of regularly arranged fiber laminae ; each of these, beginning with its 

 thalamic origin to its cortical termination, has its own course, as 

 well as its separate cortical terminal area. These fiber laminae do not 

 appreciably mix with each other. The significance of such an organi- 

 zation of the thalamo-cortical radiation appears to lie in the preserva- 

 tion of the "spatial" relationships existing in the somato-sensory 

 receptive surfaces of the body, its purpose being to isolate the con- 

 duction of somato-sensory impulses, different according to the qualities 

 and localities, up to the definite areas or segments of areas of the 

 somato-sensory^ cortex. (Compare: Visual System and Chapter XIX, 

 in the present work.) 



The present series demonstrates also the course of that portion of 

 the thalamo-cortical radiation which ascends toward the most caudal 

 segment of the somato-sensory region corresponding to areas 5 and 7 

 of Brodman (figs. 67-69, 71, compare also figs. 36, 37, 52-55). This 

 part of the somatic sensory radiation appears as the most dorsal 

 portion of the external saggital layer of the parietal lobe running close 

 to the central visual paths (vr in corresponding figures) , especially to 

 that part of it which finally enters into the upper lip of the calcarine 

 fissure. (It would appear that this portion of the thalamo-cortical 

 radiation was included by Monakow and some other investigators 

 in their too extensive "visual radiation"; see: Visual System, in 

 this work.) 



As has been mentioned before, in Experiment III not a few descend- 

 ing fibers degenerated. They enter the lateral nucleus of the thalamus. 

 The majority of these are of slight caliber, only a few being of medium 

 size, the largest being still considerably smaller than the average 

 thalamo-cortical fibers. These descending fibers, it is fairly safe to 

 assume, originate in the cortex. A considerable number of them have 

 been traced down to the roof of the midbrain, others into the cerebral 

 peduncle. The question arises whether at least some of these fibers 

 correspond with the cortico-thalamic fibers found by Ramon y Cajal 

 and accepted by others (Head, Villaverde, Hollander, Wallenberg; see 

 also Long, Melius, Probst, Minkowski 1923-24, Riese, and my paper, 

 1926). All that can be said from the present study is that some of 

 the degenerated cortico-fugal fibers which enter the ventro-lateral 

 nucleus of the thalamus and the hypothalamus (v.p. of Monakow- 

 Friedmann, ventral portion of the reticulate zone and the zona incerta) 

 might indeed terminate here ; this could not, of course, be settled by 

 Marchi's method alone. 



