70 University of California Puhlications in Anatomy [Vol. 2 



Chapter IX 



RESULTS OF THE PRESENT INVESTIGATIONS OF THE 

 SOMATIC SENSORY SYSTEMS 



1. AFFERENT SOMATO-SENSOEY TRACTS FROM LOWER 

 REGIONS OF THE NEURAXIS TO THE THALAMUS 



The first question to be answered in regard to the somato-sensory 

 system is the relation between the afferent fiber tracts from the spinal 

 cord, bulb, pons, and cerebellum, and the thalamus. Contrary to the 

 prevalent view that only the ventral nucleus of the thalamus, or that 

 nucleus preponderantly, must he reg'arded as the terminal region of 

 the above mentioned tracts, the present experiments show clearly the 

 spreading of these fibers almost over the entire ventro-lateral and 

 dorso-lateral nucleus, as far dorsally as the \ncinity of the caudate 

 nucleus. Thus the entire lateral and larger portion of the thalamus 

 receives afferent impulses from the periphery. Moreover, the lateral 

 nucleus receives afferent fibers in its entire transverse extent or width, 

 that is, in the whole region situated between the internal and external 

 medullary laminae and not merely in a narrow zone along the external 

 lamina, as commonly believed. Further it is import-ant in understand- 

 ing the internal organization and function of the entire somatic 

 sensory system to know that the afferent fibers entering the thalamus 

 do not spread here in an irregular or ' ' diffuse ' ' fashion. The course 

 of these fibers and fiber bundles is fairly regular and parallel to the 

 lateral contour of the thalamus, bundle alongside of bundle. This 

 already indicates a definite, orderly, or "spatial" arrangement of 

 special portions or subsystems of the intermediate somato-sensory 

 tracts and a morphological and functional segmentation of the thala- 

 mus. No evidence was found to support the belief in the existence of 

 a "through" or "cortical" lemniscus which would merely pass the 

 thalamus on its way to the cortex or to subcortical basal nuclei. The 

 course of all the incoming somato-sensory fibers in the thalamus is 

 approximately vertical, cutting at a right angle the horizontal bundles 

 which are the outgoing thalamo-cortical fibers. This can be explained 

 only by accepting the termination in the thalamus of all or at any 

 rate, the majority of the somatic sensory fibers from lower regions of 

 the neuraxis. Whether, nevertheless, some fibers pass the thalamus to 



