THE GREAT AFFERENT SYSTEMS 



Neuron II, with its cell body located in the posterior gray column, sends its 

 axon across the median plane into the ventral spinothalamic tract in the opposite 

 anterior funiculus. In this the fiber ascends through the spinal cord and brain 

 stem to the thalamus. This tract gives off fibers, either collateral or terminal, 

 to the reticular formation of the brain stem. Other neurons of the second order 

 in the tactile path are located in the gracile and cuneate nuclei of the medulla 

 oblongata, and their axons after crossing the median plane ascend in the median 

 lemniscus of the opposite side to end in the thalamus. All of these secondary 

 tactile fibers end within the ventral part of the lateral thalamic nucleus. 



The course of the ventral spinothalamic tract through the medulla oblongata and pons 

 is not accurately known. It has generally been figured as joining the lateral spinothalamic 

 tract dorsolateral to the olive (Fig. 230. See also Herrick, Fig. 81). But, since lesions in 

 the lateral area of the medulla oblongata may cause a loss of pain and temperature sensation 

 over the opposite half of the body without affecting tactile sensibility, it is not improbable 

 that Dejerine (1914) is correct in supposing that it follows a median course, its fibers inter- 

 mingled with those of the tectospinal tract which run, however, in the opposite direction 

 (Fig. 234; Economo, 1911; Spilkr, 1915). 



There is reason to believe that the ventral as well as the lateral spinothalamic tract 

 consists in part of short relays with synaptic interruptions in the gray matter of the spinal 

 cord and brain stem, and the two tracts are sometimes designated as the spino-reticulo-thala- 

 mic path. 



In the spinal cord there appear to be two tracts which convey tactile im- 

 pulses toward the brain, an uncrossed one in the posterior funiculus and another 

 that crosses into the opposite anterior funiculus. Since these overlap each 

 other for many segments, this arrangement would account for the fact that con- 

 tact sensibility is usually unaffected by a purely unilateral lesion (Head and 

 Thompson, 1906; Rothmann, 1906; Petren, 1902). Among the fibers of contact 

 sensibility, which ascend in the posterior funiculus to the cuneate and gracile 

 nuclei of the same side, are those that subserve the function of tactile discrim- 

 ination, or, in other words, the ability to recognize the duality of two closely 

 juxtaposed points of contact, as when the two points of the compasses or dividers 

 are applied simultaneously to the skin. Furthermore, those elements of tactile 

 sensibility, which underlie the appreciation of the form of objects or stereognosis, 

 ascend uncrossed in the posterior funiculus to the gracile and cuneate nuclei. 



Neuron III. The neurons located in the ventral portion of the lateral nucleus 

 of the thalamus, with which the tactile fibers of the second order enter into syn- 

 aptic relations, send their axons by way of the thalamic radiation through the 

 posterior limb of the internal capsule and the corona radiata to the somesthetic 

 area of the cerebral cortex in the posterior central gyrus (Fig. 220). 



