204 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



possibly some of the syntheses and associations of these sensations, 

 which we designate as perceptions or judgments, and it is possible 

 that injuries or defects here may be followed by an impairment of 

 these higher perceptive reactions, without any definite loss of sen- 

 sibility in the skin. Such a defect falls under the general head of 

 agnosia, and is illustrated by the condition of astereognosis re- 

 ferred to above, which might be defined as chiefly a tactile agnosia. 

 The part of the cortex, if any, in which the tract of pain fibers makes 

 its final terminus has not been definitely localized. 



The Histological Evidence. — Course of the "Lemniscus." — 

 On the histological side there is very strong corroborative evi- 

 dence for the view that cortical centers for the sensory fibers 

 of the skin and muscles lie in the parietal lobe in the region in- 

 dicated above. This evidence is connected with the path taken 



\ 



Fig. 91. — Cross-section through midbrain {Kolliker) to show the position of the lemniscus 

 (L, L): Nr, The red nucleus; Sn, the substantia nigra; tp, the peduncle. 



by the sensory fibers in the cord, especially those of the pos- 

 terior funiculi, after ending in the nucleus of the funiculus gra- 

 cilis and the nucleus of the funiculus cuneatus of the medulla. 

 This path is represented in a schematic way in the accompanying 

 diagram (Fig. 90). The second sensory neurons arise in the 

 nuclei mentioned. For the most part, at least, these new neu- 

 rons run ventrally, as internal arcuate fibers, cross the mid-line, 

 and then pass forward or anteriorly. The crossing occurs mainly 

 just in front of — that is, cephalad to — the pyramidal decussa- 

 tion, forming thus a sensory decussation (decussation of the 

 lemniscus), which explains the crossed sensory control, as the 



