. A text-book of physiology : for medical students and physicians . of the lemniscus terminate in thissame region. Secondly, in this region there are mediated alsopossibly some of the syntheses and associations of these sensations,which we designate as perceptions or judgments, and it is possiblethat injuries or defects here may be followed by an impairment ofthese higher perceptive reactions, without any definite loss of sen-sibility in the skin. Such a defect falls under the general head ofagnosia, and is illustrated by the condition of astereognosis re-ferred to above, which might be define
. A text-book of physiology : for medical students and physicians . of the lemniscus terminate in thissame region. Secondly, in this region there are mediated alsopossibly some of the syntheses and associations of these sensations,which we designate as perceptions or judgments, and it is possiblethat injuries or defects here may be followed by an impairment ofthese higher perceptive reactions, without any definite loss of sen-sibility in the skin. Such a defect falls under the general head ofagnosia, and is illustrated by the condition of astereognosis re-ferred to above, which might be defined as chiefly a tactile $art of the cortex, if any, in which the tract of pain fibers makesits 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 fibersof 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; Fp, 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 path is represented in a schematic way in the accompanyingdiagram (Fig. 90). The second sensory neurons arise in thenuclei 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 mainlyjust in front of—that is, cephalad to—the pyramidal decussa-tion, forming thus a sensory decussation (decussation of the 206 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. lemniscus), which explains the crossed sensory control, as thepyramidal decussation explains the cr
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