THE MAMMALIAN NERVOUS SYSTEM 71 



77. The medial lemniscus. These fibers immediately de- 

 scend from their nuclei to cross ventrally of the ventricle to the 

 other side of the brain, thus forming the decussation of the lem- 

 niscus. This tract after its decussation can be followed 

 through the series of sections as far as the thalamus. In the 

 lower part of the medulla oblongata these fibers will be found 

 in the interolivary space near the median plane (Herrick 

 CIS), Figs. 64, 72, 73). They are bounded ventrally by the 

 cortico-spinal (pyramidal) tract, and dorsally by the tecto- 

 spinal tract. Dorsally of the latter is the longitudinal medial 

 fasciculus. At the level of the pons the medial lemniscus be- 

 gins to turn laterally and in the midbrain it lies dorsally of the 

 substantia nigra (Herrick ('18), Fig. 75). In sections through 

 the midbrain and thalamus these fibers can readily be followed 

 forward to their termination in the lateral and associated 

 somesthetic nuclei of the thalamus (Herrick ('18), Figs. 77, 78, 

 and 79). The medial lemniscus carries general proprioceptive 

 nervous impulses from the spinal cord to the thalamus. 



Identify the medial lemniscus in the sections and draw it into 

 the outlines as far forward as the material provided will permit. 

 See, in addition to the figures cited in the previous paragraph, 

 Section 62 (1) to (11); Cunningham ('15), Fig. 579, p. 651; 

 Morris ('14), Fig. 632, p. 803; Piersol ('16), Fig. 964; Sabin 

 ('01), Plates V, VII; Spalteholz ('09), Fig. 752, p. 675; Villiger 

 ('12), Figs. 154, 155, pp. 167, 168. Directions for the dissec- 

 tion of the medial lemniscus of man will be found in Section 

 108. 



78. The spinal lemniscus. The ascending secondary path- 

 way for exteroceptive sensibility (touch, temperature, pain) 

 from the trunk and limbs is the spinal lemniscus, or spino- 

 thalamic tracts. There are two of these tracts in the cord, the 

 tractus spino-thalamicus dorsalis for pain and temperature, 

 and the tractus spino-thalamicus ventralis for touch and pres- 

 sure (Herrick ('18), Fig. 63). In the medulla oblongata these 

 accompany the ventral spino-cerebellar tract (Herrick ('18), 

 Fig. 73) and in the midbrain they join the lateral lemniscus 

 (Herrick (18), Fig. 75). 



It is difficult to demonstrate these tracts in either gross or 

 microscopic material. From your reference books learn their 



