. The Journal of comparative neurology and psychology. t of the Hid nerve. A, from themedial surface; B, from the lateral surface. The sulcus limitaus is indicatedby a dotted black line, and ou the outer surface and on the trigeminus theboundary line between sensory and motor structures is indicated in the sameway. The radix mesencephalica runs dorsal to the sulcus limitans throughits whole course. embryonic brains. First is the simplicity (if structure which enablesone to trace the bundle Avith perfect ease and certainty. Second isthe clearness of the general morphological relations. It is ea
. The Journal of comparative neurology and psychology. t of the Hid nerve. A, from themedial surface; B, from the lateral surface. The sulcus limitaus is indicatedby a dotted black line, and ou the outer surface and on the trigeminus theboundary line between sensory and motor structures is indicated in the sameway. The radix mesencephalica runs dorsal to the sulcus limitans throughits whole course. embryonic brains. First is the simplicity (if structure which enablesone to trace the bundle Avith perfect ease and certainty. Second isthe clearness of the general morphological relations. It is easy todetermine whether the mesencephalic root and its nucleus of originlie in the sensory or in the motor zone of the l)rain. I was fortunateto receive in 1905 a human embryo which measured after fixation JOHNSTON, The Radix Mesencephaltca Trtgemini. 617 in formalin mm. The embryo is small for its stage of devel-opment. The specimen was so fresh that the tissnes show an almostperfect histological fixation and the l)rain is in excellent condition. Fig. 14. Camera outliues of four transverse sections through the hrain ofthe embryo II. 19. These sketches show the course of the mesencephalic bundlefrom the sensory root to the level of the trochlearis. Note its relations to thesulcus limitans, the two arms of the trochlearis and the tr. tecto-cerebellaris. for stndy. The specimen was cnt in a perfect series of frontal sec-tions 10 microns in thickness and stained in Delafields bundle can be traced readily in this embryo with either high or 6l8 Journal of Comparative Neurology and Psychology. low power objectives and its course has been demonstrated to manypersons at two meetings of the American Association of Anatomistsand elsewhere. The general morphological relations are to be seen from Figs. 13,14, 15, 16. In a section through the roots of the trigeminus (trans-
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