PLATE 3 



EXPLANATION OF FIGURES 



13 Mesencephalic and trochlear roots from the same section as figure 12, 

 sufficiently magnified to allow accurate drawing of every particle of degenerated 

 myelin of any size. In both mesencephalic roots and elsewhere in this section 

 there are many more fine particles of myelin than appear in more caudal sections. 

 Observe in this figure that there are more coarse degenerated myelin sheaths 

 in the right trochlear root than in the left. Since the fibers of these roots have 

 crossed, the right root represents fibers for the left trochlear nerve. Its close 

 proximity to the trigeminal roots probably resulted in its destruction. X 48. 



14 to 27 are from a transverse series of 20;u sections through a guinea-pig's 

 brain stem, including the trigeminal roots, semilunar ganglion, and the trigemi- 

 nal nerves (ex. no. 64). The lesion in this experiment passed completely 

 through the left mesencephalic root behind the inferior colliculus and destroyed 

 about one-half of the locus cocruleus, without disturbing the trigeminal motor 

 nucleus nor any of its fibers. The sections were prepared after a modified Marchi 

 method for demonstrating medullary-sheath degenerations. 



14 Left half of a transverse section from the above-mentioned guinea-pig 

 series (ex. 64) at the level of the extreme caudal end of the trigeminal motor 

 nucleus. The lesion (Les.), which extends ventrocephalad, does not pass deep 

 enough in this section to reach the mesencephalic root and the locus copruleus, 

 and is some distance from the trigeminal motor nucleus. Observe the almost 

 complete degeneration of the fibers of the mesencephalic root, which curve 

 around the caudal end of the trigeminal motor nucleus. A few fine mesencephalic 

 root collaterals will be seen -in the trigeminal motor nucleus. The globular 

 unipolar cells of the locus cceruleus are some distance from the much larger 

 multipolar cells of the trigeminal motor nucleus. X 8. 



15 Similar view of a section ten sections cephalad of figure 14, passing through 

 the caudal end of the trigeminal motor nucleus. The lesion (Les.) is a little 

 deeper than in the previous section, 'severing some of the mesencephalic root 

 fibers, but quite remote from the trigeminal motor nucleus. In this section the 

 mesencephalic root will be seen in transverse section in the locus coeruleus a 

 little median to the brachium conjunctivum, and also as longitudinal fibers, 

 bending ventrally, between the trigeminal sensory and motor nuclei. Col- 

 laterals pass to the trigeminal motor nucleus and to some smaller cells situated 

 median and dorsal to the trigeminal sensory nucleus. X 8. 



16 Portion of the left mesencephalic root between the trigeminal sensory 

 (substantia gelatinosa) and motor nuclei from the same section as figure 15, 

 more highly magnified so that every particle of degenerated myelin of any size 

 could be accurately drawn. A comparison with figure 6 shows at least twice as 

 many large and medium-sized degenerated fibers in the mesencephalic root, but 

 not as many fibers and fine collaterals in the trigeminal motor nucleus, nor 

 among a group of small cells situated median and dorsal to the trigeminal 

 sensory nucleus. X 48. 



17 Left half of a transverse section from the same series (ex. 64) as figures 15 

 and 16, but through the extreme cephalic end of the trigeminal motor nucleus, 

 thirty-one sections further cephalad. The lesion (Les.) has severed over one- 

 half of the locus cceruleus fibers at this level, though some distance from the 

 trigeminal motor nucleus. Descending degenerated mesencephalic root fibers 

 appear in two places in this section — in the locus coeruleus between the brachium 

 conjunctivum and the fourth ventricle, and scattered through the trigeminal 

 motor root between the trigeminal sensory and motor nuclei. In the former 

 these fibers make up practically all of the tract; while in the latter they appear 

 to constitute at least two-thirds of the mesencephalic root fibers. X 8. 



208 



