ISTHMUS 183 



TRIGEMINAL TEGMENTUM 



The tegmentum of the trigeminal field forms a low eminence in the 

 floor of the fourth ventricle internally of the V roots, the erainentia 

 trigemini (figs. 2C, 90), which is evident in Necturus also ('30, figs. 

 11, 12, em.V). The gray under this eminence comprises a small-celled 

 dorsal and medial part and a large-celled ventral part (fig. 27) . The 

 small-celled part is continuous medially with a band of similar cells, 

 which extends spinalward under the ependyma of the paramedian 

 sulcus (fig. 28), and anterodorsally it is continuous with the ventral 

 border of the central nucleus of the isthmic tegmentum and more in- 

 timately with the deep periventricular gray of the isthmus (fig. 29). 

 The large-celled part is continuous anterodorsally with the large- 

 celled component of the isthmic tegmentum (figs. 28, 29, 30). Im- 

 bedded among these large tegmental cells are the motor V nuclei. 

 There are two motor V roots (fig. 27), but the two motor V nuclei 

 seen in Necturus ('30, p. 13) are here merged. Figure 40 shows three 

 of these cells in Golgi impregnation. The heavily myelinated fibers 

 of the mesencephalic V root are more dorsal, passing upward toward 

 the tectum at the outer border of the gray (figs. 13, 29-32). 



The configuration at the boundary between trigeminal and isthmic 

 tegmentum is variable, and the position of this boundary is debat- 

 able. A posterodorsal extension of the isthmic tegmentum is continu- 

 ous with a low ventricular eminence, the superior visceral nucleus. 

 The latter is separated by a shallow and variable sulcus from the nu- 

 cleus cerebelli posteriorly and a tegmental area ventrally. Compari- 

 son of the two key drawings, figures 2B and 2C, reveals differences 

 due partly to individual variations in relative sizes of parts and part- 

 ly to lack of any well-defined boundary between the large-celled com- 

 ponents of the isthmic and trigeminal tegmentum. In figure 2B the 

 dorsal part of the area marked teg.is.m. contains only large cells 

 (fig. 31), and these may be assigned to either the isthmic or the tri- 

 geminal tegmentum. More ventrally (figs. 29, 30), both large and 

 small cells occupy a similar ambiguous position. 



The large cells of the trigeminal tegmentum take various forms, 

 one of which is shown in figure 62 (others are illustrated in '14a, figs. 

 21-26). Like those of the nucleus motorius tegmenti elsewhere, they 

 are concerned with motor co-ordination in patterns as yet imperfect- 

 ly known. Some of these cells in the auricle have dendrites directed 

 forward into the posterior isthmic neuropil, where they engage ter- 

 minals of the tegmental fascicles (fig. 46). Other similar neurons di- 



L I B R A R Y 1 5 



