ISTHMUS 187 



descend from the hir^e cells of theisthmic<ind trigeminal tegmentum 

 to the motor zone of the medulla oblongata are especially clearly 

 seen in very young larvae from early swimming to early feeding stages 

 and are interpreted as provision for activation of the mandibular and 

 hyoid musculature involved in feeding. Longer fibers descend into 

 the spinal cord; but these, so far as is sliown in our material, are not 

 numerous. 



ISTIIMIC NEUROPIL 



Both the gray and the white substance of the isthmus are perme- 

 ated with dense neuropil, the axonic component of which is composed 

 mainly of terminals of afferent fibers and collaterals of fibers of 

 passage. The superficial neuropil is composed chiefiy of branched 

 terminals of the unmyelinated thalamo-tegmental and tecto-teg- 

 mental tracts and dendritic terminals from the underlying gray. The 

 intermediate neuropil is in the form of sheets or plaques insinuated 

 between the tegmental fascicles and composed largely of terminals 

 and collaterals of fascicular fibers. The deep neuropil of the alba 

 and the grisea is a c(mfused entanglement of fibers and dendrites, the 

 analysis of which is quite impossible except where clarified l^y elec- 

 tive Golgi impregnations. These preparations reveal a num})er of 

 more or less well-defined tracts imbedded within this interstitial 

 matrix. In the gray substance the cells are arranged in lamellae sepa- 

 rated by sheets of neuropil, which is continuous with that of all 

 neighboring parts. A large proportion of these axons trend dorso- 

 ventrally from tectum and dorsal tegmentum to isthmic tegmentum 

 and interpeduncular neuropil and posteroventrally, with or without 

 decussation in the ventral commissure. The latter axons accompany 

 thicker fibers of the tr. tegmento-bulbaris arising from large cells of the 

 motor tegmentum. 



POSTERIOU ISTIIMIC xVEUKOPIL 



The name "posterior isthmic neuropil" is given to a wide zone of 

 neuropil in the alba at the boundary between the isthmus and the 

 bulbar and cerebellar structures. It invades the auricle below and the 

 nucleus posterior tecti above. It is permeated by dendrites from all 

 surrounding parts, including the trigeminal tegmentum (fig. 46; 'S9b, 

 figs. 47-49, 62, 6P>), auricle ('396, fig. 53), cerebellum (fig. 47), supe- 

 rior visceral nucleus ('42, fig. 43), and dorsal and isthmic tegmentum. 

 It receives terminals of peripheral sensory fibers of the V, VH, VIII, 

 and lateral-line nerves, of a bulbo-isthmic tract (figs. 38, 39), of the 

 secondary visceral-gustatory tract; terminals and collaterals of the 



