Notes upon the Brain of the Alligator. 1 5 1 



united within the columns or not has not been determined, but 

 they seem never to communicate laterally. In the zone separating 

 this ganglion layer from the ventricle are only a few nutritive cells, 

 while outside the cellular zone are only sparsely scattered cells 

 of the same fusiform type. The peripheral fibres of these gangli- 

 onic chains can be followed no better than the central ones, but 

 appear to collect to form a tract communicating with the commis- 

 sure above the optic tract. 



A section of a somewhat higher level cuts the chiasm, showing 

 the decussation to be complete, yet, as seen in other sections, the 

 fibres intercross like the fingers of folded hands. The fascicles of 

 the optic nerve have irregular sheaths of connective tissue fibres 

 detected by their stained inoblasts. The lymphoidal cells remain- 

 ing are few as compared with those of other vertebrates examined. 

 (See Plate X., Figs. 7, 8.) The commissure consists of thick, 

 dark fibres forming its most conspicuous portion, and having, 

 apparently, no connection with the tuber, but extending upward 

 approximately parallel to the optic tract. Behind and above these 

 are more delicate fibres, apparently derived from the ganglion of 

 the tuber. These may be the fibres called by Bellonci " fibrae 

 ansulatae," but instead of being thick and prominent, as compared 

 with those of the inferior commissure, they are less conspicuous. 



It is not difficult to trace the optic tracts upward through success- 

 ive horizontal sections. The tract divides into several minor por- 

 tions, some of which reach the tectum opticum on the anterior 

 surface, some on the lateral, and some on the postero-lateral aspect 

 of the tectum. The fibres spread out on the superior surface and 

 turn abruptly downward through the curious columnar structure 

 of the tectum; the emerging fibres converge to the posterior and 

 median portions to enter the colliculi. 



The ultimate course of the fibres of the inferior commissure was 

 not discovered, although the dark fibres are followed upward with 

 the optic tracts to near the base of the optic lobes. Some fibres of 

 the optic tracts pass through the corpora geniculata, but pursue 

 their further course to the tectum opticum. No fibres from the 

 optic tracts directly to the cerebrum could be seen. (Plate X., 

 Figs. 3, 4, 5, 6; also Plates VIII. and IX.) 



(3.) The oculomotor nerves emerge from a point upon the ventral 

 surface nearly midway of the mesencephalon. The roots spring 

 from a well-defined trigonum interpedunculare. The course of 



