42 Journal of Comparative Neurology. 



die, but there is no evidence of it. The region of this Mauth- 

 ner's nidulus, where the fibres disperse, is also one rich in blood- 

 vessels. 



VI. — Optic nerves and brachia. 



The course of the two divisions of the optic nerves is per- 

 fectly simple and not difficult to follow. The two roots pass to- 

 gether for some distance dorso-caudad, until they meet the pro- 

 tuberance of the tectum. Theoretically they would spread out 

 over the entire ectal surface of the latter, but the form of the tec- 

 tum is rather lunate than hemispherical, so the fibres destined 

 to the caudal part separate and pass in a horizontal plane caudad 

 to the posterior cornu, while the cephalic division turns dorsad 

 to the anterior cornu of the crescent. In the space left by the 

 two divisions the ventral commissure passes toward the terminus 

 in the corpora posteria, while the transverse commissure follows 

 the cephalic branch toward the level of the habena. The 

 caudal optic tract spreads out within the outer or neuroglia 

 layer, radiating dorso-cephalad to fill the space not filled by the 

 fibres from the cephalic tract with dorso-caudad radiations. 

 The nature of the terminations is discussed further on, but the 

 morphological relations are absolutely simple. Perhaps they 

 could be illustrated best by the two hands, the arms of which are 

 placed parallel and represent the two divisions, while the fingers 

 of the one arch about one side of a sphere and those of the 

 other arch in the opposite direction, so that the finger-tips meet 

 above. The complications encountered in the study of the tec- 

 tum grow out of the commingling of other fibres. Yet even 

 here, when once understood, the arrangement seems very sim- 

 ple. The most superficial of these systems is the transverse 

 system, or sylvian commissure (commissure of the optic lobes) 

 which is a pure commissure or decussation of the tectum. The 

 origin of the fibres is in the reticular zone in the outer or neurog- 

 lia layer, whence they pass more or less directly across to the 

 other side, their course being greatly modified by the develop- 

 ment of the torus, divarication of the tectum by the volvula, 

 etc., in the several cases. In general these fibres converge 

 cephalad. In the drum, for example, where the roof of the 

 mesencephalon has been rotated through an angle of nearly 90°, 



