Turner, Morpliolof^y of the Avian Brain. 129 



Fourth )icrvc (Plate XIV, Figs. 8, 9; Plate XV, Figs. 

 5, 8, 9). — In the human hrain, according to Ranney,(') " the 

 deep fibres of this nerve may be traced to four diflerent 

 localities, as follows: First, some to the substance of the 

 peduncles; second, other fibres to the valve of Vieussens, 

 where they are lost, with the exception of a few which can 

 be traced to the frenulum; third, a few fibres to the tubercula 

 quadrigemina; fourth, a large bundle which passes inward 

 towards the median line and then decussate with its fellow 

 of the opposite side." 



In the avian brain I have not been able to trace, with 

 certainty, any fibres from the fourth nerve to the peduncles 

 of the cerebellum. Neither have I been able to demonstrate 

 that any of the fibres of the fourth nerve originate in the 

 valve of Vieussens. However, homologues of the two re- 

 maining tracts are constant in the avian brain. 



As in the human brain, that portion of the fourth nerve 

 which decussates constitutes the largest root of that nerve. 

 The fibres of the fourth nerve arise from their nidulus in the 

 caudo-dorsal part of the diencephalon, and, after decussating 

 in the valve, pass ventro-laterad, around the cephalad ex- 

 tremity of the pedunculi cerebelli, to the surface. 



Mesencephalic tract of the fourth nerve (Plate XV, 

 Fig. 5). — This tract arises from the fasciculus internus,at the 

 junction of that bundle with the caudo-laterad corner of the 

 mesesencephalic ventricle. It then passes, in an undulating 

 line, dorso-mesad to the valve of Vieussens. In all proba- 

 bility this tract participates in reflex actions. 



From the vicinity of the niduli of the third and fourth 

 nerves a tract passes cephalo-laterad to the prosencephalon 

 (Plate XV, Fig. 10). 



Fasciculus inter nus (Plate XV, Fig. 5). — Entad to the 

 cell layers of the tectum opticum there is a large bundle of 



upon the Brain of the Alligatoi'," by C. L. Herrick. Jimvn.-il of the Cincinnati Society 

 of Natural History, Vol. XII, Plate XIII, Fig. 6, R. III. 

 I Op. cit., pp. 395-396. 



