Turner, Morphology of the Avian Jh-ain. 113 



to contain less than the average number of Deiter's cor- 

 puscles. 



As far as I know, this nidulus has not been described 

 before. I have ventured to christen it the crescent-shaped 

 nidulus. 



Central nidnlus of the diencephalon (Plate XIV, Fig. 15). 

 — In the ventral region of the brain, near the laterad portion 

 of the diencephalon, there is a large, conspicuous nidulus. 

 It lies upon the the border of the mesencephalon and the 

 diencephalon, and is situated about half way between the 

 chiasm and the caudad surface of the thalamus. This is a 

 large nidulus. It is almost as large as the corpus genicu- 

 latum externum. It is composed of fusiform nerve cells, and 

 is traversed by medullated nerve fibres. 



NIDULI OF THE MESENCEPHALON. 



Nidulus of the third nerve. — In man, according to 

 Ranney,(') "the motor-oculi and trochlear nerves have 

 their deep origin apparently from a gray nucleus (which, 

 according to some authors, is common to both nerves) 

 within the gray matter surrounding the aqueduct of Sylvius. 

 The nucleus of the fourth nerve seems to be com- 

 posed of larger cells than that of the third nerve, however, 

 and to occupy the level defined by the line of separation 

 between the anterior and posterior corpora quadrigemina." 



From the above quotation it appears that, even in mam- 

 mals, evidence is forthcoming to indicate that the niduli of 

 the third and fourth nerves are probably distinct. In the 

 avian brain this separation becomes more complete. These 

 niduli are not only distinct, but they are separated by a 

 broad longitudinal tract of nerve fibres (Plate XIV, Fig. 12). 

 These two niduli lie near the meson in the caudo-dorsad 

 portion of the mesencephalon, the third lying a short distance 

 cephalo-ventrad of the fourth. The motor-oculi nidulus is a 



I Op. cit., p. 336. 



