268 Journal of Comparative Neurology. 



VI.— HISTOLOGY OF THE METENCEPHALON. 



Nidiili of tlic Fiftli Nerve. — The mesencephalic nidulus 

 of the trigeminal nerve has been described in connection w^ith 

 the niduli of the mesencephalon. (') One tract of the fifth 

 nerve arises in the epencephalon, but the niduli of that 

 region also have been described above. There remains then 

 to describe in this connection those niduli only of the tri- 

 geminal nerve which are located in the metencephalon. 



Lateral Motor Nidulus of the Fifth Nerve (Plate XVIII, 

 Figs. 8, 15). — This is a well-defined nidulus which is situated 

 near the lateral surface of the medulla, adjacent to the ceph- 

 alad root of the trigeminal nerve. Although situated quite 

 near to the lateral surface of the metencephalon, yet it lies 

 entad to that tract of the fifth nerve which passes to the 

 myelon. It also lies cephalad to the tract which passes to 

 the deep motor nidulus of the trigeminus. In difierent birds 

 the shape of this nidulus undergoes considerable variation. 

 As far as my observations go, the two extremes are repre- 

 sented by the young dove ( Columba livio) and by the adult 

 Swainson's thrush [Mylocichla Sivai7iso?ii). In the first case 

 it is a compact spherical nidulus; in the other it is an elon- 

 gated, straggling nidulus, which lies parallel to the deep 

 motor root of the fifth nerve. 



This nidulus is well supplied with large pyramidal nerve 

 cells. The apex of each cell is prolonged into a long pro- 

 cess, which process is often curved. In hsematoxylin and in 

 aluminium-sulphate cochineal preparations, these cells are 

 densely stained, and each one presents a densely stained 

 nucleus and nucleolus. This nidulus is well supplied with 

 Deiters' corpuscles, and nerve fibres traverse it in several 

 directions. 



Ventrad to the above nidulus there exists, in some of my 

 sections, an ill-defined cell cluster, which probably deserves 



I Supra, p. 121 ; Plate XV, Fig. 3. 



