112 Journal of Comparati\e Neukolocjy. 



geniculatum externus; a few arise from it, and others termi- 

 nate in its substance. These fasciculi will be described in 

 connection with the tracts of the optic nerve. 



Nidulus posterius (Plate XV, Fig. 8). — A short distance 

 caudo-laterad of the corpus geniculatum externum, there is a 

 small spherical cell cluster. This nidulus h:is been called by 

 Bellonci(') " nucleus posterius." For the sake of uniformity, 

 I have called it •' nidulus posterius." Compared with the 

 corpus geniculatum externum, this nidulus is quite small. 



Histologically, it might be considered a sphere within a 

 sphere, the two spheres fitting so perfectly that no space is 

 left between the inner core and the outer shell. The shell 

 is granular, and does not contain nerve cells. The core, 

 however, is composed of densely packed, distorted, fusiform 

 nerve cells. 



Crcscoit-shapcd nidulus (Plate XV, Figs. 7-9). — In the 

 same region, adjoining the corpus geniculatum externum and 

 the nidulus posterius, there is a third nidulus. In horizontal 

 longitudinal sections this nidulus is a crescent, the horns of 

 which are directed cephalo-mesad. It lies caudad to the 

 corpus geniculatum externum, and extends from the nidulus 

 posterius caudo-mesad half way to the meson. Although 

 larger than the last-mentioned nidulus, it is not so large as 

 the corpus geniculatimi. 



Histologv- — The cells of this nidulus are of two sorts, 

 fusiform and pyramidal (Plate XVI, Fig. 3). In alumi- 

 nium-sulphate cochineal and in hiematoxylin preparations, 

 each cell has a densely stained nucleus and a more densely 

 stained nucleolus. The cells are closely packed, and have 

 their major axes parallel to the longest axis of the nidulus. 

 In different bird brains the size of these cells varies. In 

 Swainson's thrush [Hyhciclila szvai'fisoni) they are from 

 sixteen to twenty-two micro-millimetres long and from six 

 to eight micro-millimetres wide. This cell cluster appears 



I Op. cit., p. 16, Fig. iii ; Plate V, Fig. ii, np. 



