no Journal of Comparativp: Neurology. 



consists of a narrow triangular slit extending along the 

 meson. The apex of this triangle terminates, upon the base 

 of the brain, in the infundibulum. Caudad, this ventricle is 

 connected with the aqueduct of Sylvius, while cephalad it 

 is connected, through the foramen of Monro, with the lateral 

 ventricles. It may be noted that the narrowness of the third 

 ventricle in the avian diencephalon corresponds to the 

 appearance of that cavity in the human brain, where, accord- 

 ing to Ranney,(') the third ventricle is " a narrow chink 

 between the optic thalami."' 



NIDULI of the diencephalon. 



Corpus geniculatuni externum (Plate XV, Figs. 5, 7, 9, 

 10, 11). — Upon passing from the avian prosencephalon into 

 the diencephalon, one of the first niduli encountered is the 

 corpus 'geniculatum externum. This is a large sub-ellipsoidal 

 cell cluster, which is situated in the cephalo-laterad portion 

 of the dorsal region of the diencephalon. Although located 

 at some distance from the meson, yet this nidulus does not 

 lie adjacent to the lateral surface of the thalamus; indeed, 

 between it and the surface there is a large fibre tract. I have 

 already stated that this nidvilus is sub-ellipsoidal. The major 

 axis of this sub-ellipsoid is oblique to the longitudinal axis 

 of the brain. The size of the corpus geniculatum externum 

 varies in different bird brains. In the brain of Swainson's 

 thrush [Hylocichla swainsoni) the major axis is about 1,250 

 micro-millimetres and the horizontal about 938 micro-milli- 

 metres long. 



Histology. — The structure of this nidulus is unique. For 

 the most part, the cells are arranged in irregular concentric 

 lamina. It looks very much as though, originally, the cells 

 had been arranged in concentric spheres, and that, by rapid 

 growth, these spheres had become contorted. 



The cells of this nidulus are fusiform (Plate XVI, Fig. 5). 



I "The Applied Anatomy of the Nervous System," by Ambrose h- Ranney. J^econd 

 edition, p. 312. 



