MILLER — CONTRIBUTIONS FROM ANATOMICAL LABORATORY. 229 



from that of the Dipnoans on the one hand and from that of 

 the Rana on the other. In both the latter cases we have the 

 optic lobes well developed. 



Anteriorly, it is sharply separated from the prosencephalon 

 by a deep groove on the dorsal surface and a shallow one on 

 the ventral; posteriorly, it is continued dorsally by the epen- 

 cephalon or the cerebellum and ventrally by the metencephalon 

 or the medulla oblongata. Measuring from the base of the 

 pineal gland to the cerebellum it is about 5.8 mm long, and 

 measured laterally it is about 3.1 mm in diameter. In cross 

 section it will be noticed that the walls are very thick laterally, 

 while dorsally they grow thinner until in the median line of 

 the thalamencephalon there is only a thin layer of cells pres- 

 ent. A cavity, the third ventricle and the Aqueduct of Sylvius, 

 extends through the thalamencephalon and the mesencephalon 

 and is of the same size throughout, except where the infundibu- 

 lum joins it and anteriorly where it connects with the lateral 

 ventricles of the cerebral hemispheres. At the place where the 

 cavity of the infundibulum joins the thalamencephalon there 

 is a V-shaped depression in the floor. At the anterior part of 

 the cavity there is a gradual widening toward the foramen of 

 Monroe. The relation of the pineal gland, pituitary body, optic 

 and occulomotor nerves to these segments of the brain will be 

 considered later. 



Epencephalon. This is a short ledge which forms part of the 

 roof of the fourth ventricle. It is thickest in its anterior part 

 and thins out at the edges. Its general form is convexo-concave, 

 when viewed from the dorsal side. 



In the dorsal view of the brain, PL 12, it will be seen that 

 the roof of the fourth ventricle extends further posteriorly than 

 is indicated by Osborn. While he gives the roof as extending 

 only as far as the superficial origin of the trigeminal, I found 

 it extending further caudal, as far as the superficial origin of 

 the facial. 



Metencephalon. I consider the metencephalon as a direct 

 posterior continuation of the ventral part of the mesencephalon, 



