ANATOMY OF A CYOLOSTOME BRAIN 637 



EXTERNAL SURFACE 



Figure 2 shows the external form of the right side of the model. 

 In front of the superficial origin of the Vth nerve is a sharp con- 

 striction, the isthmus, which separates the medulla oblongata and 

 cerebellum behind from the midbrain in front. It is occupied 

 dorsally by the decussation of the I Vth nerve. 



The roof of the mid-brain is membranous and plexiform save 

 for a post-tectal commissure immediately in front of the cere- 

 bellum and the posterior commissure at the diencephalic bound- 

 ary. Tlie attachment of this choroid plexus to the massive 

 wall is termed the taenia mesencephali (figs. 1 and 12, t.m.). 

 The lateral wall of the mid-brain is marked by two very 

 prominent eminences, a tectal eminence (tect.) dorsally and a 

 peduncular eminence ventrally (ped.). Between these is a less 

 prominent tegmental area (tegm.). On the ventricular surface 

 the sulcus limitans of His marks the boundary between the 

 tegmental and peduncular regions (cf. figs. 3 and 4). The super- 

 ficial origin of the III nerve marks the anterior border of the 

 peduncular eminence. The tectal eminence is formed by a 

 lateral evagination involving the entire thickess of the brain wall, 

 thus producing a dilation of the ventricle, the optocoele (fig. 4). 

 The tegmental eminence, on the other hand, is a solid thicken- 

 ing of the wall which extends somewhat farther caudad than 

 the tectum. The ventricle is pushed downward to form a dis- 

 tinct ventral recess in this region, but is not laterally dilated 

 (fig. 4). 



The position of the posterior commissure is indicated by a 

 distinct eminence in front of the tectum on both the lateral 

 and the ventricular surfaces (figs. 2, 3, em. pc), which extends 



fore being cut into right and left halves entirely with reference to the external 

 features of the brain, so that neither of the authors was able to see the ventricular 

 surfaces until after the stacking was finished and the completed model cut in 

 half. The observance of these precautions insures an objective result, so far as 

 the ventricular surfaces are concerned. The subsequent microscopic study of 

 the sections and their interpretation was done jointly by the two authors, though 

 the senior author assumes entire responsibility for the terminology adopted and 

 for the discussion which forms the final section of the report. 



