Notes upon the Brain of the Alligator. 133 



breadth to be near the dorsal surface, which is gently arched, pass- 

 ing by a gradual curvature into the oblique and gently convex lat- 

 eral surface, and by an abrupt flexture into the plane median 

 surface. Thus the lower surface is restricted to a narrow ridge 

 passing into the crura olfactoria. The section also reveals the 

 extent of the ventricle, which continues as a narrow cleft within a 

 short distance of the surface from the median lower angle dorsally, 

 then outward, and finally ventrally to a point near the greatest 

 external projection, and thus about two-thirds separates an axial 

 protuberance or lobe from the mantle or cortex. The extent of the 

 ventricle increases toward the posterior until the axial lobe is con- 

 nected with the mantle only by a narrow isthmus equal to less than 

 a fifth of its circumference, this line of union being limited to the 

 lower aspect. The commissures connecting the two cerebral hemi- 

 spheres are described beyond. As compared with the adult, the 

 brain in this stage is remarkable for the short globose hemispheres, 

 short rhincephalic crura, and the compactness of the entire brain. 

 The resemblance to an avian brain, except in the cerebellum, is 

 marked. 



3. The Diencephalon does not reach the surface above, but is 

 enclosed by the over-arching hemispheres and optic lobes. It is 

 bounded anteriorly by the optic chiasm, lamina terminaiis, and 

 fissura pallii : above by a membranous covering: posteriorly by the 

 commissura posterior: and below by the tuber cinereum, forming 

 the floor of the third ventricle. 



The posterior commissure forms the highest, and, at the same time, 

 most posterior portion of the thalamus. It is over-arched by the 

 anterior protuberance of the corpora bigemina. The connection of 

 the pineal body with the superior commissure could not be detected, 

 though the pillars passing into the choroid plexus from its lateral 

 aspects may represent the pineal crura. The connection of the 

 commissure with fibres from the taenia thalami is very obvious. 

 The connection of the infundibulum below with the pituitary body is 

 indicated in our figures. The latter is an oval body without ven- 

 tricle or important structural peculiarities. The median or soft 

 commissure separates the third ventricle into two portions, that 

 lying below it passing into the infundibulum. The ventricle is 

 somewhat arched. Back of the foramen of Monro it appears as a 

 narrow cleft, extending upward, as traced backward, then divid- 

 ing, as above indicated, the upper passing into the ventricle of the 



