ANATOMY OF THE FROG 63 



vascular membrane which covers this part of the head. The 

 true pineal gland is a small vesicle lying outside the skull 

 beneath the skin. In the embryo this vesicle is connected by 

 a pedicle with the roof of the thalamencephalon, but in the 

 adult frog the connection is lost, and there is no foramen 

 between the parietal bones whereby the pineal gland can make 

 communication with the brain. All that remains inside the 

 skull, therefore, is the hollow pedicle. The pineal body is 

 a structure of much interest which occurs in the brains of all 

 Craniata. The older anatomists fancifully described it as 

 being the seat of the soul, but it really is the relic of a once 

 well-developed and functional sense-organ, having the character 

 of an eye, with lens, retina and nerve, the last being re- 

 presented by the pedicle. Such an eye, in a more or less 

 degenerate condition, actually occurs in several living reptiles, 

 and in them there is a distinct foramen between the parietal 

 bones through which the nerve or pedicle passes to the eye. 

 This parietal foramen is obsolete in living Batrachia, but was 

 universally present in certain extinct Amphibia the Stego- 

 cephalia, and it is inferred that in them the pineal eye was 

 well developed and functional. It is worthy of remark that 

 the pineal organ, though it has the structure of an eye, does 

 not resemble the paired eyes of Vertebrata, but rather re- 

 sembles those of certain Invertebrata. 



In front the third ventricle is bounded by a wall of nervous 

 tissue called the lamina terminalis. Right and left of this a 

 passage leads into the cavities of the foremost division of the 

 fore-brain, the cerebral hemispheres, which together constitute 

 the prosencephalon. The hemispheres are ovoid bodies of 

 considerable size relatively to the other parts of the brain. 

 Their smaller ends are directed forward, and produced in front 

 into two rounded swellings, the olfactory lobes, or rhinen- 

 cephala. The hemispheres are united together before and 

 behind by the fusion of their inner walls, but are completely 

 separated from one another in their middle portions by a deep 

 vertical cleft extending from the dorsal to the ventral surface. 

 The rhinencephala are fused together by their inner walls, but 

 both in them and in the fused portions of the cerebral hemi- 

 spheres, shallow median furrows on the dorsal and ventral 

 surfaces mark their double origin. 



The cavities of the cerebral hemispheres are known as the 



