ANATOMY OF THE FROG 63 



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 This parietal foramen is obsolete in living Batrachia, but was 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, for example, 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 resembles 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 forwards, and produced in 

 front into two corresponding rounded swellings, the olfactory 

 lobes, or rhinencephala. 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 hemispheres shallow median furrows on the 

 dorsal and ventral surfaces mark their double origin. 



The cavities of the cerebral hemispheres are known as the 

 lateral ventricles. They are separate from one another, but 

 each opens by a short narrow passage into the third ventricle 

 just behind the lamina terminalis. The Y-shaped opening 

 thus produced is known as the foramen of Munro. The 

 lateral ventricles are ovoid cavities with rather thin walls in the 



