NERVOUS SYSTEM. VEBTEBRATA. 119 



shown. The pallium is quite thin and the lateral ventricle 

 is to a great extent occupied by the corpus striatuui, though 

 apparently not so much so as in the adult. 0. C. 1316. 



Hunterian. 

 CHELONIA. 



D. 140. The cranium of a Turtle ( Chelone mydas) with the brain 

 exposed from above. The Turtle's brain is very instructive, 

 because it readily lends itself to comparison with the 

 corresponding organ in the Amphibia and the lower 

 Ichthyopsida on the one hand, and with that of the diver- 

 gent Avian and Mammalian phyla on the other. 



The brain does not fill the roomy cranial cavity, in which 

 respect it resembles the condition found in Ichthyopsida 

 and differs from birds, mammals, and many other reptiles. 

 In this specimen, the olfactory nerves (not peduncles), the 

 olfactory bulbs, the cerebral hemispheres, the mesen- 

 cephalic roof, the cerebellum, and the medulla oblongata 

 are visible in linear succession. 



The true olfactory nerves (which are such short and 

 insignificant filaments in most Vertebrates, and in man 

 and all mammals have practically no intra-cranial course, 

 because they enter the bulb as soon as they perforate the 

 ethmoid bone) are collected into greatly elongated (40 mm.) 

 rounded cords, that arise anteriorly from the mucous mem- 

 brane of the nasal fossae and end by being inserted into the 

 olfactory bulbs. The fibres during their course are collected 

 into three bundles dorsal, ventral, and lateral which end 

 respectively in the dorsal, ventral, and lateral surfaces of 

 the olfactory bulb. The latter is a hollow, laterally com- 

 pressed vesicle, attached to the anterior extremity of the 

 cerebral hemisphere by a cylindrical peduncle, which is so 

 short that without close examination the bulb appears sessile. 



The cerebral hemisphere is also a hollow, laterally com- 

 pressed vesicle, more than thrice as long as, and propor- 

 tionately deeper than, the olfactory bulb. It completely 

 overlaps the thalamus, but does not cover the roof of the 

 mesencephalon. The epiphysis, together with the dorsal 

 sac and paraphysis, forms a large pyriform mass projecting 

 upwards from between the hinder parts of the two hemi- 

 spheres. 



