MYELON OF EEPTILES. 



295 



supports the optic lobes, is exposed from above, by their removal, 

 in fio;. 194, 2, showing the continua- 



J -s \ ' 



193 



194 



195 



tion of the ventricular cavity through 

 that segment of the brain. The base 

 of the excised ' corpus striatum ' into 

 which the ( crus cerebri ' expands, is 

 shown at i, fig. 194. The prolonga- 

 tion of the optic lobe crosses the 

 cerebral crus, externally, in its way 

 to the optic tract, fig. 195, d\ a por- 

 tion has been removed in this figure 

 to expose the crus cerebri in its ascent 

 to the hemisphere. Three tracts of 

 neurine may be traced from the pros- 

 encephalon to the rhinencephalon, of 



Which the inferior One is the mOSt DissectionB of the b^nofa Turtle, 



distinct, fig. 190. 1 



In the brain of the Crocodile a marked advance is seen in the 

 relative size of the cerebral lobes, especially in regard to their 

 breadth and height posteriorly, giving a pyriform shape to the 

 prosencephalon ; the optic lobes, also, are 

 not inferior in bulk to the cerebellum, and 

 this body shows a transverse fissure on its 

 exterior. The olfactory lobes, which are 

 situated near the hemispheres in the newly 

 hatched Crocodile, recede therefrom, and 

 advance, with a proportional prolongation 

 of the rhinencephalic crura. The optic 

 lobe shows a convex body projecting into 

 the ventricle from its posterior wall, which 

 body is serially homologous with the ' cor- 

 pus striatum ' in the ventricle of the cere- 

 bral hemisphere. In other respects the 

 brain of the Crocodile closely conforms with that of the Turtle. 



With the exception of the anourus Batrachia, the myelon 

 (spinal chord) is continued into the tail, gradually decreasing to a 

 point, and is not resolved into a ' cauda equina.' Such, indeed, 

 is its condition in the tadpole state of the frogs and toads ; but, 

 with the acquisition of the mature form, the myelon shrinks in 

 leno*th, and terminates midway between the fore and hind limbs, 

 beino* resolved in the frog, into the three pairs of nerves which 



Dissections of the brain of a 

 Turtle (Clielone). LIT. 



1 xx. vol. iii. p. 22, No. 1312. 



