150 REPTILIA. 



origin of the nerves to the extremities, but which are wanting in 

 the Ophidia ; the genus Bipes among the Sauria possesses only a 

 posterior swelling upon the cord, and Chirotes only an anterior, 

 in accordance with the position of the rudimentary extremities al- 

 ready described. The spinal cord is always traversed by a cen- 

 tral canal. The medulla oblongata is flattened like that of Fishes 

 in the naked Reptilia, and the rhomboidal sinus is broad and pa- 

 tulous ; in the Squamigerous Reptilia this part is more dilated, 

 especially in the proximity of the pyramidal tracts, and the pons is 

 absent. The cerebellum arises by two crura, and consists in the 

 naked Amphibia and Ophidia of a hollow medullary layer, which 

 passes as a small narrow band transversely over the fourth ven- 

 tricle, without covering it completely ; in the Chelonia it forms a 

 smooth spherical and hollow swelling, and in the Sauria, as in the 

 Crocodiles, it is provided with one or several lateral transverse 

 grooves. In front of the cerebellum are situated a pair of large 

 oval ganglia, hollow internally, and partly coalescing in the middle 

 line in the naked Amphibia, as the Ichthyic genera Proteus and 

 Amphiuma, but which are of largest proportional size, and distinct 

 from each other in the Frogs, and also in the Sauria ; they repre- 

 sent the corpora quadrigemina, probably incorporated with the 

 optic thalami. To these succeed the oval cerebral hemispheres of 

 considerable size, and which, smooth and devoid of convolutions, 

 give off the olfactory nerves. Within their lateral ventricles is 

 placed, at least in the Crocodiles and Tortoises, an elongated gang- 

 lion, which corresponds perhaps to the corpus striatum ; and by 

 the side of this is a distinctly developed choroid plexus, while a 

 system of commissures for uniting the double parts of the brain 

 is always met with. The pineal gland is very large in the Che- 

 Ionia, but smaller in the other orders, and lies uncovered by the 

 hemispheres in front of the corpora quadrigemina. Although this 

 gland is met with in all the Amphibia, it is difficult to determine 

 whether it is really present in the Frog. The pituitary appendage 

 is constantly present, and of noticeable size. The ventricles coalesce 

 together ; the aqueduct of Sylvius is a very wide canal, and the fourth 

 ventricle is quite open and uncovered from above, especially in the 

 Amphibia. 



The Cerebral nerves admit of being very easily reduced to their 

 analogues in the human subject and the rest of the Vertebrata, and 

 have been traced with the most special care in the Frog, Tortoise, 

 and Serpent. 



