240 NERVOUS SYSTEM. 



columns of cineritious matter have en creased in their interior. 

 The spinal chord is prolonged to a variable extent through 

 the column, being shortest in the tailless cheiroptera, qua- 

 drumana, and man, and longest in the cetacea, where it ex- 

 tends tapering through the coccygeal vertebrae, without pos- 

 terior enlargement, from the want of legs, as in apodal fishes, 

 and tadpoles, and serpents, and the human embryo. In the 

 long-tailed quadrupeds the spinal chord is extended to the 

 sacrum, and the detached nerves of the cauda equina are 

 prolonged through the coccygeal vertebrae. The ganglionic 

 enlargement of the spinal chord at the origin of each pair of 

 moto-sensitive nerves is now scarcely apparent, and the en- 

 largements corresponding to the atlantal and sacral extremi- 

 ties are of a more lengthened and fusiform shape. The 

 medulla oblongata is comparatively narrow, but it is more 

 deeply marked by the limits of the corpora pyramidalia, 

 olivaria, and restiformia. The decussations of the corpora 

 pyramidalia are more numerous and distinct, and the cine- 

 ritious matter of the corpus dentatum is generally percep- 

 tible in the corpora olivaria, which are themselves propor- 

 tionally small. The crura cerebri are traversed below by 

 the great commissure of the cerebellic hemispheres, the 

 tuber annulare, which is more or less provided internally 

 with transverse strata of cineritious matter. 



The optic lobes, reduced in size, generally without ca- 

 vities, and traversed externally by sulci, which give them 

 a quadrigeminous appearance, are largest and most exposed 

 above in the lowest mammalia, as the rodentia and edentata, 

 they are larger in herbivorous than in carnivorous quadrupeds, 

 and are least in bulk, and most concealed in quadrumana 

 and man. The anterior lobes of the corpora quadrigemina 

 are larger than the posterior in herbivorous mammalia, the 

 posterior are the larger in carnivora, and these lobes are 

 nearly equal in the highest orders of this class. The ce- 

 rebral hemispheres follow a contrary ratio in their deve- 

 lopment, being smallest and destitute of convolutions in 

 the rodentia, and becoming larger in every dimension and 

 more marked with deep convolutions, as we advance up- 

 wards to man. The optic thalami and the corpora striata 

 encrease with the expanding hemispheres, while the olfactory 

 tubercles progressively dimmish. The olfactory tubercles 



