THE SPINAL CORD OF VERTEBRATA. 327 



relatively larger, in the apes and monkeys, than in the Carnivora, and larger 

 in these, than in the Ruminants or Rodents ; it is also larger in Birds than in 

 Reptiles ; and it is larger in the active predaceous sharks, which can turn 

 themselves round and round, and even swim sideways in the water, than in 

 the ordinary and more simply swimming fishes. Again, in comparing indi- 

 vidual genera of the same Order, this organ is more developed in the Anthropoid 

 apes, than in the monkeys, the former being able to assume a more manlike 

 attitude than the latter ; and it is also larger in the bear, which can tempo- 

 rarily assume an erect posture, than in the dog. In Man, in whom the cerebellum 

 reaches its highest development, besides the innumerable and complex move- 

 ments of the upper limbs, the co-ordination of the multitudinous individual 

 motions, necessary to preserve the equilibrium of his erect body, in standing 

 and walking, is much more perfect than that required in the quadruped form 

 of locomotion ; and, again, in Birds, in which the cerebellum is still more 

 simple than in Mammalia, the movements of the wings are also more simple, 

 employ but few muscles, are maintained for a long time without fatigue, and 

 exhibit rather an automatic than a volitional character, as shown by the 

 attempted flight of birds when deprived of their cerebrum, or even when 

 decapitated. 



The pons Varolii. In proportion to the gradual diminution of the cerebel- 

 lum, especially of its lateral parts or hemispheres, the pons Varolii becomes 

 diminished in size ; and when the hemispheres are reduced to insignificant 

 appendages, as in Birds, or are absent as in Reptiles, Amphibia, and Fishes, 

 the pons does not exist, a fact which indicates the physiological use of the 

 pons, to be to establish functional relations between the two hemispheres of 

 the cerebellum. 



The Medulla Oblongata and Spinal Cord. The medulla oblongata also shares 

 in that gradual simplification, which is observed in the rest of the encephalon, 

 in the descending series of the Vertebrata. The first parts to become dimin- 

 ished, and then to disappear, are the olivary bodies, their corpora dentata and 

 the columns of white matter connected with them, these parts not being dis- 

 tinctly present below the Mammalia. The anterior and posterior pyramids, 

 and the restiform bodies, accordingly gradually preponderate, and finally 

 constitute the entire mass of the medulla oblongata, the size of which presents 

 a general correspondence with that of the body of the animal, and becomes 

 larger, in proportion to the cerebrum, in the descending vertebrate scale. 

 The triangular depression formed, on its posterior surface, by the divergence 

 of the restiform bodies, becomes plainer in the lower animals, and is more 

 directly continuous with the central canal in the spinal cord. In certain 

 Fishes, the two halves of the medulla, are actually separate from each other, 

 leaving an oblong opening in the middle line, which has been compared, though 

 incorrectly, with the ring or collar of nervous substance, which surrounds the 

 oasophagus in the Mollusca and Annulosa : for the oesophagus never perforates 

 this divided medulla oblongata, even in the lowest fishes. 



The spinal cord likewise exhibits signs of simplification. Its two enlarge- 

 ments, cervical and lumbar, are present in all the Vertebrata which possess 

 well-developed anterior and posterior limbs. In most of the Mammalia, the 

 lumbar enlargement is of greater size than the cervical enlargement. But 

 amongst Birds, those which are remarkable for their powers of night, like the 

 eagle, have the cervical enlargement larger than the lumbar ; whilst, in the 

 cursorial or running birds, as in the ostrich, the reverse is the case. In the 

 limbless Ophidian Reptiles, and also in the Fishes, the pectoral and abdominal 

 fins, or limbs, of which are so small, and have such minute muscles, both these 

 enlargements are entirely absent, and the cord is of uniform diameter, or finely 

 conical, gradually increasing from its lower end upwards. The extent to 

 which it descends within the spinal canal, is greater in the cold-blooded Ver- 

 tebrata generally, than in Mammalia and Birds. In certain Fishes, in which 

 the body is very short, the cord is equally concentrated, and the cauda equina 

 is very long. In a few rare Cycles tomatous fishes, as Orthagoriscus and 

 Trigla, the cord presents numerous constrictions, which give it a beaded ap- 

 pearance, and prove the real segmented character of the spinal nervous axis, 

 which, in the Vertebrata generally, is masked by the even fusion of its parts. 



