56 EVERS's COMPARATIVE ANATOMY. 



purpose of obviating the concussion which would arise from the 

 strong exertions in biting; for such exertions are made by all the 

 animals which possess them, even by the horse in his wild state. 



RECAPITULATION. 



1. A nervous system exists in every class of animals, though not 

 in all the animals of each class. 



2. In the invertebrate classes it has a peculiar tendency to accu- 

 mulate around the oesophagus. 



3. In all the vertebrata its principal parts are protected by osseous 

 sheaths. 



4. The spinal marrow is tubular in the human embryo, and most 

 of the lower vertebrata. 



5. This system undergoes remarkable changes in the amphibia, 

 during their metamorphosis. 



6. It is highly developed, and with great uniformity in birds. 



7. The chord presents enlargements corresponding in size to the 

 members most used. 



8. The spinal chord bears a large ratio to the size of the body in 

 most mammalia. 



9. Man's superiority is due to his mental faculties. 



10. The brain is larger in proportion to the size of the nerves 

 connected with it in man than in any other animal. 



11. The fifth cerebral, or the nerve of instinct, is very large in 

 most mammalia, and in the foetal state of the human subject. 



CHAPTER IX. 



ORGANS OF SENSE. 



General observations. — The organs of sense are those instru- 

 ments which are placed upon the distal extremities of certain cere- 

 brospinal sensitive nerves, whose office being to establish a relation 

 between the internal sentient principle and the external objects of 

 surrounding nature, are necessarily placed in connection with the 

 external surface of animals, and generally in the neighbourhood of 

 the entrance of the alimentary canal. They are more numerous, 

 more varied, and more perfect in the higher than in the lower tribes 

 of animals, but they are more developed in the active insects and 

 others of the articulata, than in the slow and torpid mollusca, and 

 they attain their greatest degree of perfection in the vertebrata where 

 the most complicated and delicate forms of organisation are con- 

 signed to their careful watch. 



Vision. — Organs of vision have been figured and minutely de- 



