520 EXTERNAL SENSES. 



" 7. In different species of animals, and in individuals of the 

 same species, the nerves of sense are developed and decline at 

 very different periods. 



" 8. No decussation of any other nerve than the optic is at 

 present known, and its decussation is not found in all species of 

 animals. 



" 9. The corresponding nerves of each side communicate toge- 

 ther by commissures, and in other parts of the brain by branches.'' 



Again, that, 



" 1. To the functions of the senses, material instruments are 

 indispensable. 



" 2. That the nerves merely communicate the irnpfession of 

 the external world to the brain that it may be modified by this. 



" 3. Every nerve of sense can receive but certain impres- 

 sions, and the functions of one sense cannot be performed by an- 

 other. 



" 4. The delicacy of every sense is ordinarily proportionate to 

 the perfection and development of the apparatus, and probably 

 also to the number of apparatus. 



" 5. The particular functions of the senses have not the same 

 force in different species of animals, nor in different animals of the 

 same species : the animal which has acute sight may have dull 

 hearing. 



" 6. The nervous system of the senses may, like other systems, 

 acquire a higher activity by unusual irritants, from inflamma- 

 tion, &c. 



" 7. The derangement of the functions of the senses that follow 

 lesion of the brain do not affect the opposite side any more 

 than those of the spinal chord, at least according to my present 

 experience. 



"' 8. The functions of the different senses manifest themselves 

 at different periods, according to the development of these organs. 

 It is asked how, and for what purpose, some animals are born 

 with senses perfectly developed, at least with the eyes and ears 

 open, and others with them closed. This peculiarity is not 

 always in relation to the power of using the extremities more or 

 less promptly ; for the new-born child is as incapable of loco- 

 motion as the new-born puppy. 



" 9. All the functions of the senses gradually decline in old 

 age." According to some physiologists this is the result only of the 



