252 ANATOMY, PHYSIOLOGY, &C. 



The Optic Nerve. 



If the optic nerves of an animal be divided, it be- 

 comes instantly blind, although the eyes remain un- 

 touched, and capable of acting on light as before. 



Insects are the last in the scale of animated nature 

 possessing eyes, and we have reason to suppose they 

 see very obscurely. 



Although old persons lose the power of distin- 

 guishing near objects, and require for this purpose 

 convex glasses, they usually retain the sight of those 

 that are distant as well as when they were young. 



The Liver. 



The weight of the liver, which in healthy subjects 

 is about 3j pounds, in liver disorders sometimes in- 

 creases to 18, and even 24 pounds. 



The Ribs. 



The ribs in general are 12 on each side, but in 

 some 13, and in others, though rarely, only 11. 



The Teeth. 



At full maturity, we usually find 32 teeth in both 

 jaws, viz. 16 above, and 16 below. The number 

 varies in different subjects, but it is seldom seen to 

 exceed 32, and is very rarely found to be less than 

 28 teeth. 



Grinders above 10 



Incisores, cutting, or front teeth 4 

 Canine or dog-teeth .... 2 



Above . . 16 

 Below 16 



