SENSE OF SIGHT. 



malia, {Fig 32.) Each) of these points, (which are placed one 

 above, and the other below,) communicate with a little curved 

 canal, which runs inwards, and opens into a vertical conduit, that 

 is larger in size, called the nasal canal, and which empties into the 

 nasal fossae. The function of these lachrymal pnucta is to pump up 

 and receive the tears as fast as they are poured over the eye: in 

 this way the fluid is carried off as fast as it is formed. Under par- 

 ticular circumstances, the equilibrium between these two pheno- 

 mena is destroyed ; and either that the tears are secreted in too 

 large a quantity, or the lachyrymal puncta do not pump them off 

 with proportioned activity, or they are obstructed in their passage 

 through the lachrymal ducts and nasal canal, this fluid overruns 

 the eye-lids and falls in considerable quantity along the cheeks. 



38. The eye-brow*, which form a ridge above the orbit and are 

 garnished with hairs, also belong to the protecting organs of the 

 eye, but their use is less important than that of those organs of 

 which we have just spoken. They assist in shading the eyes 

 wlien exposed to strong light. 



39. The niofor organs of the eye Fig. 33. 

 consist of six muscles which are 



fixed, by their anterior extremities 

 into the sclerotica, and by their 

 posterior extremities to the bottom 

 of the orbit, ( Fig. 33, //*.) By con- 

 tracting they direct the ocular 

 lobe, to the side where their mus- 

 cular tibres are placed. 



40. The apparatus of vision pre- 



sents nearly the same structure in the mammalia, birds, reptiles, 

 and fishes, but in insects, the organization of the eyes is very 

 different, as we shall see when we come to the history of these 

 animals. 



41. Through the medium of the senses we take cognizance of 

 all that surrounds us ; but our relations with the external world 



Explanation of Fig. 33. The orbit, opened to show the position of the eye 

 in this cavity, and the musc'e* which move it, cr.the cranium, wo. the 

 orbitary arch, po. the floor of the orbit, p. and pi. the upper and lower 

 eye-lids, s. the globe of the eye, c. the conjunctiva, n. the optic nerve, 

 m. m. two muscles of the eye, ml. a third muscle cut so as to afford a view 

 of the optic nerve. 



38. What are the eye brows? 



39. What are the motor organs of the eye ? 



40. Is the organ of vision the same in all animals 



,11 1171 



.~. . v< uigini ui vision me same in an animals ' 



41. What powers are requisite besides the senses, to complete OUT rela 



IMS With thu r>v*r>->nl ,,,~.1,>1 



ions with the external world? 





