PO SENSE OF SIGHT. 



man there are two eye-lids, one superior, and the other inferior. 

 The superior eye-lid is larger than the inferior 



33. Each eye-lid has two edges or borders; one is continuous 

 with the skin, the other is free. The free border of the eye lids, 

 is bristled with delicate hairs, called cilia, or eye-lashes. The 

 use of the cilia is to form a kind of little grating in front of the 

 eye, to arrest foreign bodies, the presence of which would inter- 

 fere with the exercise of vision. 



34. The eye-lids perform the double office of protecting the 

 ball of the eye, by closing in front of it, and of rendering it inac- 

 cessible to luminous rays, the brillrancy of which might disturb 

 sleep. Besides, the eye-lids by their alternate movement ot 

 depression and elevation, spread over the front of the globe of 

 the eye, the tears, an aqueous liquid, which prevents the cornea 

 from drying, and also favours the motion of the eye-lids. 



35. The lachrymal apparatus, which secretes the tears is com- 

 posed of several organs, some of which are destined to form this 

 liquid, and pour it over the front of the eye; and as the presence 

 of the tears, if too long continued, would become troublesome, 

 other organs convey them from the eye. The first organs, are : 



Fio. 32. 26. 1st. The lachrymal ylaiid, a small body, 



the size of an almond, placed at the exterior 

 anc ^ su P er i r P art f tne globe of the eye, be- 

 tween it and the orbitary cavity, (fig. 32, gl.)i 

 it serves to secrete the tears. 2nd. Several 

 'en sma u canals which arise in this gland, and open 

 upon the internal face of the adhering; border of 

 the upper eye-lid, where they constantly pour upon the conjunc- 

 tiva the lachrymal fluid, or tears. 



37. The organs destined to carry away those tears which have 

 been spread over the front of the eye, and to convey them into 

 the nasal fossae, or nostrils, are two little canals which open upon 

 the free border of the eye-lids, near the internal angle of the eye, 

 by two small orifices called the lachrymal points, puncta lachry- 



Explanation of Fig. 32. The eye seen in front, p. the pupil, and i. the 

 iris, seen through the transparent cornea, gl. the lachrymal gland, pi. the 

 lachrymal points or puncta lachrymalia, en. nasal canal. 



33. What is the use of the eye lashes ? 



34. What is the use of the eye lids? 



35. What is the use of the lachrymal apparatus ? 



36. Of what use is the lachrymal gland ? How do th* tears pass from 

 Ihis gland? 



37 What are the puncta lachrymalia ? What becomes of the tears after 

 they have moistened the ball of the eye ? What is the nasal canal ? Explain 

 the phenomenon of crying ? 



