ORGANS OF THE SENSES. 



organized for seeing in the erect position of the trunk, that 

 we find the most complete protection of the orbits by solid 

 osseous parietes and the most parallel direction of their 

 axes. Shaded externally by the eye-brows which are moved 

 by corrugator muscles, and protected by two highly moveable 

 eye-lids formed by the common integuments which continue 

 thin and transparent, as a conjunctiva, over the fore part of 

 the organ, the human eye presents in form of a plica semi- 

 lunaris, only a small rudiment of the third eye-lid or 

 membrana nictitans so highly developed in most of the 

 inferior vertebrata. The upper eye-lid is now the largest 

 and the most moveable ; both eye-lids are supported by tarsal 

 cartilages, they are provided with long cilia symmetrically 

 formed and disposed, arid with numerous glandular follicles 

 or meibomian glands, which pour out their secretion by 

 minute pores along the inner margins of the eye-lids, and 

 the eye-lid is perforated within by the several ducts pro- 

 ceeding from the lobules of a large lachrymal gland situate 

 in the upper and outer part of the orbit. There are three 

 or four irregular rows of cilia in each eye-lid, which are more 

 numerous and larger in the upper than in the lower, and in- 

 crease in size from the two angles to the middle of the eye-lids, 

 the large upper eye-lid has its proper levator muscle which 

 is not required in the lower, and both are closed by the 

 orbicularis palpebrarum. The glandula Harderi so large in 

 the inferior quadrupeds and birds, where the third eye -lid 



PART ill. T 



