SENSE 01? SIGHT. 185 



developed (Fig. 150). (3) The inner wall is formed by the 

 lateral nasal process, in which the nasals, lachrymals lateral 

 mass of the ethmoid, are formed. The optic nerve enters the 

 orbit between the orbito- and pre-sphenoids- — derivatives of the 

 trabeculae cranii, both of which help to form the orbit. The 

 great wings of the sphenoid are also derived from the trabeculae 

 (p. 168). The orbital plate of the malar cuts the orbit off from 

 the temporal fossa ; it is developed in higher primates only. 

 The nasal duct is formed between the maxillary and nasal pro- 

 cesses (Figs. 1 and 150). In lower primates and mammals 

 generally the hamular process of the lachrymal appears on 

 the margin of the orbit ; the pars facialis lachrymalis is some- 

 times seen in the human skull (Fig 20, p. 26). 



The Eyelids are formed by folds or ridges of epiblast which 

 rise above and below the cornea. Mesoblast grows into the 

 folds and forms the tarsal plates. The upper eyelid is formed 

 from the capsule of the fore-brain, the lower from the maxillary 

 process. The epiblast on the deep surface of the folds forms the 

 palpebral conjunctiva. It is continuous with the epiblast of 

 the cornea. The lid-folds meet in front of the cornea during the 

 third month and adhere by their edges. The edges become again 

 separated about the 7th month. From the epiblast between their 

 adherent edges, buds grow into the lids and form the cilia, Meibo- 

 mian and other glands in the same manner as hairs and sweat 

 glands are developed. 



The plica semilunaris (Fig. 151), a fold of conjunctiva in 

 the inner canthus of the eye, is a vestige of the third eyelid 

 (membrana nictitans) which is so fully developed in birds and 

 reptiles. It is well seen in the cat, partially crossing the cornea 

 as the lids are shut. The lachrymal papillae rub in the grooves 

 at the outer and inner margins of the fold. 



The Lachrymal Gland arises as a number of epiblastic buds 

 which spring from the fornix of the conjunctiva beneath the 

 upper lid and grow into the tissue of the outer and upper 

 segment of the orbit. The outer buds form the orbital part 

 of the gland; the more internal buds form the palpebral part. 

 Smaller lachrymal glands may occasionally be found at the 

 •outer angle of the eye. This is the position occupied by the 

 lachrymal glands of birds and reptiles (Wiedershiem). The 



