SENSE OR RECEPTOR ORGANS 853 



The cornea of the eyeball is formed mainly from the mesenchyme of the 

 sclerotic coat which extends forward in front of the developing anterior cham- 

 ber of the eye (fig. 360H). The overlying skin forms the corneal epithelium. 



4) Contributions of the Pars Caeca. The pars caeca or non-nervous part 

 of the primitive optic cup gives origin to the smooth muscle tissues of the iris. 

 These muscles are derived probably from the pigmented layer of the original 

 optic vesicle. The zonula ciliaris or suspensory ligament of the lens also is 

 derived from this source. 



5) The Origin of the Ciliary Muscles. The smooth muscle tissue of the 

 ciliary muscle together with the connective tissue of the ciliary bodies, and the 

 stromal tissue of the iris, are derived from the mesenchyme of the primitive 

 choroid coat which overlies the pars caeca of the optic cup. 



6) Accessory Structures of the Eye. The upper and lower eyelids develop 

 as folds of the integument about the eyeball and circumscribing the corneal 

 area (fig. 360H). In the chick these folds are apparent on the seventh day 

 of incubation, and in the human embryo at about the seventh week. In mam- 

 mals the eyelids normally fuse after their formation, and in many they do 

 not reopen until some time after birth. In the dog the eyelids reopen at about 

 10 to 15 days after birth, while in the human and guinea pig they reopen 

 before birth. Complete fusion of the eyelids does not occur in the chick. A 

 third or rudimentary eye structure, the plica semilunaris, is present at the 

 inner angle of the human eye. This structure may represent the nictitating 

 membrane in the cat and dog, and possibly also the nictitating membrane of 

 the chick. The real homology of the plica semilunaris with these structures, 

 however, is questionable. 



Accessory eye glands arise in land vertebrates. The lacrimal glands arise 

 as epidermal ingrowths from the inner aspect of the developing upper (man, 

 cat, dog) or lower (urodeles) eyelid. The lacrimal gland is developed typically 

 in mammals. The racemose harderian gland arises as a solid ingrowth of epi- 

 dermal cells at the inner ingle of the nictitating membrane. The secretion of 

 the harderian gland found in reptiles, birds, and also in certain mammals is 

 oily while that of the lacrimal gland is watery. The tarsal (Meibomian) glands 

 of the human eyelid arise as epithelial invaginations. 



The naso-lacrimal duct in mammals arises from the naso-lacrimal groove 

 formed in the area of the lateral nasal and maxillary processes; it extends 

 from the nasal sac to the angle of the eye (fig. 256). During the formation 

 of the face this groove sinks inward and forms a duct which establishes a 

 definite connection with the inner edges of each eyelid. It opens into the nasal 

 chamber. 



8. Structure and Development of the Ear 

 a. Structure 



The functions of hearing and equilibration (balance) in the gnathostomous 

 vertebrate group involve the structure known as the membranous labyrinth 



