EYES 219 



two groups. One becomes aggregated near the inner angle and forms 

 what is known as Harder's glands (glandula membrana nictitans) ; 

 the other migrate toward the outer angle of the eye and constitute the 

 true lacrimal or tear gland. In the mammals the migration continues 

 until the gland comes to lie beneath the upper Ud, where it shows its 

 multiple nature by the numbers of ducts by which it pours its secre- 

 tions into the conjunctival sac. In most mammals Harder's gland 

 degenerates. The tears secreted by the glands pass over the conjunc- 

 tiva and are collected at the inner angle of the eye, where they are 

 drained by the lacrimal duct into the cavity of the nose. This duct 

 is formed as a thickening of the epidermis which later becomes per- 

 forated. It follows the course of an earher groove (fig. 217) leading 

 from the orbit to the nasal invagination and which was formerly 

 thought to form the duct. 



The eyes of the cyclostomes are degenerate. In the larval (Ammocoetes) 

 stage of Petromyzon the eye is buried under a thick skin, but this thins out in the 

 adult. In the myxinoids the lens and eye muscles are lacking, and iris, cornea 

 and sclera are not differentiated. 



Fishes have eyes with a very flattened cornea, a spherical lens and very long 

 retinal rods. A peculiar feature in many fishes is the falciform process, a vas- 

 cular and muscular structure which enters the retinal cup through the chorioid 

 fissure and extends to the lens where it bears an expansion, the campanula 

 Hailed. The whole is supposed to act as a means of accommodation, there 

 being no ciliary muscles. In most fishes the eyes are so placed on the sides of 

 the head that there must be monocular vision. In the flat fishes (Heterosomata) 

 one of the eyes migrates during development, so that both eyes come to lie on 

 one side of the head. 



Most sauropsida are characterized by the development of a process from the 

 inner retinal surface which reaches its extreme in the pecten of the birds. In the 

 reptiles it is a small conical process arising from the point of entrance of the optic 

 nerve, but in the birds this expands distally into a quadrangular plate, folded like 

 a fan, to which various functions have been ascribed. It has recently been 

 shown to be rich in sense cells. The shape of the eye of the bird is peculiar, but 

 ii not easily described. It consists of a hemispherical posterior part, followed 

 by a conical portion, and this surmounted by a hemispherical corneal region, 

 the whole being somewhat telescopic in shape. The whole is very large in pro- 

 portion to the size of the animal. 



The pecten is said to be outlined in the foetal stages of some mammals. The 

 pupil of the mammals is not always circular, but is a vertical slit in the cats, a 

 horizontal opening in the whales, many ungulates, etc. During development 

 the lids fuse for a time, separating in some cases, only after birth. The edges of 

 the lids are fringed with short hairs, the eye-lashes or cilia, and internal to these 

 are the ducts of sebaceous glands (tarsal or Meiobomian glands), the glands them- 

 selves being in the substance of the lids. The whales have an enormously thick 



