204 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



There are no glands connected with the eyes in cyclostomes or 

 fishes, but in the urodeles a series of glands is developed from the con- 

 junctival lining of the lower lid. In the amphibia they show little 

 differentiation, but in all sauropsida (glands are lacking from a few 

 reptiles crocodile tears are non-existent) they become divided into 

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

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

 the others 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 lid, where it shows its 

 multiple nature by the numbers of ducts by which it pours its secretions 

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

 ates. The tears secreted by the glands pass over the conjunctiva 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 perforated. It 

 follows the course of an earlier groove (fig. 194) 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 vascular 

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

 extends to the lens where it bears an expansion, the campanula Halleri. 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 mi- 

 grates 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 been recently shown 

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

 size of the animal. 



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



