504 HISTORY OF THE HUMAN BODY 



amphibians, form as dorsal and ventral folds of the integument, 

 which may become stiffened, either by connective tissue or by 

 cartilage, as in mammals (tarsal cartilages). That portion of 

 integument which forms the inner face of the folds and is con- 

 tinued over the front of the eyeball is very thin and sensitive, 

 and forms the conjunctiva. A nictitating membrane is formed 

 in some vertebrates by an inner fold of this last ; it attains in 

 birds and in some mammals the dignity of a third eyelid; in 

 Man it is represented by the plica semilunaris, a delicate fold 

 situated in the inner corner. 



! The lubricating fluid, the " tears," is furnished by two 

 groups of glands which arise as invaginations of the conjunc- 

 tiva and retain their connection with that layer, supplying the 

 pockets formed by the lids. These are ( i ) the harderian 

 glands, which are located about the anterior (inner) corner and 

 are associated with the nictitating membrane, and (2) the 

 lacrimal glands, located near the posterior (outer) corner. 

 This differentiation is not found in the amphibians where the 

 glands are all alike and are evenly distributed, but appears in 

 reptiles, from which point the two groups are distinct, both in 

 location and structure. The harderian glands are well de- 

 veloped in reptiles, birds and most mammals, but are rudi- 

 mentary in the Anthropoidea. The lacrimal gland is asso- 

 ciated in reptiles and birds with the lower eyelid, beneath which 

 its ducts empty, but migrates in mammals to a more dorsal 

 position and thus becomes almost exclusively associated with 

 the upper lid. In some mammals a few ducts occur in the 

 lower fold; indications of its former location. The lacrimal 

 fluid, supplied by both of these glands, is continually being se- 

 creted and is as constantly spread in an even layer over the 

 outer surface of the eyeball by the movement of the lids. The 

 excess is conveyed to the nasal cavities through the nasolacrimal 

 duct, which appears in amphibian larvae as an integumental 

 groove extending from eye to nostril. This eventually closes 

 up, sinks into the interior, and gains its independence from the 

 integument, thus forming an internal canal connecting the con- 

 junctival sac with the anterior end of the nasal cavity. 



