EYE OF THE ENGLISH SPARROW 357 



The structure of the two hds is in general very similar. The 

 upper lid is somewhat thicker than the lower. The thickness of 

 the marginal folds varies from .324 mm. to .432 mm. The thin- 

 nest portion, which is just above the marginal folds, is from .144 

 mm. to .198 mm. in thickness. Corresponding measurements 

 of the lower lid show the marginal folds to be from .243 mm. to 

 .400 mm. and the thinnest place about the center of the tarsus- 

 like plate is from .09 mm. to .27 mm. 



The skin over the front of the lids is very thin and delicate. 

 It becomes much thicker at the marginal folds and is thickest at 

 the inner margins where it merges into the conjunctiva (fig. 4). 

 Over this portion the basal cells are long and cylindrical and are 

 covered by stratified epithelium. Nmnerous pigment cells are 

 scattered among the epithelial cells of the free margins of the 

 lids and are especially abundant in the long cylindrical cells 

 where the transition to conjunctiva occurs (fig. 4, and plate 

 10, figs. 58, 59, 60, and plate 11, figs. 63, 67). 



The conjunctiva is thickest near the margin of the lids. This 

 thickened region extends for about 1 mm. from the margin. 

 From this point on over the inner surface of the lids, the con- 

 junctiva becomes rapidly thinner until at last it has a uniform 

 thickness. In the region of the inferior fornix conjunctivae, this 

 membrane, as well as the underlying connective tissue, is greatly 

 folded and convoluted (plate 10, fig. 59). This convoluted 

 appearance is scarcely noticeable in the superior fornix conjunc- 

 tivae (fig. 58). This is what one would expect when the move- 

 ment of the two hds is considered. The folds make it possible 

 for the two lids to open and close without putting the conjunc- 

 tiva on a stretch and injuring it. The conjunctiva not only 

 lines the lids, but, at the fornices, is reflected over the whole of 

 the front of the eyeball and completely envelopes the nictitating 

 membrane (fig. 61, plate 10). 



In the region of the inferior fornix the conjunctiva contains 

 numerous goblet cells. Doenecke ('99) has demonstrated these 

 goblet cells in both hds of the hen. In the sparrow I have found 

 them only in the conjunctiva of the inferior fornix. They are 

 equally numerous on both the bulbar and the palpebral portions. 



