14 
Casey A. Wood. 
The histology and 'physiology of the free border of the third lid 
of Birds have to do with the cleansing of the cornea, so that it shall 
be free of foreign matter and continuously moist and transparent. 
The accompanying semi-diagrammatic cut shows the free margin of 
the Sparrow’s nictitating membrane, both at the center and toward 
the upper cul-de-sac. In this situation it preserves its usual struc- 
tural characters, that of a plate of dense connective tissue covered 
before, behind and at its border by conjunctival epithelium. In 
the first figure the slightly pigmented, irregular border presents the 
general outline of the barb of a fish-hook, or of an anchor with one 
of its flukes cut off close to the shaft. The relations of the pro- 
cesses (really a marginal band) of the free border to the surround- 
Fig. 8 — Left Eye of the Sparrow with the Lower Lid Dissected Loose 
and Turned Down to Show the Opening of Harder’s Gland (H) Beneath the 
Nictitating Membrane (N) and the Attachment of the Tendon (T), which 
Moves the Membrane, x 8. (Wood and Slonaker.) 
Cj, conjunctiva; L, lachrymal gland; p, pigment portion of the nictitat- 
ing membrane (shaded portion); Ta, outline of tarsus. 
ing parts are interesting. It would seem as if the space behind the 
process or band is obliterated by the traction on it of the pyramidal 
muscle when the membrane is swept over the cornea. The corneal 
detritus (mixed with the Harderian secretion) is then pushed 
before the shelving margin of the membrane — like a rubber scraper, 
or the sharp margin of the lids in Mammals. 
On the return journey, when the pyramidalis and quadratus are 
passive and the elastic fibres are in action, a quite different condi- 
tion is apparent. The margin of the pectinate fold now rises and 
