Eyelids and Lachrymal Apparatus of Birds. 
9 
is composed of (1) an anterior epithelial layer, (2) a middle con- 
nective tissue layer and (3) a posterior epithelial layer. 
The anterior epithelial layer has the appearance of typical pave- 
ment epithelium. The deep layers are more cylindrical in form, 
while the superficial are more flattened and show oval nuclei. 
Pigment cells begin in the deep cells but get less and less in amount 
as the surface is reached. 
The middle layer constitutes the true substance of the nictitating 
membrane and is largely made up of elastic fibres interwoven with 
connective tissue fibres. The former are most numerous immedi- 
ately under the epithelial layer. The middle layer is provided with 
numerous blood-vessels, nerves, and a number of tubular, solitary 
glands. Slonaker and the writer have not been able to find the 
latter in Passer, but we do recognize there glands which are some- 
times straight, and sometimes globular, like sweat glands. Their 
openings are on the anterior surface of the membrane. 
• 
The posterior epithelial layer is composed of cylindrical epithe- 
lium two and three layers deep. The deepest cells are polyhedral in 
shape, while the more superficial have long prismatic elements. 
This layer of the conjunctiva is finally continued as modified ante- 
rior corneal epithelium. 
According to Fumagalli, the elastic fibres of the third lid run in 
all possible directions through the connective tissue bundles to 
form a thick network, which may be resolved into three layers. 
Furthermore, a bundle of these fibres is shown extending from the 
base to the apex of the lid. It lies in the deep portions of the con- 
nective tissue, directly on the posterior epithelial layer. From this 
deep, basement or foundation layer of larger fibres there stretch at 
right angles to it more delicate fibres through the whole width and 
thickness of the membrane, and terminate in the cells of the ante- 
rior epithelial layer. 
This strong, deep-lying bundle becomes thicker the nearer one 
approaches the free border, until it forms two or three fibrous 
bundles measuring 123 microns wide that eventually becomes part 
of the tendon of the pyramidalis muscle. 
Fumagalli finds that posteriorly the elastic connective nerve-fibre 
bundles are so disposed as to form a subepithelial network from 
which still finer fibrils extend, some of which terminate in end- 
corpuscles. 
Slonaker and the writer, after considerable time spent in an in- 
vestigation of the subject, conclude that the nictitating membrane 
is a conjunctival duplication — a thin, translucent membrane com- 
