EYE OF THE ENGLISH SPARROW 359 



They rapidly diminish in number as one leaves this region, and 

 are wanting in the main portion of the lower lids and the balance 

 of the conjunctival surface. The tarsal or Meibomian glands 

 of mammals are entirely lacking in birds. 



An approximatal oval plate, 2,43 mm. broad and 1.98 mm. in 

 height, resembling the tarsal cartilage in the lid of man, occurs 

 in the lower lid, but there is none in the upper. It covers the 

 greater part of the extent of the lower lid (figs. 5 and 8). It is 

 almost uniform in thickness over the greater portion with the 

 exception of the edges which diminish quickly. A slight dif- 

 ference is noted in the thickness between the upper part and the 

 lower, to which the inferior palpebral muscle is attached, the 

 upper part being .073 mm. and the lower .099 mm. in thickness. 

 This extra thickness evidently compensates for the strain due 

 to the pull of the muscle. The whole plate is saucer-like, con- 

 forming to the curvature of the cornea. At first sight this 

 tarsal-like plate resembles cartilage, but there are no cartilage 

 cells present. It is composed of a firm and closely connected 

 tissue mass. This plate lies immediately adjacent to the pal- 

 pebral conjunctiva. 



About midway between the two surfaces of the lids are large 

 lymph spaces, more or less connected and crossed by connective- 

 tissue bundles, blood-vessels, and nerves. These spaces extend 

 from the marginal folds to the region of the distal attachment of 

 the palpebral muscles. A vertical section through the Uds 

 shows the cross-section of a few muscle fibers, constituting the 

 orbicularis muscle which serves to close the lids. Unlike all the 

 other muscles of the eye, the orbicularis is composed of non- 

 striated fibers. 



Fig. 4 Enlarged semidiagrammatic drawing of a vertical section of the adult 

 sparrow eye. B, Briicke's muscle; Bv, blood-vessels; C, cornea; Ch, chorioid; 

 Cj., conjunctiva; Cm, circular muscles of the iris; Cp, ciliary processes; Cr, 

 Crampton's muscle; Cs, canal of Schlemm; E, epithelium; F, feather follicle; 

 IP, inferior palpebral muscle; L, lenticular portion of the lens; LI, lower lid; 

 Ls, lymph spaces; M, Midler's muscle; N, nictitating membrane; Ob, orbicularis 

 muscle fibers of the lid; Pc, pars ciliaris retinae; Rm, radial muscles of the iris; 

 Rp, ring-like pad of the lens; S, sclera; Scl, scleral plates; SP, superior palpebral 

 muscle; T, tarsus; U, uvea; Ul, upper lid. 



