686 



Comparative Morphology of Chordates 



Fig. 512. Diagrammatic section of half a mammalian eye. {ac) Anterior cham- 

 ber; (c) eyelash (cilium); (ca) ciliary arteries; (cj) conjunctiva; (co) cornea; (cp) 

 ciliary process; (cs) conjunctival sac; (ct) choroid tunic; (cv) central retinal artery 

 and vein; (d) dura of optic nerve; (i) iris; (on) optic nerve; (os) ora serrata; (p) 

 posterior chamber; (pe) pigmented epithelium; (r) retina; (sc) sclera; (tg) tarsal 

 gland; (vv) vorticose vein; (zz) zonula zinii. (Courtesy, Kingsley: "Comparative 

 Anatomy of Vertebrates," Philadelphia, The Blakiston Company.) 



across the eye, to a minimum in man, where it appears as an apparently 

 useless rudimentary fold, the plica semilunaris, at the anterior 

 (nasal) angle of the lids. It is not found in whales. 



The lacrimal apparatus (Fig. 513) is usually well developed, 

 except in aquatic mammals. As in reptiles, there is a Harderian gland 

 at the anterior (nasal side) region of the eyeball and in close relation 

 to the nictitating membrane, and a posterior lacrimal gland. The 

 lacrimal gland, however, becomes more important in mammals, and 

 in higher primates the Harderian gland is rudimentary or lacking. 

 Further, whereas in reptiles the several ducts of the lacrimal gland 

 all open within the lower lid, in mammals they open at the region of 

 the posterior angle between the lids, some below and some above, or 

 (as in man) all on the upper lid. Beneath the anterior (nasal) angle of 

 the lids, the very thin space (conjunctival sac) between either lid 

 and the eyeball communicates with a small sac, the lacrimal sinus, 

 leading into a nasolacrimal duct whose other end opens into the an- 

 terior region of the lower respiratory part of the adjacent nasal cavity. 



The secretion of the lacrimal gland is a watery fluid containing 

 (in man) small quantities of various salts such as occur in blood (mostly 

 sodium chloride), and very small amounts of protein substances and 

 sugar. Also, a mildly antiseptic agent is present. A little mucus in the 

 fluid probably comes from small conjunctival glands. The secretion of 

 the Harderian gland ordinarily resembles that of the lacrimal, but in 

 some cases the secretion may be more or less mucous or, in some 

 aquatic mammals, oily. 



