. The chordates. Chordata. 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,"
. The chordates. Chordata. 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. Als
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