CHAP. IX.] NERVOUS SYSTEM AND ORGANS OF SENSE. 289 



on the inner surface of each palpebral fibrous membrane. Each 

 tube is lined by mucous membrane, and forms an oily secretion. 



The coiijidicfira is vascular, red, opaque, and somewhat thick 

 where it lines the eyelids, but where it is reflected from them (over 

 the ball of the eye) it is transparent, almost colourless, and has but 

 very few blood- vesscls._ Over the coloured part of the eye and the 

 pupij, the conjunctiva is quite transparent, and "without vessels, and 

 consists almost entirely of epithelium. 



The lachrymal gland, which secretes the tears, lies at the 

 upper and outer part of the orbit. It is a very small racemose 

 gland, vnih. ducts which open on the inner surface of the upper eye- 

 lid, just above the outer canthus. Tears, are a clear, saliiae, 

 alkaHne fluid, with a minute portion of albuminoid matter. About 

 1 per cent, is the quantity of solid matter which they contain. 

 Having traversed the surface of the conjunctiva, the tears enter 

 the puncta lachrymalia (before noticed), which are the orifices of 

 two short membranous canals (the lachrymal canah) which pass 

 inwards (towards the nose), and open into a membranous tube, the 

 ■nasal duct, which descends, through the bony lachrymal canal before 

 described, to empty itself into the anterior part of the inferior meatus 

 of the nose. This canal, formed of fibrous and elastic tissue, adheres 

 closely to the bones it adjoins in its passage. It is lined with mucous 

 membrane, which is thus continuous with the conjunctiva above and 

 with that of the nasal fossa below. The epithelium which coats its 

 mucous membrane is ciliated. 



The Uarderian gland is a small organ situated at the inner 

 canthus of the orbit. It is oval and somewhat like a small bunch 

 of grapes. It secretes a thick, whitish fluid, which escapes from 

 one or two orifices which open beneath the membrana nictitans. 



The eyeball is embedded in fat, from which it is separated by a 

 layer of fascia. It is spheroidal and composed of three investing 

 membranes, one within the other, enclosing certain fluid and solid 

 contents. 



The first membrane is the sclerotic and cornea, the second is the 

 choroid and iris, and the third is the retina. 



The fluid contents is made up of the aqueous and vitreous humours. 



The solid contents is the lens, with the capsule enclosing it. The 

 shape of the eye-ball is really that of a large segment of one sphere, 

 with a small segment of a lesser sphere afiixed to it in front. The 

 proportion between these spheres is as eleven to seven. 



The FIRST or outer membrane is partly opaque — the sclerotic 

 — while that part of it which is called the cornea is transparent. 



The sclerotic invests the larger part of the eyeball. It is formed 

 of very dense fibrous tissue, with elastic tissue and with stellate and 

 fusiform nucleated cells. By its solidity and toughness the sclerotic 

 maintains the globe of the eye in its proper shape. It is white 

 externally and smooth, except as regards the insertion of the orbital 

 muscles into it. It is thickest at the back and thinnest near the 

 margin of the cornea. The optic nerve pierces the sclerotic slightly 



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