THE EYE AS AN OPTICAL INSTRUMENT. 507 



The eyelashes are short curved hairs, arranged in one or two 

 rows along each lid where the skin joins the conjunctiva. 



The Lachrymal Apparatus consists of the tear-gland in 

 each orbit, the ducts which carry its secretion to the upper eye- 

 lid, and the canals by which the tears, unless when excessive, 

 are carried off from the front of the eye without running down 

 over the face. The lachrymal or tear gland, about the size 

 of an almond, lies in the upper and outer part of the orbit, 

 near the front end. It is a compound racemose gland, from 

 which twelve or fourteen ducts run and open in a row at the 

 outer corner of the upper eyelid. The secretion there poured 

 out, is spread evenly over the exposed part of the eye by the 

 movements of winking, and keeps it moist; finally the tear is 

 drained off by two lachrymal canals, one of which opens by a 

 small pore (punctum lachrymalis) on each lachrymal papilla. 

 The aperture of the lower canal can be readily seen by ex- 

 amining the corresponding papilla by the aid of a looking- 

 glass. The canals run inwards and open into the lachrymal 

 sac, which lies just outside the nose, in a hollow where the 

 lachrymal and superior maxillary bones (L and Ms, Fig. 

 30) meet. From the sac the nasal duct .proceeds to open 

 into the nose-charnber, below the inferior turbinate bone 

 and within the nostril. 



Tears are constantly being secreted, but ordinarily in 

 such quantity as to be drained off into the nose, from which 

 they flow into the pharynx and are swallowed. When the 

 lachrymal ducts are stopped up, however, their continual 

 presence makes itself unpleasantly felt, and may need the aid 

 of a surgeon to clear the passage. In weeping the secretion 

 is increased, and then not only more of it enters the nose, 

 but some flows down the cheeks. The frequent swallowing 

 movements of a crying child, sometimes spoken of as " gulp- 

 ing down his passion/ 7 are due to the need of swallowing the 

 extra tears which reach the pharynx. 



The Muscles of the Eye (Fig. 141). The eyeball is 

 spheroidal in form and attached behind to the optic nerve, n, 

 somewhat as a cherry might be to a thick stalk. On its ex- 

 terior are inserted the tendons of six muscles, four straight 

 :and two oblique. The straight muscles lie, one (superior 

 rectus), s, above, one (inferior rectus) below, one (external 

 rectus), a, outside, and one (internal rectus), i, inside the 

 eyeball. Each arises behind from the bony margin of the 



