544 STUDIES IN ADVANCED PHYSIOLOGY. 



gland) , run quite a number of ducts which open in a row 

 under the upper eyelid. From this point the secretion is 

 washed over the front of the eye by the ordinary movements 

 of the eyelids, such as those of winking, and so the cornea 

 is kept moist and clear and free from dust. The secretion 

 is finally led off by two lachrymal canals, which arise each as 

 a small pore called the punctum lachrymalis found on each 

 lachrymal papilla. These lachrymal papillae are readily 

 recognized as small reddish elevations at the inner angle of 

 the upper and lower eyelids. The lachrymal canals soon 

 unite, opening into the nasal duct, the upper portion of 

 which is somewhat enlarged and called the lachrymal sac. 

 The lower portion of the nasal duct opens into the back 

 part of the nose chamber, and from this place the tears find 

 their way into the throat and are swallowed. Ordinarily the 

 amount of the secretion so poured into the throat is so slight 

 that we take no notice of it. In violent weeping, however, 

 the secretion may become so plentiful as to necessitate re- 

 peated acts of swallowing. 



4. The Muscles of the Eyeball. The rapidity and ex- 

 actness with which the eyeball may be moved attest the 

 presence of a system of eye muscles. Each globe is pro- 

 vided with six muscles ; four of these are straight muscles 

 and two are oblique. The straight muscles lie, one on top 

 of the eye the superior rectus, one below the eyeball, 

 the inferior rectus, one on the outer side external rec- 

 tus, and the fourth on the inner side internal rectus. 

 It is readily seen that by means of these four muscles the 

 globe of the eye may be turned in any direction save a 

 rotary one. To make this motion possible there are two 

 oblique muscles. One of these is fastened to the top of the 

 eyeball, runs inward towards the nose, and there passes 

 through a tendonous loop like a pulley, to be inserted near 

 where the recti are. This is the superior oblique muscle. 

 The inferior oblique muscle arises at the back of the orbit, 

 near the lachrymal sac. From this point it passes slightly 

 outwards and backwards beneath the eyeball, and is inserted 



