Chap. XX] INTERNAL AND EXTERNAL SENSES 423 



Seven to twelve minute ducts lead from the gland to the surface 

 of the conjunctiva of the upper lid. The secretion (tears) is 

 usually just enough to keep the eye moist, and after passing over 

 the surface of the eyeball is sucked into two tiny canaliculi 

 through the punctae and is conveyed into the lacrimal sac, which 

 is the upper dilated portion of the nasal duct. 



The nasal duct is a membranous canal, about three-quarters 

 of an inch (19 mm.) in length, which extends from the lacrimal 

 sac to the nose, into which it opens by a slightly expanded orifice. 



The tears consist of water containing a little salt and albumin. 

 They are ordinarily carried away as fast as formed, but under 

 certain circumstances, as when the conjunctiva is irritated, or 

 when painful emotions arise in the mind, the secretion of the 

 lacrimal gland exceeds the drainage power of the nasal duct, 

 and the fluid, accumulating between the lids, at length overflows 

 and runs down the cheeks. 



The conjunctiva. — The conjunctiva is the mucous membrane 

 which lines the eyelids and is reflected over the front of the eyeball. 

 It is often considered part of the lacrimal apparatus as it secretes 

 a fluid like that of the lacrunal gland. 



Muscles of the eye. — For purposes of description the muscles 

 of the eye are divided into two groups : (1) intrinsic, and 

 (2) extrinsic. The intrinsic muscles are the ciliary muscle, and 

 the muscles of the iris. (See page 426.) The extrinsic muscles 

 are those which move the eyeball and include the four straight, 

 or recti, and the two oblique. They have been described in 

 Chapter VII. 



Nerves of the eye. — The nerves which are supplied to the eye 

 are: (1) the optic nerve, which is concerned with vision only; 



(2) the motor oculi nerve controls the internal rectus, the superior 

 rectus, the inferior rectus, and the inferior oblique muscles; 



(3) the pathetic nerve controls the superior oblique muscle; 



(4) the abducens controls the external rectus; and (5) the 

 ophthalmic, which is a branch of the trifacial nerve, supplies gen- 

 eral sensation. 



The orbits. — The orbits are the bony cavities in which the 

 eyeballs are contained. 



Seven bones assist in the formation of each orbit, namely, frontal, 

 malar, maxilla, palate, etlmioid, sphenoid, and lacrimal. As three 



