THE LACRIMAL GLAND 343 



substance secreted by their own oil glands in the margin of the 

 lids. The cilia of the upper lid curve upward, those of the lower 

 lid curve downward; they never interlace. They guard the eye 

 from foreign bodies as coal dust, etc., floating in the surrounding 

 air. 



The space between the margins of the eyelids is called the 

 inter palpebral slit (palpebral fissure). It varies with the action 

 of the lids; the opening and closing of the slit is done by the 

 upper lid mainly, the lower one moving but very little. (Muscles 

 orbicularis closes, levator palpebrce opens . (See p. 89.) 



Lacrimal gland. The gland which secretes the tears. It is 

 situated in the lacrimal fossa of the frontal bone, beneath the 

 lateral end of the orbital arch, and has several ducts for the dis- 

 charge of the tears under the upper eyelid. The tears flow across 

 the eyeball and bathe the conjunctiva, washing away the dust and 

 other fine particles of foreign substances, which would be injurious 

 if allowed to attach themselves to the conjunctiva. They are con- 

 ducted by tiny canals (canaliculi] into the lacrimal sac and nasal 

 duct (see Fig. 221) thence to the nasal fossa. Being a thin saline 

 solution they are unirritating to mucous membranes. 



Clinical note. The conjunctiva is supplied with blood-vessels 

 most of which are invisible except when they become congested. 

 In active inflammation or conjunctivitis they are so enlarged as to 

 give the membrane a bright red color. 



Motions of the eyeball. The eyeball is moved by six slender 

 muscles, which have their origin at the apex of the orbit and their 

 insertion upon the sclera at a little distance from the cornea. 

 These are the orbital muscles. 



The superior rectus rolls the ball upward. (Third nerve.) 

 The inferior rectus rolls the ball downward. (Third nerve.) 

 The internal rectus rolls the ball inward. (Third nerve.) 

 The external rectus rolls the ball outward. (Sixth nerve.) 

 The superior oblique rolls the ball downward and outward. 

 (Fourth nerve.) 



The inferior oblique rolls the ball upward and outward. (Third 

 nerve.) 



Clinical note. If these muscles are well balanced the pupil is 

 directed straight forward while they are at rest, but if they are of quite 

 unequal strength the eye will be turned habitually in some special 



