THE EYE AND VISION 



chiefly by the elevation and depression of this lid that the eye is 

 opened and closed. The eyelids are closed by the contraction of 

 muscular fibres which are arranged in the form of a ring around 

 the eye. This circular muscle is called the orbicularis (Lat. orbi- 

 culus, a little circle). The lower lid has no special muscle to 

 depress it, but the upper eyelid is raised by the contraction o*' a 

 muscle which comes forward from the back of the orbit. 



The eyeball is moved by six muscles, four of which are called straight 

 muscles, and the other two oblique. The four recti (Lat. rectus, straight) 

 come forwards from the back of the 

 orbit, and are named superior, in- 

 ferior, external, and internal, from 

 their positions. The superior rec- 

 tus muscle turns the front of the 

 eyeball upwards, the inferior rectus 

 turns it downwards ; and the external 

 and internal recti turn the front of 

 the eyeball outwards and inwards 

 respectively. 



The superior oblique muscle 

 comes forward from the back of 

 the orbit, and then, becoming ten- 

 dinous, passes through a kind of 

 pulley of fibres attached to the frontal 

 bone. It then turns backwards and 

 outwards, and is attached to the 

 outer and back portion of the eye- 

 ball. Hence, when this muscle 

 contracts, it turns the eyeball ob- 

 liquely outwards and downwards. 

 The inferior oblique muscle springs 

 from the lower and front portion of 

 the orbit, and passes obliquely back- 

 wards and outwards. By its con- 

 traction the front of the eyeball is 

 turned upwards and outwards. 



The front of the eyeball is 

 kept clean and moist by a saline 

 fluid which is secreted continu- 

 ously by the lachrymal gland 

 (Lat. lachryma, a tear). This 

 gland is about the size of a small 

 almond, and is situated in the 

 upper and outer portion of the 

 orbit. It is very similar to the salivary glands in structure. It is pro- 

 vided with several small ducts by which the lachrymal fluid is con- 

 ducted to the upper surface of the eyeball. The fluid gradually moves 

 over the surface of the ball, assisted by every motion of the eyelid, 

 till it reaches the inner angle formed by the eyelids, having carried 



Figs. 1 86 and 187. The Muscles of 

 the Bight Orbit. 



, sphenoid bone ; i, muscle which raises the 

 upper lid ; 2, pulley and tendon of the superior 

 oblique ; 3, tendon of the superior rectus ; 

 4, external rectus partly removed in fig. 

 188 ; 5, inferior oblique muscle ; 6, inferior 

 rectus ; 7, internal rectus ; 8, optic nerve. 



