CHAP, xix IHI. I VI. AND ill! SENSE OF SIGHT 203 



it, forms a delicate layer on the front of it. There is thus 

 no break in the continuity of the epidermis ; it becomes 

 much thinner and transparent on the front of the eyeball. 

 Lying in the eyelids are striated muscle fibres, placed circu- 

 larly round the eye. Contraction of these closes the eye. 

 The eye is opened by the action of a muscle which raises 

 the upper lid. Blinking, the sudden closure of the eyes 

 which occurs unconsciously every few seconds, may be a 

 reflex act. The sensory nerve concerned is a branch of the 

 fifth cranial ; this carries impulses, due to an irritation, it 

 may be a very slight one, on the surface of the eye, up to 

 the brain, and motor impulses are sent out along the seventh 

 cranial nerve to the muscle fibres of the eyelids. 



The eyeball is attached behind by the optic nerve, which 

 is large, and forms, as it were, the stalk of the organ, passing 

 from the brain through a hole at the back of the orbit. Six 

 striated muscles connect the eyeball with the wall of the orbit. 

 Four of these muscles, attached to the back of the orbit around 

 the entrance of the optic nerve, pass straight forward, and are 

 inserted into the front part of the eyeball, just behind what we 

 shall presently describe as the cornea ; they are the straight or 

 rectus muscles, and are named the external, internal, superior, 

 and inferior rectus muscles, according to their position. The 

 external rectus muscle turns the eyeball outwards, the internal 

 rectus inwards, the superior rectus upwards, and the inferior 

 rectus downwards. Two other muscles, called oblique 

 muscles, inferior and superior, attached to the sides of the 

 orbit, and pursuing a slanting, or even bent course, the 

 tendon of the superior passing through a pulley, are inserted 

 into the eyeball behind the place of insertion of the straight 

 muscles. By means of the action of one or more, usually of 

 two or more, of these muscles, the superior oblique and in- 

 ferior rectus usually working together, and the inferior oblique 

 and superior rectus together, the eyeball can be turned in many 

 directions. When we look at an object both eyes are directed 

 to it, and if the object is near, the muscles of both eyes are so 

 balanced as to turn both eyeballs inwards and to keep them 

 steadily directed on the object. 



The rest of the cavity of the orbit is occupied by fat and 

 the blood-vessels and nerves connected with the eye and its 



