MOVEMENTS OF THE EYEBALLS. 591 



lated, and the excitations produced, by the impulses passing 

 along the nerve, in the nerve cells of the seeing centre, there 

 must be further a psychical action in other cells of the cortex of 

 the brain. This psychical action of the brain consists of a series 

 of conclusions drawn from the experiences gained by our visual 

 and other sensations. 



Our ideas of external objects are not in exact accord with the 

 image produced on the retina and transmitted to the brain, but 

 are the result of a kind of argument carried on unconsciously in 

 our minds. Thus, when no light reaches the retina, we say (with- 

 out what we call thought) that it is dark ; our retina being un- 

 stimulated, no impulse is communicated, and the sensation of 

 blackness arises in our sensorium. When luminous rays are 

 reflected to the retina from various objects around us, the physio- 

 logical impulse starts from the eye, but in the brain, by uncon- 

 scious psychical activity, it is referred in our minds to the objects 

 around us, so that mentally we project into the outer world what 

 really occurs in the eye. So also, from habit, we reinvert in our 

 minds the image which is thrown on the retina by the lens, 

 upside down, and so unconscious are we of the psychical act that 

 we find it hard to believe that our eyes really see everything 

 inverted, and our minds have to reinstate them in the upright 

 position. 



One of the most important means employed to enable us to 

 form accurate visual perceptions is the varied motion which the 

 eyeballs are capable of performing. 



MOVEMENTS OF THE EYEBALLS. 



The eyeballs may be regarded as spherical bodies, lying in 

 loosely fitted sockets of connective tissue padded with fat, in 

 which they can move or revolve freely in all directions, in a 

 limited degree. The muscles which act directly on the eyeball 

 are six in number. Four recti muscles, which pass from the 

 back of the orbit and are attached to the eyeball, one at each 

 side and one above and below, not far from the cornea, can move 

 the front of the eye to the right or left, and up or down respec- 

 tively ; and two oblique muscles which pass nearly horizontally 



