178 THE HUMAN BODY. 



Sometimes there is an apparent displacement of objects, 

 although neither the objects nor the eyes are in motion, but 

 in a normal condition it is always after a movement of the 

 body that this phenomenon appears. As when the body is 

 whirled round rapidly and then suddenly stopped, every- 

 thing seems to turn in an inverse direction. It is probable 

 that the illusion then depends on the impulse to movement 

 in a certain direction imparted to the brain; in fact, if we 

 stop after turning round, the sensation of turning persists for 

 some moments, especially in the head; and if we refer it 

 instinctively to external objects, it is in consequence both of 

 the persistence of the previous sensation, and of the idea of 

 our actual immobility. We turn still, just as after having 

 laid down a burden we continue to feel its weight upon us. 



Gratiolet ascribes the apparent motion of objects under 

 these circumstances to insensible oscillations, which displace 

 to a limited extent the ocular axes, but he does not indicate 

 the cause of these oscillations. 



Optic nerve. The visual impressions are transmitted from 

 the retina to the brain by means of the optic nerve, of which 

 that membrane appears to be the expansion. The two optic 

 nerves converge from the base of the orbit toward the centre 

 of the base of the skull, where there is an interlacement of 

 their fibres in such a manner, that a portion of the right 

 nerve goes to the left side of the brain, and a part of the left 

 nerve to the right side; this is called the chiasma, or commis- 

 sure of the optic nerves. Physiological theories, which are 

 no longer tenable, have been deduced from this crossing of 

 the nerves, and nothing positive is yet known of the relation 

 between this disposition and the visual function. Mechani- 

 cal irritation of the nerve seems to develop luminous impres- 

 sions as in the retina, but it causes no pain whatever. 



Movements of the eye. The ocular globe is put in motion 

 in the orbit by six muscles, grouped two by two, which raise 

 or lower the eye, turn it inward or outward, or on its antero- 

 posterior axis. In these movements the centre of the 

 globe is immovable, and the eye moves around its transverse 

 and vertical diameters. These, three orders of movements 

 are independent of each other, and may be made singly, or in 



