118 ZOOLOGY. 



in the retinal picture, the proof is easy. Let the reader 

 shut his left eye and regard these two asterisks, fixing his 

 gaze intently upon the left-hand one of them. 



At a distance of three or four inches from the paper, both 

 stars will be f ocussed on his retina, the left one in the centre 

 of vision, and the right one at some spot internal to this, and 

 he will see them both distinctly. Now, if he withdraw his 

 head slowly, the right spot will of course appear to approach 

 the left, and at a distance of ten or twelve inches it will, in 

 its approach, pass over the blind spot and vanish, to reappear 

 as he continues to move his head away from the paper. 

 The student will probably ask what purpose the blind 

 spot serves. The answer is none : its existence is a 

 necessary result of the peculiar arrangement of the retina. 

 In all the other sense-organs of the rabbit, the actual 

 sensory element is always nearer the external stimulating 

 agent than are the conducting fibres, such being the most 

 obviously reasonable arrangement ; but the rods and cones 

 are turned away from the light. This arrangement is 

 characteristic of the eyes of vertebrata ; whereas the most 

 highly-developed eyes of invertebrata have the sensory 

 elements nearer the light and have consequently no blind 

 spot. When we come to study Amphioxus and also the 

 development of the eye in the rabbit, we shall find the 

 clue to this anomalous arrangement. 



11. Eye-muscles and Glands. A series of muscles in 

 the orbit (eye-socket) move the eye, and so enable the rabbit 

 to vary its field of view. There is a leash of four muscles 

 rising from a spot behind the exit of the optic nerve from 

 the cranium, and attached to the upper, under, anterior, 

 and posterior sides of the eyeball. These are called the 

 " straight " muscles (recti), because each one, acting alone, 

 causes motion of the eye in the vertical or horizontal plane. 

 They are distinguished as superior, inferior, anterior, and 

 posterior rectus. Running from the front of the orbit 

 obliquely to the underside of the eyeball is the inferior 

 oblique muscle. Corresponding to it above is a superior 



