548 OF SENSATION. 



which lies between the black pigment and the vitreous humour. It is 

 in this structure, that the presence of cells at the peripheral as well as 

 the central extremities of the afferent nerves ( 381), may be most 

 clearly demonstrated. They can scarcely be distinguished, in many 

 animals, from the cells of the vesicular matter of the brain ; and, like 

 the latter, they lie in the midst of a plexus of capillary blood-vessels 

 (Fig. 164), which supplies the materials requisite for their growth and 

 activity. For the maintenance of the due nutrition of this organ it is re- 

 quisite that it should be occasionally called into use. If its functional 

 power be destroyed, by opacity of the anterior portion of the eye, the 

 nutrition of the retina and optic nerve suffers to such a degree that 

 these parts cease, after a time, to exhibit their characteristic structure ; 

 thus showing that the general rules already stated (CHAP, vii.) in regard 

 to the connexion between the functional activity and the due nutrition, 

 of tissues and organs, hold good with respect to the Nervous structure. 

 The fibres of the Optic nerve when they diverge to form the Retina, lose 

 their tubular structure ; their central axes only being continued, in the 

 form of gray fibres ( 375), and some of these becoming directly conti- 

 nuous with the caudate vesicles ( 378). 



961. The picture of external objects, which is formed upon the 

 Retina, closely resembles that which we see in a Camera Obscura. It 

 represents the outlines, colours, lights and shades, and relative posi- 

 tions, of the objects before us ; but these do not necessarily convey to 

 the mind the knowledge of their real forms, characters, or distances. 

 The perception of the latter, as already remarked ( 936), is a mental 

 process; and it may be intuitive, or acquired, the latter, it would 

 seem, being the general condition of the function in Man, the former in 

 the lower animals. The Infant is educating his perceptive powers, 

 long before any indications present themselves, of the exercise of higher 

 mental faculties. By the combination, especially, of the sensations of 

 sight and touch, he is learning to judge of the surfaces of objects as 

 they feel, by the appearance they present, to form an idea of their 

 distance, by the mode in which his eyes are directed towards them, 

 and to estimate their size, by combining the notions obtained through 

 the picture on the retina, with those he acquires by the movement of 

 his hands over their different parts. A simple illustration will show, 

 how closely the ideas excited by the two sets of sensations, are blended 

 in our minds. The idea of smoothness is one which has reference to 

 the touch ; and yet it constantly occurs to us, on looking at a surface 

 which reflects light in a particular manner. On the other hand, the 

 idea of polish is essentially visual, having reference to the reflection of 

 light from the surface of the object; and yet it would occur to us from 

 the sensation conveyed through the touch, even in the dark. 



962. That this sort of combination is not intuitive in Man, but is the 

 result of experience, is evident from the numerous observations made 

 upon those who had acquired the sense of Sight for the first time, after 

 long familiarity with the characters of objects as perceived through the 

 Touch. Thus a boy of four years old, upon whom the operation for 

 congenital cataract had been very successfully performed, continued to 

 find his way about his father's house, rather by feeling with his hands, 



