EXAMINATION OF SOLIDITY. 419 



ternal objects upon a surface, but the hand examines the so- Examination 

 lidity of bodies. The former is occupied with length and of solidity by 

 breadth ; the latter with all three dimensions, length, breadth, the hand ' 

 and thickness conjointly. Our notions of solidity are to no little extent 

 obtained in this way, as was proved in the case of Cheselden's patient, 

 who had been blind from birth, and to whom vision was given by a suc- 

 cessful operation for cataract, and still more recently by a similar case 

 of Franz. In this instance, "a solid cube and a sphere, each of four 

 inches diameter, were placed before the patient, at the distance of three 

 feet, and on a level with the eye. After attentively examining these 

 bodies, he said he saw a quadrangular and a circular figure, and, after 

 some consideration, he pronounced the one a square and the other a disk. 

 His eye being then closed, the cube was taken away, and a disk of equal 

 size substituted, and placed next to the sphere. On again opening his 

 eye, he observed no difference in these objects, but regarded them both 

 as disks. The solid cube was now placed in a somewhat oblique posi- 

 tion before the eye, and, close beside it, a figure cut out of pasteboard, 

 representing a plain outline prospect of the cube when in this position : 

 both objects he took to be somewhat like a flat quadrate. A pyramid 

 placed before him, with one of its sides turned toward his eye, he saw as 

 a plain triangle. This object was now turned a little, so as to present 

 two of its sides to view, but rather more of one side than of the other : 

 after considering and examining it for a long time, he said that this was 

 a very extraordinary figure ; it was neither a triangle, nor a quadrangle, 

 nor a circle he had no idea of it, and could not describe it : 'in fact,' 

 said he, 'I must give it up.' An example of the close association which 

 exists between the sense of touch and that of sight, in enabling the mind 

 to form a correct idea of an object, is afforded in the statement of this 

 patient, that, although by the sense of sight he could detect a difference 

 in the cube and sphere, and perceive that they were not drawings, yet 

 he could not form from them the idea of a square and a disk until he 

 perceived a sensation of what he saw in the points of his fingers as if he 

 really touched the objects/ When he took the sphere, cube, and pyra- 

 mid into his hand, he was astonished that he had not recognized them asf 

 such by sight, being well acquainted with them by touch." 



The mechanism for touch, as distinguished from the general dermoid 

 sensibility, is the papilla, which may be described as conical Structure of 

 eminences on the cutis, at once solid and flexible, sometimes papillae of 

 clavate in form, and sometimes having numerous points. They 

 are about the T J^ of an inch in height, and the -% J^ of an inch in diam- 

 eter at their base, these dimensions varying, however, very greatly with 

 the situation. They contain a loop of blood-vessels and a twig of a 

 sensory nerve, for all the centripetal nerves, with the exception of those 



