422 ON SENSATION, AND THE ORGANS OF THE SENSES. 



side of the globe, and fall upon the point of the retina, which has just been 

 mentioned. The phenomenon is not confined to that spot, however ; nor is 

 it correct to say, as is sometimes done, that the retina is not sensible to light 

 at that point ; since, if such were the case, we should see a dark spot in our 

 field of view, whenever we use only one eye. The fact is, that a similar 

 phenomenon may occur under somewhat different conditions, in any division 

 of the retina, especially in its lateral parts. Thus, if we fix the eye for some 

 time, until it is fatigued, upon a strip of coloured paper lying upon a white 

 surface, the image of the coloured object will in a short time disappear, and 

 the white surface will be seen in its place ; the disappearance of the image, 

 however, is only of a few seconds duration. The truth seems to be, that 

 there is a tendency in the retina, to the propagation over neighbouring parts, 

 of impressions which occupy a large proportion of its surface ; and that this 

 tendency is the strongest, around the point at which the optic nerve enters, 

 so that the state of this part will generally become similar to that of the sur- 

 rounding portion of the retina. Hence, when we are using one eye only, 

 we do not perceive any dark spot in the field, but only a certain degree of in- 

 distinctness in a portion of the image. 



555. Under particular circumstances, we may receive a visual representa- 

 tion of the retina itself; as is shown by the experiments of Purkinje. "If, 

 in a room otherwise dark, a lighted candle be moved to and fro, or in a circle, 

 at a distance of six inches before the eyes, we .perceive, after a short time, a 

 dark arborescent figure ramifying over the whole field of vision; this appear- 

 ance is produced by the vasa centralia distributed over the retina, or by the 

 parts of the retina covered by those vessels. There are, properly speaking, 

 two arborescent figures, the trunks of which are not coincident, but on the 

 contrary arise in the right and left divisions of the field, and immediately 

 take opposite directions. One trunk belongs to each eye, but their branches 

 intersect each other in the common field of vision. The explanation of this 

 phenomenon is as follows : By the movement of the candle- to and fro, the 

 light is made to act on the whole extent of the retina, and all the parts of the 

 membrane which are not immediately covered by the vasa centralia are feebly 

 illuminated ; those parts, on the contrary, which are covered with those vessels 

 cannot be acted on by the light, and are perceived, therefore, as dark arbore- 

 scent figures. These figures appear to lie before the eye, and to be suspended 

 in the field of vision."* We have thus another demonstration of the fact 

 that, in ordinary vision, the immediate object of our sensation is a certain 

 condition of the retina, which is excited by the formation of a luminous 

 image. 



6. Sense of Hearing. 



556. In the Ear, as in the Eye, the impressions made upon the sensory 

 nerve are not at once produced by the body which originates the sensation ; 

 but they are propagated to it, through a medium capable of transmitting them. 

 Here too, therefore, we take cognizance by the mind, not of the sonorous ob- 

 ject, but of the condition of the auditory nerve ; and all the ideas we form of 

 sounds, as to their nature, intensity, direction, &c., must be based upon tin 

 changes which they produce in it. The complex contrivances, which we 

 meet with in the organ of Hearing among higher animals, are evidently in- 

 tended to give them greater power of discriminating sounds, than is possessed 

 by the lower tribes ; in which last it is reduced to a form so simple, that it may 

 be questioned whether they can be said to possess an organ of hearing, if 

 by this term we imply anything more than the mere consciousness of sono- 



* Midler's Physiology, p. 1103. 



