390 ON SENSATION, AND THE ORGANS OF THE SENSES. 



tion ; and beings which cannot be made conscious, in this manner, of the 

 existence of something external to themselves, do not deserve to be ranked 

 in the Animal kingdom. Our difficulty lies (as heretofore remarked, 1), 

 in ascertaining what are to be regarded, in such beings, as unequivocal indi- 

 cations of consciousness. Those animals which are fixed to one spot, can 

 have few other ideas of matter than this most general one ; but in those 

 which have the power of locomotion, the general sensibility of the surface 

 doubtless communicates to them some notion of the character of the body 

 over which they move, in the same manner as we learn it by passing the 

 hand over its exterior. We shall presently see, however, that the idea of 

 the shape of a body which we form from the touch, results from a very com- 

 plex process ; which animals of the lowest grade can scarcely be supposed 

 to exercise. There can be no doubt that, next to the mere sense of resistance, 

 sensibility to temperature is the most universally diffused through the Ani- 

 mal kingdom ; and probably the consciousness of luminosity is the next in 

 the extent of its diffusion. There is good reason to believe, from observation 

 of their habits, that many animals are susceptible of the influence, and are 

 directed by the' guidance of light ; whilst their organs are not adapted to re- 

 ceive true visual impressions, or to form optical images ; and such would 

 seem to be the function of the red spots, frequently seen on prominent parts 

 of Animalcules, the lower Articulata and Mollusca, and even of some Radiata. 

 Wherever these are of sufficient size to allow their structure to be examined, 

 they are found to be largely supplied with nerves, but to be destitute of the 

 peculiar organization which alone constitutes a true eye. The sense of Taste 

 may be considered as a refined modification of that of Touch ; and it is 

 probable that this exists very low down in the animal scale, being obviously 

 of great importance in the selection of food ; but the Anatomist has no means 

 of ascertaining where this refinement exists, and where it does not ; since the 

 organs of taste and touch are so similar. The sense of Hearing does not 

 seem to be distinctly present among the Invertebrate animals, except in such 

 as approach most nearly to the Vertebrata ; it is not improbable, however, 

 that sonorous vibrations may produce an effect upon the system of those ani- 

 mals which do not receive them as sound; and this would appear, from a 

 fact subsequently to be mentioned ( 526), to be not improbably the case, 

 with regard especially to aquatic animals. The sense of Smell, which is con- 

 cerned with one of the least general properties of matter, appears to be the 

 least widely diffused among the whole; being only possessed in any high 

 degree by Vertebrated animals, and being but feebly present in a large pro- 

 portion of these. 



515. Besides the various kinds of sensibility which have been just enume- 

 rated, there are others which are ordinarily associated together, along witli 

 the sense of material resistance (and its several modifications), and the sense 

 of temperature, under the head of Common Sensation ; but several of them, 

 especially those which originate in the body itself, can scarcely be regarded 

 in this light. Such are the feelings of Hunger and Thirst; that of Nausea; 

 that of distress resulting from suspended aeration of the blood ; that of " sink- 

 ing at the stomach," as it is vulgarly but expressively described, which results 

 from strong mental emotion ; that of the venereal excitement, and perhaps 

 some others. Now in regard to all these, it is impossible in the present state 

 of our knowledge to say, whether their peculiarity results from the particular 

 constitution of the nerves that receive and convey them, or only from a modi- 

 fication in the impressing causes, and in the mode in which they operate. 

 Thus we have no evidence that the nervous fibrils, which convey from the 

 lungs the sense of distress resulting from deficient aeration, may not be of a 

 different character from those which convey from the surface of the air-pas- 



