852 OF SENSATION, AND THE ORGANS OF THE SENSES. 



Matter, on which, in fact, our notion of it is chiefly founded ( 804), is its 

 occupation of space, producing a more or less complete resistance to displacement; 

 and this quality is that through which alone any knowledge of the external 

 world can be obtained by a large proportion of the lower Animals ; contact 

 between their own surface and some material body being required to produce 

 sensation. We shall presently see, however, that the idea of the shape of a 

 body which we form from the touch; results from a very complex process, such 

 as animals of the lower grades can scarcely be supposed to exercise. There can 

 be little doubt that, next to the mere sense of resistance, sensibility to tempera- 

 ture is* the most universally diffused through the Animal kingdom ; and probably 

 the consciousness of luminosity is the next in the extent of its diffusion. 1 It is 

 probable that the sense of taste (which has a close affinity to that of touch) 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 very 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 pro- 

 duce an effect upon the system of those animals, which do not receive them as sound. 

 The sense of smell, which is concerned 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 pre- 

 sent in a large proportion of these. 



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

 rated, there are others which are ordinarily associated together, along with the 

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

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

 cially 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 " sinking at the 

 stomach," as it is vulgarly but expressively described, which results from strong 

 mental emotion ; the sexual sense, 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 modification in the impressing causes, 

 from the particular endowments of their ganglionic centres, and from the mode 

 in which they operate. Thus we have no evidence whether the nervous fibrils, 

 which convey from the lungs the sense of distress resulting from deficient aera- 

 tion, are of the same or of a different character from those which convey from 

 the surface of the air-passages the sense of the contact of a foreign body. But 

 as we know that all the trunks, along which these peculiar impressions travel, 

 do minister to ordinary sensation, whilst the nerves of truly special sensation 

 are not sensible to common impressions, it is evident that the probability seems 

 in favor of the identity of the fibres, which minister to these sensations, with 

 those of the usual sensory character. We shall see that, with regard to the 

 sense of Temperature, there is strong evidence that its peculiarity depends on 

 the speciality of the apparatus by which impressions are received at the peri- 



1 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 receive 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 the lower Articulata and Mollusca, and even of some lladiata. Wherever these are of 

 sufficient size to allow their structure to be examined, they are found to be largely sup- 

 plied with nerves, but to be destitute of the peculiar organization which alone constitutes 

 a true eye. 



