. ' Notes, ii. lO. 173 



ariimal be laid bare, as in A.'s vivisection of the chamseleon {H. A^ ii. 11), the hemispheres 

 may be cut witliout any signs of pain whatsoever, and without any struggling on the 

 part of the animal. This difficulty was insuperable to Aristotle. i3. He could find no 

 brain, nor anything apparently analogous to a brain, in any of the invertebrata, excepting 

 in the Cephalopods ; the cephalic ganglia in other animals having, owing to their 

 minute size, escaped his unaided vision.' Yet sensation was the special characteristic of 

 an. animal. The absence of a brain, then-, from numerous sentient creatures was quite 

 imcompatible with the notion that the brain was the sole organ of sensation. 7. The 

 brain he erroneously thought (cf. Note 18) to be bloodless, as also did Hippocrates ; and 

 all experience taught him that those parts alone were sensitive that contained blood 

 (cf. Note 20). 5, "It is manifest," he says (ii. 7, 4), "on inspection, that there is no 

 anatomical connection between brain and sense-organs." It has indeed been supposed 

 that he knew of the optic, auditory, and olfactory nerves (cf. Note 19). If he really 

 knew of them, he did not know that they united the sense-organs to the brain itself, 

 but thought that they led to the vascular membrane round it, or to the supposed empty 

 space behind it. They would thus be in connection with the blood, and so with the 

 heart, but not with the actual brain. It is true that in his account of the vivisection 

 of the chamseleon {H. A. ii. 11, 9) he says that the eyes are "continuous with the 

 brain," and elsewhere {De Sensu, 2, 20) that the eyes are "made out of the. brain"; 

 but these, passages must be read in connection with others, such as D. G. ii. 6, 33, where 

 he says that "the purest part of the fluid which is about the brain is separated from it, 

 and passes to the eyes by the channels {ir6poi) which visibly connect them with the 

 membrane that surrounds the brain." «. Lastly, he believed that he had good grounds 

 for supposing another part, viz. the heart, to be the sensory centre (cf. ii. 7, Note 27). 



10. The heart, according to A., is the sensorium commune ; but while three of the 

 five senses have also external organs, viz. the eyes, ears and nostrils, the remaining two, 

 viz. touch and its variety taste, have no such external organs, but are lodged in or close 

 to the heart itself, which is thus their first and also their final organ. "The question," 

 says A., " whether the sense-organ lies internally or not, being on the latter supposition 

 the flesh itself, is not to be answered off-hand by adducing the fact that a sensation is felt 

 the moment the body is touched. For even if one were to stretch a piece of membrane 

 tightly over the surface of the body, a touch would equally call forth immediate signs 

 of sensation ; and yet it is plain that the membrane is not the sense-organ. Indeed 

 the fact adduced proves the contrary, for "the direct contact of an object with an organ 

 of sense in no case produces a sensation. Thus if a white body be placed in contact 

 with the eye, there is no sensation of whiteness. This shows plainly that the sense- 

 organ of touch lies deeper than the flesh. For the same must occur in the 'organ of 

 touch as in the organs of other senses. That is to say it must be, as they are, insensible 

 to objects in direct contact with it. The fact, then, that objects are felt when in contact 

 with the flesh shows that the flesh must be the medium which separates the sense-object 

 from the sense-organ ; the flesh and the tongue standing to touch and taste apparently 

 in the same relation as do air and water to sight and .hearing" {D. A. ii. il). So, again, 

 ' ' If the coloured body be placed in direct contact with the eye, the eye will not see it. 

 There must be a transparent medium, such' as air, which is set in motion by the colour, 

 and which in turn sets in motion the sense-organ, with which it is in continuity. For 

 Democritus was mistaken in supposing that, if the interval* between the eye and the 

 object were a vacuum, vision would be so distinct, that we should be able to discern 

 even an ant though it were at the distance of the heaven. ... In reality, with a 

 vacuum for a medium, there would be no vision at all. The same is true in the case 



