108 PHYSIOLOGY CHAP. 



finger-tip. Why then do we not perceive the figures traced by 

 the finger in our 'joints and muscles, but in a different and far-off 

 spot which is not the seat of any excitation ? Evidently because 

 from long habit we explore the objects which surround us with 

 our eyes and fingers, so that the memory of the things felt on the 

 skin and the extent of the space seen are necessarily revived each 

 time the contraction of the muscles moves the articulations of 

 a limb, even when we do not see them move, and when the sense 

 of touch is hardly, if at all, excited. 



This observation is of great importance, because it proves 

 incontestably that the so-called " muscular sensations," which as 

 we have seen are principally due to the sensibility of the joints, are 

 in themselves only forms of the common sensibility with which all 

 the internal organs supplied with afferent nerves are provided. 

 Accordingly, we are unable to objectify the muscular sensations 

 or transform them into perceptions, without the collaboration, 

 direct or indirect, actual or mnemonic, of tactile and visual 

 sensations. And those authors are wrong who hold the muscular 

 sense to be a special sense, like the tactile sense, the visual sense, 

 and so on. 



Just as the tactile sense is complementary to the muscle sense, 

 so we may say that muscular sensations reinforce tactile sensations 

 and contribute in developing their capacity of localisation. 



As Weber pointed out, we may reasonably hold that all 

 our sensations, including the cutaneous, are at first devoid of 

 the power of localisation. They represent simple states of con- 

 sciousness, differing in quality and intensity, but giving no notion 

 of place, and having no local sign. On the strength of certain 

 researches of Preyer on the chick embryo, it may be asserted that 

 in the ontogenetic development of the senses common sensibility 

 in the obscure form of internal bodily sensation appears first ; 

 cutaneous sensibility only begins to appear at the tenth day of 

 incubation. Before that it is possible to apply every kind of 

 mechanical, chemical, and electrical stimulus to the skin without 

 evoking the slightest reflex movement ; but at the end of the fifth 

 day the embryo exhibits automatic movements due to internal ex- 

 citations. The same facts were noticed in the mammalia embryo 

 also. So that of foetuses in general, including the human, it may 

 be admitted that obscure muscular sensations precede those of the 

 special senses. This is highly important from the psychological 

 point of view. 



How does the capacity of tactile and cutaneous localisation 

 develop in the new-born animal? We may legitimately assume 

 that its development is promoted by the activity of the muscles 

 and the resulting muscular sensations. If Weber's tables for the 

 delicacy of tactile sensibility are consulted, it will be found that 

 he gives first place as surfaces of extreme sensibility to the tip of 



