GENERAL SENSIBILITY AND TACTILE PERCEPTIONS. 813 



led to connect the cognizance of these qualities with a particular part of our 

 own body. 



737. Like the more specific senses previously studied, the sensations of 

 which we are now speaking, and which may be referred to under the name of 

 touch, using that word for the present in a wide meaning, require for their 

 production terminal organs ; and the chief but not exclusive organ of touch 

 is to be found in the epidermis of the skin and certain underlying nervous 

 structures. For the development of specific tactile sensations these terminal 

 organs are as essential as are the terminal organs of the eye for sight or of 

 the ear for hearing. Contact of the skin with a hard or with a hot body 

 gives rise to a distinct sensation, whereby we recognize that we have touched 

 a hard or a hot body. But the application of either body or of any other 

 stimulus to a nerve-trunk gives rise to a sensation of general feeling only, 

 corresponding to the simple sensation of light which is produced by direct 

 stimulation of the optic nerve. We have no more tactile perception of a 

 body which is in contact with a nerve-trunk than we could have visual 

 perception of any luminous object, the rays proceeding from which were 

 strong enough to excite sensory impulses when directed on to the optic 

 nerve instead of on to the retina, supposing such a thing to be possible. It is 

 further characteristic of these ordinary nerves of general feeling, that the 

 sensations caused by any stimulation of them beyond a certain degree de- 

 velop that state of consciousness which we are in the habit of speaking of 

 as " pain." Putting aside the general feeling which many parts of the eye 

 possess, a very strong luminous stimulation of the retina is required to pro- 

 duce a sensation of pain, if indeed it can be at all brought about ; whereas 

 a very moderate stimulation of the skin, and almost every stimulation of an 

 ordinary nerve-trunk, is said by us to be painful. 



Though the skin is the chief organ of touch, the mucous membrane 

 lining the various passages of the body also serves as an instrument for 

 the same sense, but only for a short distance from the respective orifices. 

 We can recognize hard or hot bodies with our lips or mouth, but a hot 

 liquid, when it has reached the oesophagus or stomach, simply gives rise to 

 a sensation of pain ; we cannot distinguish the sensation caused by it from 

 the sensation caused by a draught of a too acid fluid. 



From parts and tissues of the body other than the skin and the portions 

 of mucous membrane just mentioned we have obscure sensations of general 

 feeling, by which we are made vaguely aware of the general condition of our 

 body, though our -judgments in this matter are chiefly influenced by what 

 we shall have to speak of directly as a muscular sense." In all parts of the 

 body, however, on occasions all too frequent, this general feeling may become 

 prominent as pain. 



738. The stimuli which, when applied to the skin, give rise to tactile 

 perceptions are of two kinds only : (1) mechanical, that is, the contact of 

 bodies exerting varying degrees of pressure ; and (2) thermal, i. e., the rais- 

 ing or lowering of the temperature of the skin by the approach or contact 

 of hot or cold bodies. We can judge of the weight and of the temperature 

 of a body, because we can, through touch, perceive how much it presses when 

 allowed to rest on our skin or how hot it is. But we can through touch de- 

 rive no other perceptions and form no other judgments. An electric shock 

 sent through the skin will give rise to a sensation, but the sensation is an 

 indefinite one, because the electric current acts not on the terminal organs 

 of touch, but on the fine nerve-branches of the skin. We cannot distinguish 

 the sensation so caused from a mechanical prick of similar intensity ; we 

 cannot perceive that the sensation is caused by an electric current. Sim- 

 ilarly, certain chemical substances, such as a strong acid, will give rise to a 



