338 SPECIAL PHYSIOLOGY. 



being the excitable anatomical seat of what is known to our conscious- 

 ness, as sensation. The presence and the degree of sensibility in a 

 part, depend, other things being equal, on the existence and the num- 

 ber of nerve-fibres distributed to it; thus parts and tissues destitute of 

 nerves, such as the nails, hairs, and cuticle, are absolutely insensible; 

 the tendons and bones, which have but few nerves, are but moderately 

 sensitive, excepting in cases of disease; whilst the skin, lips, and 

 tongue, are highly sensitive, being provided with an abundance of 

 nerve-fibres. Stimuli act especially, and most easily and effectively, 

 on the extremities of the nerve-fibres. 



Each point, or definite part, of a recipient sensory surface is inde- 

 pendent in its action, as regards all other points or parts; and so there 

 must be corresponding points, or parts, in the sensory nervous centre ; 

 otherwise there could be no distinctness of local impression, on which 

 depend, to a great extent, if not entirely, the accuracy of certain sen- 

 sations, for example, those of touch and sight, and also the power 

 of comparing different, or repeated similar sensations. The inde- 

 pendence of the nerve-fibres, in their course from the sensory organs 

 to the great nervous centres, is supposed to account for this indepen- 

 dence, so far as regards the internuncial nerve-cords. 



Another interesting general fact, concerning sensation, is, that the 

 mind, i. e., the mental perception, refers sensations not to the senso- 

 rial centre, or actual seat of conscious sensation, but either to the 

 stimulated part, or seat of the primary impression, or even to the 

 outer world. Thus the prick of a pin, is at once referred to the skin, 

 and not to the sensorial portion of the cerebrum, where the sensation 

 is completed; and, again, the impressions which produce the sensations 

 of taste and smell, are also located in the tongue and nose. The sen- 

 sations belonging to hearing and sight, are likewise not felt in the 

 brain, and only appear to take place through the ear and eye; but 

 they are referred exclusively, to their external causes, outside of, and 

 more or less remote from, the body. It is also to be noted, that touch, 

 taste, and smell, are excited by bodies of various kinds, whilst hearing 

 and sight are stimulated, each by one special agent. The reference 

 of sensations to the peripheral extremities of a nerve, even though 

 this be stimulated in its trunk, is illustrated by the tingling of the 

 little finger, or the feeling of pins and needles, when the ulnar nerve 

 is struck, or compressed, at the elbow, or funny bone; and the same 

 phenomenon is observed after amputations of the limbs, when sensa- 

 tions are felt, as it is expressed, in the amputated toes or fingers, 

 owing to irritation in the cut ends of the nerves of the stump. This 

 local reference of the sensations, to the extremity of the nerves con- 

 cerned, is also exemplified in the phenomena of the transference and 

 radiation of painful sensations (see page 274). 



The nature of some sensations, as we have just shown, is so purely 

 objective, that we habitually refer them entirely to the outer world, 

 without being conscious of any internal or local changes in the sensory 

 apparatus ; this is the case in sight and hearing. A less completely 

 objective sensation is that of pressure, since we refer it to a part of 

 the body, as well as to its external cause. Still less apparently objec- 



