PHYSIOLOGY. 43 



which excite the action of irritable fibres, and which we call 

 stimulants ; but they are only causes of motion, and not of 

 greater mobility." 



LVII. Different parts of the body are sensible, and sensible 

 only by means of nerves present in them ; but anatomy does 

 not always determine certainly with regard to the presence or 

 absence of nerves ; and, therefore, the sensibility of several 

 parts can be determined by experiment only ; which, however, 

 is also fallacious. 



LVIII. Particular sensations arise from impressions on cer- 

 tain parts only: 1. Because the sentient extremities in these 

 parts are so situated as to be exposed to the action of certain 

 external bodies only. 2. Because the sentient extremities are 

 connected with an organ that increases the force of the external 

 agent, or modifies its action in the manner necessary to a deter- 

 mined impression. 3. Because the fibres of certain sentient 

 extremities are, by their size or tension, fitted to be acted upon 

 by certain external bodies only. 4. Because certain sentient 

 extremities are so constantly preserved in a certain state, as to 

 render them more sensible to a change. 



These circumstances determine the mode of impulse, but do 

 not account for the sensation arising from it. 



LIX. Different sensations are accompanied with different 

 judgments concerning the bodies making impression, and the 

 part of the human body upon which it is made. Some 

 sensations are referred to bodies at a distance; others, to 

 external bodies in contact; and others, to the feeling body 

 itself. 



LX. When sensations are referred to our own bodies, it is in 

 three several ways : 1. They are most commonly referred to 

 the part on which immediately the impression is made, and this, 

 with regard to the external parts, very accurately ; but, with 

 regard to the internal, much less so : and commonly the sensa- 

 tions arising from internal parts, are referred to the incumbent 

 external part, with some obscure distinction between superficial 

 and more deep. " We may observe this in the stitch of the 

 side. The patient commonly does not know whether it is ex- 

 ternal or internal, till he tries if it can be affected by external 

 pressure. In many cases we know that the pain is ou the ex- 



