530 OF SENSATION IN GENERAL. 



parts where the impression is first made on the nerves, they are really 

 felt in the brain. This is evident from two facts ; first, that if the 

 nervous communication between the part and the brain be interrupted, 

 no impressions, however violent, can make themselves felt ; and second, 

 that if the trunk of the nerve be irritated or pinched, anywhere in its 

 course, the pain which is felt is referred, not to the point injured, but 

 to the surface to which these nerves are distributed. Hence the well- 

 known fact that, for some time after the amputation of a limb, the 

 patient feels pains, which he refers to the fingers or toes that have been 

 removed ; this continues until the irritation of the cut extremities of the 

 nervous trunks has subsided. 



931., It would seem probable that, among the lower tribes of Ani- 

 mals, there exists no other kind of sensibility, than that termed general 

 or common; which pervades, in a greater or less degree, nearly every 

 part of the bodies of the higher. It is by this, that we feel those im- 

 pressions, made upon our bodies by the objects around us, which pro- 

 duce the various modifications of pain, the sense of contact or resistance, 

 the sense of variations of temperature, and others of a similar character. 

 From what was formerly stated ( 403) of the dependence of the im- 

 pressibility of the sensory nerves, upon the activity of the circulation 

 in the neighbourhood of their extremities, it is obvious that no parts 

 destitute of blood-vessels can receive such impressions, or (in common 

 language) can possess sensibility. Accordingly we find that the hair, 

 nails, teeth, cartilages, and other parts that are altogether extra-vascu- 

 lar, are themselves destitute of sensibility ; although certain parts con- 

 nected with them, such as the bulb of the hair, or the vascular membrane 

 lining the pulp-cavity of the tooth, may be acutely sensitive. Again, 

 in tendons, ligaments, fibro-cartilages, bones, &c., whose substance con- 

 tains very few vessels, there is but a very low amount of sensibility. 

 On the other hand, the skin and other parts, which are peculiarly 

 adapted to receive such impressions, are extremely vascular ; and it is 

 interesting to observe, that some of the tissues just mentioned become 

 acutely sensible, when new vessels form in them in consequence of 

 diseased action. It does not necessarily follow, however, that parts 

 should be sensible in a degree proportional to the amount of blood they 

 may contain ; for this blood may be sent to them for other purposes, 

 and they may contain but a small number of sensory nerves. Thus, 

 although it is a condition necessary to the action of Muscles, that they 

 should be copiously supplied with blood ( 359), they are by no means 

 acutely sensible ; and, in like manner, Glands, which receive a large 

 amount of blood for their peculiar purposes, are far from possessing a 

 high degree of sensibility. 



932. But besides the general or common sensibility, which is dif- 

 fused over the greater part of the body, in most animals, there are cer- 

 tain parts, which are endowed with the property of receiving impres- 

 sions of a peculiar or special kind, such as sounds or odours, that would 

 have no influence on the rest ; and the sensations which these excite, 

 being ^of a kind very different from those already mentioned, arouse 

 ideas in our minds, which we should never have gained without them. 

 Thus, although we can acquire a knowledge of the shape and position 



