

OP SENSATION IN GENERAL. 849 



several viscera, which, in the ordinary condition, are much less sensible than 

 the membranes that cover those viscera, although so plentifully supplied 

 with blood for their especial purposes. The most sensible of all parts of 

 the body is the Skin, in which the sensory nerves spread themselves out into a 

 minute network ; and even of this tissue, the sensibility differs greatly in dif- 

 ferent parts. The organs of special sensation become, by the peculiar charac- 

 ter of the nerves with which they are supplied, the recipients of impressions 

 of a particular kind : thus the eye is sensible to light, the ear to sound, &c. ; 

 and whatever amount of ordinary sensibility they possess, is dependent upon 

 other sensory nerves. The eye, for example, contrary to the usual notions, is a 

 very insensible part of the body, unless affected with inflammation ; for, though 

 the mucous membrane which covers its surface, and which is prolonged from 

 the skin, is acutely sensible to tactile impressions, the interior is by no means 

 so, as is well known to those who have operated much on this organ. And there 

 are many parts of the body that are supplied with the common sensory nerves, 

 which receive and convey to the mind impressions of particular kinds, with 

 much greater readiness than they communicate those of a different description. 



851. An active Capillary Circulation being essential to the Sensibility of 

 every part supplied with nerves, any cause which retards this deadens the sen- 

 sibility, as is well seen with regard to Cold ; and, on the other hand, an increase 

 in its energy produces a corresponding increase in the sensibility, as is peculiarly 

 evident in the " active congestion" which usually precedes and accompanies In- 

 flammation. A diminution or increase of sensibility to external impressions may 

 arise, however, not only from an abnormal state of the circulation in the organ or 

 part itself, but from the similar conditions affecting that part of the sensorium in 

 which the impressions are received. Thus, in those various conditions of the 

 Encephalon, in which either a stagnation of the circulation, or an abnormal state 

 of the blood, occasions a diminished functional activity in the Sensorial centres, 

 this is marked by obtuseness to sensory impressions j on the other hand, in 

 active congestion of the brain, the most ordinary external impressions produce 

 sensations of an unbearable violence ; and there are some peculiar conditions 

 of the nervous system, known under the name of hysterical, in which the 

 patients manifest the same discomfort, even when the circulation is in a feeble 

 rather than in an excited state. 1 It is remarkable that the sensibility of the mu- 

 cous membranes lining the internal organs is less exalted by the state of inflamma- 

 tion, than is that of most other parts ; and in this arrangement we may trace 

 a wise and beneficent provision ; since, were it otherwise, the functions neces- 

 sary to life could not be performed without extreme distress, with a very mode- 

 rate amount of disorder in the viscera. If a joint is inflamed, we can give it 

 rest ; but to the actions of the alimentary canal we can give little voluntary 

 respite. 



852. The feelings of Pain or Pleasure which are connected with particular 

 sensations, cannot (for the most part at least) be explained upon any other prin- 

 ciple than that of the necessary association of these feelings, by an original law 

 of our nature, with the sensations in question. As a general rule, it may be 

 stated that the violent excitement of any sensation is disagreeable, even when 

 the same sensation in a moderate degree may be a source of extreme pleasure. 

 This is the case alike with those impressions which are communicated through 

 the organs of sight, hearing, smell, and taste, as with those that are received 

 through the nerves of common sensation; and there can be no doubt that the 

 final cause, or purpose, of the association of painful feelings with such violent 



1 The influence of toxic agents introduced into the blood, in producing Anaesthesia and 

 Hypersesthesia, constitutes a very wide field of inquiry, which is well deserving of careful 

 cultivation. It is remarkable that Lead should be capable of inducing either of these states. 

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