722 OF THE ORGANS OF THE SENSES, AND THEIR FUNCTIONS. 



of the body, unless affected with inflammation ; for though the mucous mem- 

 brane which covers its surface, and which is prolonged from the skin, is 

 acutely sensible to tactile impressions, the interior, with the exception of the 

 Iris, is by no means so, as is well known to those who have operated much 

 on this organ. And the common sensory nerves which supply certain parts 

 of the body, are adapted to receive and convey to the mind impressions of 

 particular kinds, with much greater readiness than they communicate those 

 of a different description ; thus the sensibility to tickling is much greater on 

 some parts of the surface than on others ; and this kind of excitement, 

 applied to the genitals or to the nipple, produces sensations of a most pecu- 

 liar order. 



589. An active Capillary Circulation being essential to the sensibility of 

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

 sensibility, 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 inflammation. A diminution or increase of sensibility to ex- 

 ternal 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 stag- 

 nation of the circulation, or an abnormal state of the blood (such as that 

 produced by anaesthetic agents), occasions a diminished functional activity 

 in the Sensorial centres, this is marked by obtuseuess to sensory impressions ; 

 on the other hand, in active congestion of the brain, the most ordinary ex- 

 ternal impressions produce sensations of an unbearable violence; and in that 

 peculiar condition of the nervous system known under the name of Hysteri- 

 cal, the patients often manifest the same hyperresthesia, even when the cir- 

 culation is in a feeble, rather than in an excited state. 1 It is remarkable 

 that the sensibility of the mucous membranes lining the internal organs, is 

 less exalted by the state of inflammation, 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 necessary to life could not be performed 

 without extreme distress, whenever a very moderate amount of disorder 

 might exist 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. 



590. It is through the medium of Sensation that we acquire a knowledge 

 of the material Universe around us, by the psychical operations which its 

 changes excite in ourselves. The various kinds or modes of Sensation sug- 

 gest to us various ideas regarding the properties of matter ; and these prop- 

 erties are known to us, only through the changes which they produce in (he 

 several organs. It is well known that instance's exist, in which, from some 

 imperfection of the organization, there is an incapacity for distinguishing 

 colors or musical tones, whilst there is no want of sensibility to light or 

 sound ; and that some persons are naturally endowed with a much greater 

 range of the sensory faculties, than that possessed by others. Hence it does 

 not seem at all improbable, that there are properties of matter of which none 

 of our senses can take immediate cognizance; and which other beings might 

 be formed to perceive, in the same manner as we are sensitive to light, 

 sound, etc. Thus many animals are affected by atmospheric changes, in 



i The influence of toxic ngonts introduced into the Mood, in producing Anaesthesia 

 and II y|M'i-;i'sthesia, constitutes a very wide field of inquiry, the investigation of 

 which has been ably commenced hy Dr. Anstie, in his interesting work on Stimu- 

 lants and Narcotics '(London, 1804). It is remarkable that Lead and Alcohol should 

 be capable of inducing either of these states. 



