386 ON SENSATION, AND THE ORGANS OF THE SENSES. 



and the term excitor, proposed by Dr. M. Hall, is much preferable. Every 

 afferent spinal nerve, therefore, is made up of sensory and of excitor fibres ; 

 and these may be distributed in very different proportions to different parts. 

 Of the excitor fibres, enough has been already said. Those parts of the body 

 which are endowed with sensory fibres, and impressions on which, therefore, 

 give rise to sensation, are ordinarily spoken of as sensible, and different parts 

 are spoken of as sensible in different degrees, according to the strength of the 

 sensation which is produced by a corresponding impression on each. 



507. In accordance with what was formerly stated ( 250) of the depend- 

 ence of all nervous action on the continuance of the capillary Circulation, 

 especially at the extremities of the fibres, it is found that the sensory nerves 

 are distributed pretty much in the same proportion as the blood-vessels ; that 

 is to say, in theTion-vascular tissues, such as the epidermis, hair, nails, car- 

 tilage, and bony substance of the teeth, no nerves exist, and there is an en- 

 tire absence of sensibility ; and in those whose vascularity is trifling, the sen- 

 sibility is dull, as is the case with bones, tendons, ligaments, fibrous mem- 

 branes, and other parts whose functions are simply mechanical, and even with 

 serous and areolar membranes. Many of these textures are acutely sensible, 

 however, under certain circumstances ; thus, although tendons and ligaments 

 may be wounded, burned, &c., with little or no consciousness of the injury, 

 they cannot be stretched without considerable pain ; and the fibrous, serous 

 and areolar tissues, when their vascularity is increased by inflammation, also 

 become extremely susceptible of painful impressions. All very vascular parts, 

 however, do not possess acute sensibility; the muscles, for instance, are fur- 

 nished with a large supply of blood, to enable them to perform their peculiar 

 function ; but they are not sensible in by any means the same proportion. 

 Even the substance of the brain and of the nerves of special sensation, ap- 

 pears to be destitute of this property ; and the same may be said of the mu- 

 cous membranes, lining the interior of the several viscera, which, in the ordi- 

 nary condition, are much less sensible than the membranes which 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 net- work ; and even of this tissue, 

 the sensibility differs greatly in different parts. The organs of special sensa- 

 tion are, by the peculiar character of the nerves with which they are sup- 

 plied, rendered sensible to 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 some kinds of impressions, the interior is by no means so, as is 

 well known to those who have operated much on the eye. And there are 

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

 which convey to the mind impressions of particular kinds, with much greater 

 readiness than they communicate those of a different description. 



508. It appears, then, that the vascularity of a part is an essential condition 

 of its sensibility; but it does not follow that a tissue should be peculiarly sen- 

 sible, because it is highly vascular; since its large supply of blood may be 

 required for other purposes. It is not simple vascularity, however, which is 

 necessary, but rather an active capillary circulation; any cause which retards 

 tliis, deadens the sensibility, as is well seen in 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 pre- 

 cedes inflammation. Acute sensibility to external impressions may arise, 



