OF SENSATION IN GENERAL. 225 



grees, according to the strength of the sensation which is produced by a cor- 

 responding impression on each. 



303. In accordance with what was formerly stated ( 118) 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 the non-vascular tissues, such as the epidermis, hair, nails, car- 

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

 entire absence of sensibility ; and in those whose vascularity is trifling, the 

 sensibility 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 network ; 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 supplied, 

 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 com- 

 municate those of a different description. 



304. 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 

 this, 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 

 precedes inflammation. Acute sensibility to external impressions may arise, 

 however, not only from abnormal activity of the circulation in the organ or 

 part itself, but from the same condition affecting that part of the sensorium in 

 which the impressions are received. Thus, in active congestion and inflam- 

 mation 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 



