OF SENSIBILITY IN GENERAL. 721 



CHAPTER XIV. 



OF THE ORGANS OF THE SENSES, AND THEIR FUNCTIONS. 



1. Of Sensibility in General. 



588. WE have seen that the conscious Mind is affected by impressions 

 made upon the corporeal organism, or, in other words, that Sensation is pro- 

 duced, through the instrumentality of a certain part of the Eucephalon 

 termed the Sensorium, which is the general centre of the nerves both of 

 "special " and of " common " sensibility: the former connect it with the 

 special Organs of Sense, the latter with the body generally, to the several 

 parts of which they are by no means uniformly distributed, some tissues 

 being altogether destitute of them. Those parts of the body which are en- 

 dowed with sensory fibres, and impressions on which, therefore, give rise to 

 sensation, are ordinarily spoken of as sensible ; and different parts are said 

 to be sensible in different degrees, according to the strength of the sensation 

 produced by a corresponding impression on each. In accordance with the 

 general fact of the dependence of all Nervous action on the continuance of 

 the Circulation of the blood, it is found that the sensory nerves are distrib- 

 uted pretty much in the same proportion as the bloodvessels : that is to say, 

 in the non-vascular tissues, such as the epidermis, hair, nails, cartilage, and 

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

 of sensibility ; and in those whose vascularity is trifling, as in the case with 

 bones, tendons, ligaments, fibrous membranes, and other parts whose func- 

 tions are simply mechanical, and even with serous and areolar membranes, 

 there are few nerves, and the sensibility is dull. Many of these textures are 

 acutely sensible, however, under certain circumstances; thus, although ten- 

 dons and ligaments may be wounded, burned, etc., without much conscious- 

 ness of the injury being aroused, they cannot be stretched without the pro- 

 duction of 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 furnished 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, appears to be 

 destitute of this endowment ; and the same may be said of the mucous mem- 

 branes lining the interior of the several viscera, which, in the ordinary con- 

 dition, 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 Avhich the sensory nerves 

 spread themselves out into a minute network ; and even of this tissue, the 

 sensibility differs greatly in different parts ( 594). The organs of Special 

 Sensation become, by the peculiar character 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, etc. ; 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 



