848 OF SENSATION, AND THE ORGANS OF THE SENSES. 



interest, that, during the present century, notwithstanding the large amount of anatomical 

 and experimental inquiry which has been directed to the Nervous System both in France 

 and Germany, and the vast addition to our knowledge of details which has hence arisen, 

 the great advances in the general doctrines of this department of the science should have 

 been made by British Physiologists.] 



CHAPTER XV. 



OF SENSATIONS, AND THE ORGANS OF THE SENSES. 



1. Of Sensation in General. 



850. BY the term Sensation is rightly understood that change in the condition 

 of the Mind by which we become aware of an impression made upon some part 

 of the Body; or, in a briefer form of expression, it may be denned to be the 

 consciousness of an impression. Some Physiologists have, it is true, spoken of 

 a sensation without consciousness; but it seems very desirable to limit the term 

 to the mental change ; since the word impression serves to designate the change 

 produced in the afferent nerves by an external cause, up to the point at which 

 the mind becomes conscious of it. We have seen reason to believe that the 

 impressions communicated to the Spinal Cord may there excite motor actions, 

 without occasioning true sensation ; and it would seem to be with a certain part 

 of the Encephalon only, that the Mind possesses the relation necessary for the 

 production of such a change in it. Hence this organ is spoken of as the Sen- 

 sorium. For the reasons already given ( 732-4), it seems probable that the 

 ganglia of Special Sensation are the essential instruments of this function, rather 

 than the Cerebral Hemispheres. The afferent nervous fibres, which connect the 

 various parts of the body with the Sensorium, are termed sensory ; and these 

 are distributed in very different proportions to different parts. Those parts of 

 the body which are endowed with sensory fibres, and impressions on which, there- 

 fore, 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. In 

 accordance with what was formerly stated ( 355) of the dependence of all 

 Nervous action on the continuance of the Circulation of blood, it is found that 

 the sensory nerves are distributed pretty much in the same proportion as the 

 bloodvessels ; that is to say, in the non-vascular tissues such as the epider- 

 mis, hair, nails, cartilage, and bony substance of the teeth no nerves exist, 

 and there is an entire absence of sensibility; and in those whose vascular- 

 ity is trifling, the sensibility is dull, as is the case with bones, tendons, 

 ligaments, fibrous membranes, and other parts whose functions are simply me- 

 chanical, and even with serous and areolar membranes. Many of these tex- 

 tures are acutely sensible, however, under certain circumstances ; thus, although 

 tendons and ligaments may be wounded, burned, &c., without giving rise to 

 much consciousness of the injury, 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 membranes lining the interior of the 



