214 On the Anatomy and Physiology of the Organs of the Senses. 



Whenever an organ of sense is of size sufficient to admit of its examina- 

 tion, which it is in all the higher and in many of the lower tribes of animals, 

 a number of medullary or nervous filaments may be detected in its struc- 

 ture ; the attenuated extremities of which are expanded upon, or rather, 

 form the sentient surface of the organ, and are finally collected into a single 

 trunk or nerve, by Avhich the connexion between the organ and the senso- 

 rium, and by its intervention, between one organ and another, is established 

 and preserved. 



Among the provisions, of which the animal oeconomy is full, there is 

 none more striking than that by which the different nerves are adapted to 

 receive and convey different classes of impressions. Thus the optic nerve, 

 that of vision, appreciates the rays of light, but is insensible to a grosser 

 injury, and feels no pain when wounded. The nerves of the skin, liable to 

 a thousand accidents by heat, injuries mechanical and chemical, are alive 

 to such impressions j but light and sound can never injure the skin, and 

 therefore their impressions, felt so acutely by the eye and ear, are in no 

 degree felt by the skin. The tendons and ligaments, when divided beneath 

 the knife of the surgeon, give no pain ; but place these ligaments on the 

 stretch, an injury unfelt for obvious reasons by the flexible integument, and 

 a sensation of pain will be produced, calling upon the sufferer to attempt 

 the relief of the part. 



It is found moreover that these nerves are so organised, as to refer all 

 their impressions, at whatever part of their course received, to the extre- 

 mity, that is to the part of the body for the supply of which they were 

 provided ; so that if the nerve supplying the inside of the hand be struck, 

 as it often is, where it passes over the elbow, a tingling sensation is felt, 

 referable, not expressly, nor in any great degree to the elbow, but to the 

 fingers below. If the nerve injured supply an organ of sense, the impression 

 produced is not that of pain, but that which the organ is calculated to re- 

 ceive, so that if the optic nerve be pressed upon as it enters the orbit, a 

 sensation of a flash of light is produced. 



The minuteness of the ultimate filaments in which the nerves by which 

 the integument is supplied, terminate, is very considerable ; every part of 

 the surface has its particular nerve, and as every part of the body is liable 

 to aggression, every nerve must be competent to produce a distinct impres- 

 sion upon the sensorium, vvhich must thus present an epitome of the 

 superficies of the body, as the acorn is said to contain all the ramifications 

 of the future oak. 



When an organ of sense has received an impression, that impression is 

 conveyed along the nerve to the sensorium, producing there a physical 

 change, by which it is presented to the mind as a sensation, and its recog- 

 nition, its apprehension by the mind, constitutes a perception. These two 

 faculties being alone of comparatively little value, we find a third power 

 added, by means of which certain measures are determined by the mind, 

 and this is called volition. 



