170 



ON THE EXTERNAL SENSES 



facts. If we divide, or tie, or merely compress a nerve of any kind, the 

 muscle with which it communicates becomes almost instantly palsied ; 

 but upon untying or removing the compression the muscle recovers its 

 feeling and mobility. If the compression be made on any particular por- 

 tion of the brain, that part of the body begomes motionless which derives 

 nerves from the portion compressed. And if the cerebrum, cerebel, or 

 oblongated marrow be irritated, excruciating pain or convulsions, or both, 

 take place all over the body,* though chiefly where the irritation is applied 

 to the last of these three parts. 



The matter of sensation, or nervous fluid, as for want of a more precise 

 knowledge upon this subject we must still continue to call it, is probably 

 as homogeneous in its first formation as the fluid of the blood ; but, like 

 the blood, it appears to be changed by particular actions, either of parti- 

 cular parts of the brain, or of the particular nervous fibres themselves, into 

 fluids of very diffierent properties, and producing very different results. 

 And it is probably in consequence of such changes alone that it is capable 

 of exciting one set of organs to communicate to the brain the sensation 

 of sound alone, another set that of sight alone, and so of the rest. While 

 branches from the spinal marrow, or fountain-nerve of touch, are diffused 

 over every portion of the body, sometimes in conjunction with the local 

 nerves, as in the organs of local sense, and sometimes alone, as in every 

 other part of the system.* 



Such an idea leads us naturally to a very curious and recondite subject, 

 which has never, that I know of, been attended to by physiologists, and 

 will at the same time throw no small degree of light upon it : — 1 mean the 

 production of other senses and sensorial powers that are common to the 

 more perfect animals, or such a modification of some one of them as may 

 give the semblance of an additional sense. 



What, for example, is that wonderful power by which migratory birds 

 and fishes are capable of steering with the precision of the expertest 

 mariner from climate to climate, and from coast to coast ; and which, if 

 possest by man, might perhaps render superfluous the use of the magnet, 

 and considerably infringe upon the science of logarithms ? Whence comes 

 it that the field-fare and red-wing, that pass their summers in Norway, or 

 the wild-duck and merganser, that in like manner summer in the woods 

 and lakes of Lapland, are able to track the pathless void of the atmos- 

 phere with the utmost nicety, and arrive at our own coasts uniformly in 

 the beginning of October ? or that the cod, the whiting, and the herring, 

 should visit us in innumerable shoals from quarters equally remote, and 

 with an equal exactness of calculation ? the cod pursuing the whiting, 

 which flies before it, from the banks of Newfoundland to the southern 

 coasts of Spain ; and the cachalot, or spermaceti whale, driving vast 

 armies of herrings from the arctic regions, and devouring thousands of 

 those that are in the rear every hour. 



We know nothing of this sense, or the means by which all this is pro- 

 duced ; and knowing nothing of it, and feeling nothing of it, we have no 

 terms by which to reason concerning it. 



Yet it is a sense not limited to migratory animals. A carrier-pigeon 

 lias been brought in a bag from Norwich to this metropolis, constituting 

 a distance of 120 miles ; and having been let off* with a letter tied rounJ 

 its neck, from the top of St. Paul's, has returned home through the air in 

 a stj-aight fine, in four or five hours. 



* See Hunter's Aaim. Eeonoinyj p. 261? 262 



