OF ANIMALS. 



16. 



quamted with the reason of this conformation, and of the respective share which 

 each division taices in producing the general effect. 



The nerves imiformly issue in pairs, one for each side of the body, and the 

 number of the pairs is thirty-nine ; of which nine rise immediately from the 

 great divisions of the brain, under which we have just contemplated it, and 

 are chiefly appropriated to the four local senses ; and thirty from the spinal 

 marrow, through different apertures in the bone that encases it, and are alto- 

 gether distributed over the body, to produce the fifth or general sense of 

 touch and feeling, as also irritability to the muscles. 



That these nervous or pulpy fibres are the organs by which the various sen- 

 sations are produced or maintained is demonstrable from the following facts : 

 If we divide, or tie, or merely compress, a nerve of any kind, the muscle with 

 which it communicates becomes almost mstantly palsied ; but upon untying 

 or removing the compression, the muscle recovers its feeling and mobility. 

 If the compression be made on any particular portion of the brain, that part 

 of the body becomes motionless which derives nerves from the portion com- 

 pressed. 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 particular parts 

 of the brain, or of the particular nervous fibres themselves, into fluids of very 

 different 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 : — I mean the pro- 

 duction of other senses and sensorial powers than 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 possessed by 

 man, might, perhaps, render superfluous the use of the magnet, and consider- 

 ably 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 atmosphere with the utmost nicety, 

 and arrive on 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 New- 

 foundland to the southern coasts of Spain ; and the cachalot, or spermaceti 

 whale, driving vast armies of herrings from the arctic regions, and devourmg 

 thousands of those that are in the rear every hour. 



We know nothing of this sense, or the means by which all this is produced : 

 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 has 

 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 round its neck, from 



• See Hunter's Anim. Economy, p. 261, 262. 



