M A M MALI A. 



12f) 



tinual danger of carrying the matter further than ar.i- 

 nial physiology in the proper view of it will warrant. 

 This is unavoidable, i'rotn the circumstance of our 

 naturally taking ourselves as the standard of judgment 

 in these cases ; and from there being a mental process 

 in addition to the merely animal one, in every case 

 ,in which a hnoivn impression is made upon our senses. 

 There mav be slight momentary affections of the 

 body as matter, which pass away without mental cog- 

 nizance, just as there are mental impressions once 

 known which may never again return in those sug- 

 gestions to which we give the name of rememberings ; 

 but in every case, where the impression on the sense 

 is sufficiently powerful and continued, for the mind 

 taking note and forming a judgment concerning it, 

 there is an element between the initial impression 

 on the sense and the final thought, whether that 

 thought does or does not ripen into action, vt hich 

 can belong to no other animal. This mental part of 

 the succession is inferred rather than originally felt 

 even by ourselves, and therefore the separation of it 

 is in all cases exceedingly difficult. 



There is another source of difficulty ; we can say 

 very little of the mode of action in nerves, because 

 there is nothing to which we can compare their 

 structure. It does not appear that there is any cir- 

 culation in it ; and it does not appear to be a sub- 

 stance calculated for transmitting any powerful kind 

 of action of a mechanical or chemical nature. The 

 most careful examiners are not agreed as to the struc- 

 ture of nervous matter, and even though that were 

 made out, it would not explain nervous action. That 

 it is a means for the very rapid production of animal 

 action, vte have no reason to doubt ; and the analogy 

 would lead us to suppose that that which it resembles 

 the most, though we cannot say that they are iden- 

 tical or even alike, is that display of action to which 

 we give the name of galvanic electricity. We know 

 that this electricity is one of the most powerful sol- 

 vents that can be applied in the arts ; arid as the arts 

 cannot, of course, introduce a principle, or even a 

 mode of action, further than as it depends on the use 

 of apparatus, we may naturally suppose that this 

 mode of action enters very largely into the operations 

 of nature; and more especially in the -more delicate 

 functions of life in the mammalia as the most highly 

 developed of animals. It. is known that the division 

 of nerves, connected with organs of secretion, impairs 

 the functions of those organs, though the way in 

 which this takes place is not so clear a matter. There 

 is one other analogy which would lead us to conclude 

 that there is such a connexion between the excite- 

 ment of nervous action and that, of galvanic electricity 

 as has been stated ; for those fishes, such as the tor- 

 pedo and the gymnotus which have electric organs, 

 which resemble galvanic batteries, and are capable of 

 giving severe shock?, have always those organs very 

 copiously supplied with nerves. We must be on our 

 guard, however, against laying too much stress upon 

 this analogy, or supposing that our galvanic batteries 

 can point out to us how such batteries are worked 

 by nature ; because too much credulity on this sub- 

 ject, or too much of some other means of bringing it 

 forward, is has occasionally introduced much electri- 

 cal quackery into the art of pretended medicine. 



That the excitability of the nerves, even in man, is 



not necessarily connected with or dependent upon 



sensation, in the common meaning of the term, to say 



nothing of mind, is proved by many direct <>xperi- 



NAT. HIST. VOL. III. 



ments ; and indeed the actual existence of life does not 

 appear to be absolutely essential to the one any more 

 than to the other; for it may be called into 'action 

 not only after the part has" lost what is properly 

 termed sensibility, but after it is divided from the 

 body. A certain condition of the organs in which it. 

 is excited, as well as a certain composition in those 

 organs, appears to be necessary to the production of 

 this excitement ; and in the mammalia and other 

 warm-blooded animals there must be a connexion of 

 nervous substance, freedom to act, and a continued 

 supply of blood ; but from some experiments detailed 

 by Dr. Philip and Mr. Brodie, in the Philosophical 

 Transactions for 1822, it appears that absolute con- 

 tinuity in the nervous substance is not essential ; for 

 when the ends of a divided nerve are brought very 

 near to each other, the communication by means of 

 that nerve is not wholly destroyed. The whole 

 doctrine of the nerves and their modes of action is, 

 however, very imperfectly known ; and what is known 

 is not well adapted for popular explanation. 



Sensation, the last function of the animal system to 

 which we deem it necessary to advert, is so closely 

 connected with nervous excitement, and the obscuri- 

 ties of the one are so much involved in those of the 

 other, that we cannot speak confidently beyond what 

 we feel in ourselves, and there are doubts and diffi- 

 culties even there. The word too has been used in 

 different senses, and in some of them with great lati- 

 tude of expression, though in other instances it has 

 been narrowed beyond its proper meaning. 



Generally speaking sensation, or rather sensibility, 

 for sensibility is the function and sensation the exer- 

 cise of the function, is applied to the quality of re- 

 ceiving impressions from objects or causes external 

 of the organ immediately impressed. Those impres- 

 sions are very different in their results, even in the 

 same animal, or part of an animal. They may be 

 pleasurable, they may be painful, or they may be 

 decidedly neither the one nor the other, but merely 

 what we may term informatory, and their effect may 

 be to produce, terminate, or suspend the actions of 

 the body both external and internal. 



Sensation, in this view of it, extends almost to every 

 part of the animal system ; for if the part is considered 

 as not immediately capable of receiving an impression 

 itself, it always can transmit the impression to some 

 other part which can. In a perfectly healthy state 

 of the body, and in undisturbed repose, as when it is 

 in dreamless sleep, it is all insensible to slight im- 

 pressions ; and if the sleep is very profound, it may 

 be seriously injured without sensation till the injury 

 is done. In disturbed sleep, which is broken by 

 dreams, there is always some remnant of sensation, 

 arising from a disturbing cause ; and thus dreaming 

 is not, in its primary state, an affection of the mi-ad 

 even in man ; for, from the very nature of the mind, 

 it cannot sleep, and in those animals with which we 

 are familiar in their sleeping hours, there are evidences 

 of dreaming as well as there are in man, though we 

 are as ignorant of the nature of their dreams, or of 

 how these differ from our own, as we are of the dif- 

 ference between sensation accompanied by mental 

 action and sensation not so accompanied. Various 

 diseased conditions of different parts of the body will 

 also bring very acute sensation into parts which do 

 not possess it when in their natural state of health, 

 as for instance tendons, ligaments, hones, and even 

 fatty substances may experience most acute pain from 



