700 



SMELL. 



than to the Cetacea, the olfactive apparatus is 

 formed after the usual type. 



In Fishes, however, the plan is altogether 

 changed, the organ of smell being no longer 

 connected with the respiratory passages, but 

 disposed in a cavity peculiar to itself, which 

 opens externally by anterior nares, but has no 

 internal communication by means of posterior 

 orifices. 



No distinct organ of smell has yet been dis- 

 covered in the Dibranchiate Cephalopoda; 

 but in the Nautilus, a peculiar laminated 

 organ, strongly resembling the olfactive organ 

 of fish, has been considered by Prof. Owen 

 as an olfactory apparatus. The inferior Mol- 

 lusca would seem to be altogether destitute of 

 special organs of smell ; but as there is much 

 reason to believe that some of them, espe- 

 cially the terrestrial Gasteropods, are guided 

 to their food by its scent, it would not seem 

 improbable that some part of the soft spongy 

 glandular mantle, in which the entire body is 

 enveloped, may be adapted to take cogni- 

 zance of odorous emanations ; or that in the 

 air-breathing species, the entrance to the re- 

 spiratory sac should be endowed with a low 

 degree of this power. 



There is ample reason to infer, from ob- 

 servations of the actions of Insects, that these 

 animals possess the olfactive power in no in- 

 considerable degree ; and yet no special organ 

 for this sense has hitherto been satisfactorily 

 made out. That many insects are guided to 

 their food, to the proper nidus for their eggs, 

 and to the opposite sex of their own species, 

 and are even informed of the proximity of their 

 natural enemies, by odorous emanations, can 

 scarcely be doubted by any one who watches 

 their habits, and who experiments upon their 

 actions under a variety of circumstances. 

 Thus, the flesh-fly will be attracted by the 

 odour of decomposing meat, when this is 

 completely hid from its sight ; and will depo- 

 sit its eggs on the envelope with which it may 

 be covered. On the other hand, the same 

 insect is deceived by the odour of the Stapelia, 

 or carrion-flower, and is led to deposit its 

 eggs in its petals. Again, many male insects 

 will show that they are a^are of the prox- 

 imity of their females, when the latter are shut 

 up in boxes, so as to he hid from their sight, 

 and utter no sound. And in like manner, 

 when a predaceons insect or spider is shut 

 up in a box that gives a sufficiently free pas- 

 sage to air, the small insects on which it preys 

 will manifest their alarm at its proximity, and 

 will endeavour to make their escape. Some 

 entomologists have supposed the seat of the 

 olfactory sense of insects to be in their an- 

 tenna;, others in the palpi, and others in the 

 entrances to the air-tubes. No evidence can 

 be adduced in favour of either of these sup- 

 positions that is satisfactory enough to prove 

 it, and we have little other guide at present 

 than a priori probability. In regard to the 

 last of the three suppositions, however, it may 

 be remarked that all analogy opposes the idea 

 that the true olfactory apparatus should be 

 thus scattered amongst the several segments of 



the hotly ; and the experiments which appear 

 to favour it really lead to no other conclusion 

 than this, that acrid or irritating vapours, 

 taken in through the breathing-pores, may 

 excite reflex movements which seem destined 

 to expel them, or to withdraw the body from 

 them. Such movements resemble those of 

 coughing and sneezing in man, which are ex- 

 cited through the nerves of common sensa- 

 tion, and not through the first pair ; and they 

 do not in the least indicate, therefore, that 

 the sense of smell is in any way connected 

 with the respiratory apparatus of insects, 

 myriapods, &c. The use which many insects 

 may be seen to make of their palpi, in taking 

 cognizance of their food without actually 

 touching it, suggests the idea that they are 

 the true olfactive organs; and this idea is 

 borne out by the fact, that these organs ter- 

 minate, in the living state of many insects, 

 in soft bulbous expansions, which shrivel up 

 and become horny in the dead specimen, 

 thereby obscuring their real character. On the 

 other hand, many insects are furnished with 

 soft membranous appendages at the base of 

 their antennas, which seem equally adapted to 

 perform this function. And it is asserted by 

 Duges*, that insects whose antennae had been 

 cut oil' did not manifest the same cognizance 

 of the neighbourhood of odorous substances, 

 as did others of their kind whose antennae had 

 been left entire. It would seem not a very 

 improbable supposition that, as the antenna? 

 and palpi are organs of a similar class, the 

 sense of smell may not be localised in one or 

 other of, them constantly ; but that it may be 

 assigned to one or the other, according to the 

 modifications they may respectively require 

 for the performance of their other offices. 

 The same doubt exists in regard to the olfac- 

 tive organ of the Crustacea. The manner in 

 which crabs and lobsters are attracted by 

 odorous bait placed in closed traps, makes it 

 almost certain that they must possess some 

 sense of smell ; and the most probable locality 

 of the organ would seem to be a cavity dis- 

 covered by Rosenthal at the base of the first 

 pair of antenna?. 



As to the existence or absence of the sense 

 of smell in the lower Invertebrata, nothing 

 can be definitely stated. 



Nerve of smell. That the first pair of cra- 

 nial nerves is the true olfactive, and that 

 through it alone are the proper odorous 

 emanations perceived, would seem a legitimate 

 inference from the fact, that its development 

 in vertebrated animals is constantly propor- 

 tionate, CfEleris paribus, to the acuteness of 

 the sense ; and that it is chiefly distributed 

 to that part of the nasal cavity, which is most 

 distinguished by the possession of this endow- 

 ment. This inference is fully borne out by 

 the facts supplied by experiment and patho- 

 logical observation. The division of the olfac- 

 tory nerves in animals evidently produces a 

 complete destruction of the power of per- 

 ceiving odours ; although they are st.ll affected 



* riiyhOilo^iu Comparte, torn. i. \i\<. 100, 1C1. 



