SMELL. 545 



All animals exposed to the air have perhaps the sense of smell. Its seat has 

 been referred in the air-breathing annelides arid insects to the mouths or lateral 

 pores of the air sacs ; or in the latter "to the delicate extremities of their long 

 flexible antennae, and the inner pair of those organs in the Crustacea have been 

 considered as the seat of the same sense. The labial appendices of the conchi- 

 fera, the entrance to the respiratory sacs of pulmonated gasteropods, the highly 

 sensitive tentacula covered with a delicate mucous membrane, and even the 

 whole surface of the skin in the more elevated molluscous classes, have been 

 considered as the organs through which these animals receive impressions from 

 odorous emanations. In the class of fishes, we observe the organ of smell to be 

 only a depression excavated on the anterior part of the face, but it does not 

 communicate behind with the mouth, or the respiratory organs, or the interior of 

 the body." " By the motions of fishes through the water they are sufficiently 

 exposed to receive impressions of odorous substances diffused through that 

 medium, without drawing dense water through those delicate organs for the 

 purposes of smell. Perhaps the volumes of water necessary to be carried con- 

 tinually through the mouth of fishes for respiration are too great, and would 

 prove too powerful a stimulus to have passed through such an organ of smell, 

 and to have allowed that organ to preserve its necessary delicacy, and therefore 

 it is quite apart from the passage through which that element is taken for re- 

 spiration in all water-breathing animals. It is, obviously, however, in fishes an 

 organ of great delicacy and importance, and is of great size, provided with very 

 large olfactory nerves, and large olfactory tubercles, coming off alone from the 

 hemispheres of the brain." " In the amphibious animals, where the respiration 

 of air begins to take place through the nostrils, the olfactory apparatus begins to 

 be more complicated and concealed." In fish a plate of cartilage sometimes 

 divided the impervious olfactory cavity into two : in the amphibia this " begins 

 to assume now the more compact and convoluted form which the osseous plates 

 in the higher animals present. The surface of the organ thus increases in 

 extent, as we ascend through the reptiles and through the birds to the mam- 

 malia. In the perenni-branchiate amphibia the nostrils form still on each side 

 a simple sac, scarcely complicated internally, and having their posterior opening 

 so far formed in the mouth as to be immediately under the upper lip. In the 

 salamanders and frogs the nostrils are still, in the larva state, confined to the 

 exterior of the head, as in fishes j but in the adult form, the posterior openings, 

 though within the cavity of the mouth, are still much advanced in their position, 

 and remote from the median line." " In the serpents the internal surface is 

 extended by the rudimentary turbinated bones, and by an enlarged nasal cavity, 

 opening posteriorly by a common orifice on the median line." " In the sauria 

 the turbinated bones begin to be strengthened by ossific matter and to assume 

 a more complicated form : both the anterior and posterior openings of the nares 

 present enlarged dimensions, and the whole organ is more internal and more 

 protected by the expanded nasal bones. The organs of smell are more protected 

 and concealed in the solid head of the chelonian reptiles, where their surface is 

 increased in extent and their posterior openings are placed further back from 

 their primitive anterior aspect. The anterior openings of the nostrils are here 



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