844 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



In birds the sense of olfaction is less strongly developed than in 

 mammals, its place probably being taken by the higher degree of devel- 

 opment of the sense of sight. As a consequence their nasal chambers 

 are simple ; at the most three turbinated plates (anterior, middle, and 

 posterior) are present, and these are simple in form. The olfactory nerve 

 is distributed alone to the posterior turbinated bone. The external 

 nares show great variations in shape, while the posterior nares commu- 

 nicate b} r a small, slit-like opening with the aural cavity. The olfactory 

 lobes are most highly developed in birds of prey and in palmipedes who 

 feed on living fish. 



In amphibia the nasal chambers are even less complicated than is 

 the case in birds. The turbinated plates are rudimentary and usually 

 reduced to one in number. In reptiles generally the nasal chambers are 

 limited in extent and are formed of two canals opening externally, and 

 internally communicating with the mouth by two canals passing through 

 the palatine arch. 



In the fish the olfactory apparatus is not so arranged as to be trav- 

 ersed by a current of air. In them the olfactory organ consists of two 

 small cavities terminating in a cul-de-sac and opening externally by two 

 nostrils. The bottom of these sacs is generally thrown up into folds, 

 arranged as radii from a central point to which fibres coming from the 

 olfactory lobe have been traced. Water carrying odors to this olfac- 

 tory membrane can affect it but slightly, unless we can conclude that 

 their method of olfaction differs entirely from what holds in air-breathing 

 animals ; for we find that if in the latter the nasal chambers be filled 

 with liquid all impression on the sense of smell is impossible. 



In the invertebrates no organ of smell can be recognized in the ar- 

 ticulates or in the mollusks. It is, nevertheless, certain that in some of 

 the invertebrates, and particularly in insects, the sense of smell is highly 

 developed. It has been supposed that here the antennae or tentacula are 

 the seat of the sense of smell. 



For any substance to be odorous it must possess two properties. In 

 the first place, it must be volatile that is, be capable of passing into the 

 atmosphere; and, in the second place, it must to a certain extent be 

 soluble in water, that it may pass by imbibition into the fluid which 

 invariably moistens the olfactory membrane. Odorous substances in- 

 haled with the air are brought into contact with the olfactory mucous 

 membrane and act on the terminal cells of the olfactory nerve and not 

 directly upon the nerve-fibres; for we may conclude that just as neither 

 the optic nerve-fibres are affected by waves of light, nor the auditory 

 nerve-fibres by waves of sound, the fibres of the olfactory nerve are 

 equally insensitive to odors. Smell consists, therefore, in the production 

 of some change, probably of a chemical nature, in the terminal apparatus 



