ORGANS OF SENSE. 63 



closed by a raembrana tympani, which is concave externally, except 

 in the whale. In the ornithorhynchus, which in so many points of 

 view constitutes the connecting link between reptiles and the higher 

 classes of animals, the external passage is long and singularly tor- 

 tuous, the ossicula auditus anchylosed, the cochlea represents a 

 curved horn, as in birds and reptiles, and the semicircular canals 

 project into the cranium. The Eustachian tube is wide in the ceta- 

 cea, it opens at the blowing hole, where it is furnished with a valve; 

 the cochlea is short and convoluted in one plane in the cetacea, long, 

 narrow, and spiral in the rodentia, and in the guinea-pig, porcupine, 

 and aguti, it forms three turns and a half; the other essential parts 

 of the labyrinth agree on the whole with those of the human 

 subject. 



Smell. — No organs subservient to this sense have been detected 

 in the cyclo-neurose animals, but in the annelidesof the diplo-neura, 

 the sense is said to reside in the parietes of their mouths, or the pores 

 of their air-sacs, and in the palpi, and autennag of insects. The 

 entrance to the respiratory passages, the sensitive tentacula, and 

 even the whole surface of the body in the higher orders of the mol- 

 lusea, are capable of receiving odorous impressions. The lami- 

 nated organs of smell in fishes are placed in two small depressions on 

 the anterior part of the face ; they are protected by cartilage, lined 

 by a delicate mucous membrane disposed in radiated folds in the 

 pike: tufted in the cyprinus, and arranged like the barbs of a quill 

 in the carp, ray, and shark. These cavities have no communication 

 with the mouth or pharynx ; they are closed in front by a valve 

 formed of skin and cartilage, and a similar structure partly divides 

 each into two. 



In the perennibranchiate amphibia, as the proteus and siren, the 

 organs of smell in every respect resemble those of fishes, and in the 

 more highly developed genera, as frogs and salamanders, they 

 approach in characters to those of reptiles. The nostrils are partly 

 cartilaginous, partly osseous, and open into the cavity of the mouth. 

 The olfactory nerves enter by two foramina in the ethmoid bone, 

 and are distributed to every part of the soft pituitary membrane. In 

 serpents the nasal cavities present rudimentary turbinated bones, 

 and open posteriorly by a single orifice. The turbinated bones are 

 larger and more curved in the sauna, and in the chelonia the olfac- 

 tory surface is greatly extended and concealed by a strong osseous 

 covering. 



In birds the external nostrils vary much as to size, shape, and 

 situation, they are generally free and wide, but in some instances 

 very small, as in the heron and gannet. The septum narium is 

 partly bony, partly cartilaginous, and covered by a highly vascular 

 pituitary membrane in the swan. The nasal cavities in birds con- 

 tain three turbinated bones and open separately into the pharynx, 

 except in the cormorant, where they join previous to their opening. 

 The olfactory nerves are distributed exclusively to the membrane 

 covering the septum narium and superior spongy bone. It is gene- 



