OF THE SENSE OF HEARING. 113 



distributed. Finally, between these turbinated bones there 

 are longitudinal grooves, called meatuses. These fossae com- 

 municate with cavities, called sinuses, hollowed out in 

 the thickness of the frontal bone, the superior maxillary, &c. 

 The pituitary membrane is thick, and has a velvety appear- 

 ance ; on its surface may be observed a vibratile movement, 

 produced by microscopic cilia ; finally, it is continually mois- 

 tened by a viscous liquid, called nasal mucus, and it receives 

 many nervous filaments from the olfactory and fifth pair of 

 nerves. 



220. The membrane must be moist, in order that odours 

 may be perceived ; and the sense is strongest in the upper 

 part of the nostril. The extent of membrane seems greatly 

 to influence the power of the organ ; and in this respect the 

 carnivora, the ruminants, and some pachydermata, are re- 

 markable for the development of the turbinated bones, and 

 consequently for the extent of mucous membrane covering 

 them. In reptiles the organ is extremely simple. 



221. In fishes, the nasal fossae do not communicate 

 with the pharynx, but are merely cavities shut in at the 

 back, and the pituitary membrane presents in them a num- 

 ber of folds, arranged like rays around a central point, or 

 in parallel rows like the teeth of a comb, on each side of a 

 median band. 



But there are many animals which have a fine sense of 

 smell, as insects, Crustacea, and mollusca, and in which no 

 special organ of smell has ever been discovered. 



OF THE SENSE OF HEARING. 



222. The apparatus of hearing is very complex, minute, 

 and difficult to describe, and its essential part is enclosed 

 within the petrous portion of the temporal bone (Fig. 73 e). 



We divide the ear of man into three portions the external 

 ear, the middle ear, and the internal ear. The external ear is 

 composed of the pavilion or figured part of the ear, and the 

 auditory canal. 



The figured part of the ear is composed of a fibro- cartila- 

 ginous lamina, adhering to the edge of the auditory canal, and 

 covered with a very fine and thin skin, dry and tense, and is 

 so arranged as to form an acoustic tube. The lobe of the ear 

 is not supported by any cartilage. The auditory tube is 

 partly cartilaginous and partly osseous ; the integuments 

 I 



