876 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



Many insects are apparently free from any evidences of anything 

 resembling an organ of hearing, yet it is clear that these animals are 

 sensible of external sounds. It is possible that in these animals, as in 

 certain radiates and many mollusks, the vibrations of sound are not 

 appreciated as sound, but, perhaps, as some modification of the sense of 

 touch. 



In fishes there is no external ear, no tympanic cavity, and no 

 cochlea. The ear is reduced to the membranous part of the vestibule 

 and the semicircular canals, the latter varying from two to three in 

 number, while the vestibule and semicircular canals represent a closed 

 cavity, since there is neither oval nor round window, no tympanic 

 cavity, nor auditory ossicles. Sometimes, as in cartilaginous fishes, the 

 membranous internal ear is lodged in the cartilaginous substance of the 

 bones of the head, while in osseous fishes it is sometimes in part within 

 the bones of the cranium, and part free in the cephalic cavit} 7 , resting 

 against the brain. The internal ear is in all cases supplied with terminal 

 fibres of the auditory nerve, and is filled with a liquid in which are 

 found various calcareous concretions of greater or less volume. 



In reptiles there is no external ear, neither a pinna nor an external 

 auditory canal, the tympanic membrane being flush with the head and 

 lying directly below the skin, although in some instances a drum mem- 

 brane is absent. When present, as is the case in the majority of instances, 

 it communicates generally by a large opening — the Eustachian tube — 

 with the pharynx. The auditory ossicles are usually reduced to two in 

 number. When the tympanic membrane is absent, the ossicles, attached 

 at one side to the oval window, are fastened on the other directly against 

 the external integument. 



In lizards, crocodiles, and serpents the internal ear is composed of 

 the vestibule, semicircular canals, and cochlea. In them, consequently, 

 the internal ear communicates with the cavity of the tympanum by the 

 oval window and by the round window. The cochlea in them is not con- 

 voluted, but almost straight. 



In the batrachians no cochlea is present, and, as a consequence, no 

 round window, the internal ear being reduced to the vestibule and the 

 semicircular canals, the only communication with the tympanum being 

 by the oval window. 



In birds the apparatus of hearing is almost as complete as in mam- 

 mals, with the single exception of the external auditory pinna, which is 

 absent. The external auditory canal is formed by a bony canal travers- 

 ing the temporal bone, and the tympanic cavity, separated from this 

 canal by the tympanic membrane, is well developed. It communicates 

 •with bony cavities in the interior of almost all the cranial bones, and by 

 the intermediation of the Eustachian tube with the pharynx. The 



