424 



ON SENSATION, AND THE ORGANS OF THE SENSES. 



the ear consists of a small cavity excavated in the solid frame-work of the 

 head ; this cavity is lined with a membrane, on which the nerve is distri- 

 buted ; and it is filled with a watery fluid. In some instances, the cavity is 

 completely shut in by its solid walls ; and the sonorous vibrations can then 

 only be communicated through these : but in the higher forms of this appa- 

 ratus, there is a small aperture covered with a membrane, upon which the 

 external medium can at once act. In tracing this most simple into the more 

 complex forms, it is at once seen, that the cavity corresponds with the vesti- 

 bule of the ear of higher animals, and its opening with the fenestra ovalis. 

 In the lowest Cyclostome Fishes, the organ is but little more complicated ; 

 from the vestibule proceeds a single annular passage, which may be consi- 

 dered as a semicircular canal ; and the auditory nerve is distributed minutely 

 upon its lining membrane, as upon that of the vestibule itself. In species 

 a little higher in the scale, two such canals exist ; these are present in the 

 Lamprey. And in all the rest of the class, three semicircular canals are 

 found; holding the same direction in regard to each other, as they do in Man. 

 Within the vestibular sac of Fishes are found calcareous concretions, which 

 are pulverulent in the Cartilaginous, but hard and stony in the Osseous tribes; 

 to these the name of Otolithes has been given. Some rudiments of a tympa- 

 nic cavity may be found in Fishes ; but there is no vestige of a cochlea : in 

 several tribes, the organ of hearing possesses a peculiar connection with the 

 air-bladder, which appears to be a foreshadowing of the Eustachian tube of 

 higher classes. 



558. In the true Reptiles, a considerable advance is constantly to be found 

 in the character of the Ear; a tympanic cavity being added, with a drum and 

 a chain of bones ; and a rudiment of the cochlea being generally discoverable. 

 Among the Amphibia, however, which are in so many respects intermediate 



ig. 181. 



Diagram of the inner wall of the tympanun after maceration, the outer wall anil ossicles being re- 

 moved, o. Fenestra ovalis. b. Fenestra rotunda, c. Promontory. <l. Pn r:\mid, with the orifice at its 

 apex. e. Projection of the aqueductus Fallopii. f. Some of the mastoid cells communicating with the 

 tympanum, g. Processus cochleariformis, bounding i. the canal for the tensor lympani muscle : the an- 

 terior pyramid is broken off, if it existed, h. Commencement of the Bustacnian tube, j. Jugular-fossa, 

 immediately below the tympanum, k, k. Carotid canal, with the artery in outline, to show its course in 

 relation to the tympanum and Kustachian tube. I. Portio dura of the seventh pair of nerves, as it would 

 be seen in the terminal part of the aqueduct of Fallopius. m. Chorda tympani, leaving the portio dura, 

 and entering a short canal, which opens in the tympanum, at the base of the pyramid, n. Grooves for 

 the tympanic plexus.] 



