ORGAN OF HEARING. 813 



its proportionate size, very accurately resembles the auditory organ of 

 the lower Vertebrata. The semicircular canals exhibit nearly the same 

 arrangement, and in like manner communicate with the vestibule by five 

 orifices. The vestibule itself is small, and no longer contains any chalky 

 concretions : it communicates on the one hand with the cavity of the 

 tympanum, by means of the foramen ovale ; and on the other sends off 

 a canal (scala} to form the cochlea, an organ which in the Mammifer 

 assumes its full development and perfection. 



(2390.) In Reptiles and Birds, as the reader will remember, the cochlea 

 was a simple canal bent upon itself (fig. 367, e), one end of which (scala 

 vestibuli} opened into the vestibule, while the other (scala tympani) 

 terminated at the tympanic cavity, from which it was separated by the 

 membrane of the fenestra rotunda ; but in the Mammalia the two scalae 

 of the cochlea are considerably elongated, and wind in a spiral direction 

 around a central axis (modiolus), so as very accurately to resemble the 

 whorls in the shell of a snail, whence the name of the organ is derived * . 



(2391.) It is in the increased complexity of the cochlea, therefore, 

 that the chief character of the labyrinth of the Mammal consists. But 

 in the tympanic cavity the differences between the Mammiferous ear 

 and that of the Bird are still more striking and decided. 



(2392.) The cavity of the tympanum in the class before us is very 

 extensive, and not unfrequently its extent is considerably enlarged by 

 the addition of capacious mastoid cells. By means of the Eustachian 

 tube it communicates freely with the throat. Upon its inner wall it 

 offers the fenestra ovalis and the fenestra rotunda, closed by their 

 respective membranes ; and externally is the membrana tympani, the 

 vibrations of which are to be conveyed to the labyrinth. 



(2393.) In Reptiles and Birds the communication between the drum 

 of the ear and the membrane of the fenestra ovalis was effected by the 

 interposition of a single ossicle, called the " columnella ;" but in Mammals 

 a chain of four ossicles, named respectively the malleus, the incus, the 

 os orUculare, and the stapes, intervenes between the labyrinth and the 

 membrana tympani : these ossicles, both in their disposition and con- 

 nexions, are precisely similar to those of Man, and, moreover, are acted 

 upon by little muscles in every respect comparable to those of the 

 human subject. 



(2394.) However remote the structure of the tympanic chain of 

 ossicles in the Mammal may appear to be from that of the simple 

 columnella of the Bird, it is interesting to see how gradually the trans- 

 ition is effected from one class to another even in this particular of their 

 economy ; for in the Ornithorhynchus, the Echidna, and the Kangaroo, 



* In Man, and by far the greater number of Mammals, the scalae of the cochlea 

 make two turns and a half around the modiolus ; but in a few Eodent quadrupeds, 

 as for example in the Guinea Pig, the Cavy, and the Porcupine, there are as many 

 as three turns and a half. 



