412 SPECIAL PHYSIOLOGY. 



ferred by the sensorium to the exterior; whereas those conducted 

 through the cranial bones appear to proceed from the head itself. 



As in the other organs of sense, here also there are after-sensa- 

 tions and subjective sensations. For example, the noises in the ear 

 which remain after certain sounds, when these have excited the audi- 

 tory nerve for a lengthened period, are analogous with the after-sen- 

 sations of touch, taste, and smell. The noises in the head and ear, 

 such as musical phrases, and the singing, buzzing, ticking, and hum- 

 ming sounds heard by persons suffering from disease of the brain or 

 auditory nerve, are examples of subjective sensations. Delusions of 

 the auditory sense are not uncommon, especially amongst excitable 

 persons. 



The various uses of the sense of hearing are sufficiently obvious. 



The Organs and Sense of Hearing in Animals. 



The organ of Hearing, in Mammalia generally, is, in all particulars, con- 

 structed on the same plan as that of Man. The external ear consists of a 

 cartilaginous pinna, and of a partly cartilaginous and partly osseous meatus. 

 The former is often large, and provided with numerous powerful muscles ; in 

 the beaver, otter, and other diving animals it is but slightly developed ; in 

 the seals, the mole, the Cetacea, the armadillo, and the ornithorhynchus, it is 

 absent. The external meatus is sometimes provided with a fold of the auri- 

 cle, by means of which it can be closed, as the ear-flap of the elephant and 

 the valve-like antitragus of the water-shrew. The general development of the 

 external ear appears to be proportional to the acuteness of hearing. It at- 

 tains its highest development in the bats, in which its forms are often remark- 

 able. The entrance to the tympanum is usually surrounded by a separate 

 bone, the os tympanicum ; but in the monkey, as in man, this is blended with 

 the petrous part of the temporal bone. The cavity of the tympanum fre- 

 quently extends widely into the adjacent osseous structures. The tympanic 

 ossicles are three in number ; they present great variety of shape, although 

 they always resemble those of man. In some marsupials, the stapes is sim- 

 ply style-shaped with a broad base, or is divided into two short crura only, 

 a condition which somewhat approaches the representative bone, the colu- 

 mella, in birds. In the Cetacea the walls of the tympanum are very thick, 

 and when detached from the rest of the petrous bone, form the remarkable so- 

 called ear-bone of those animals ; in them the Eustachian tube is membran- 

 ous. In different species of Mammalia, the cochlea forms from 1^ to 5 turns. 

 The labyrinth is completely embedded in the petrous portion of the temporal 

 bone ; in the mole, however, the vertical semicircular canals project into the 

 cavity of the skull. In some Mammalia, no otoliths, nor even otoconia are 

 present. 



In Birds the external meatus is present ; there is, however, no pinna, but 

 only a radiated arrangement of the feathers, or a few naps of skin around the 

 aperture ; these are very large in the owl tribe. Otherwise, the organ of hear- 

 ing is highly developed. The membrana tympani is oval, and projects ex- 

 ternally, instead of sinking inwards as in Mammalia. The tympanum 

 communicates with the mouth by a very large .Eustachian tube, and also, by 

 different foramina, with air cells in the cranial bones ; these cells are very capa- 

 cious, and generally even extend across the middle line, so that the two tym- 

 panic cavities are connected with each other. There is only one tympanic 

 bone, a modified stapes, here named the columella, which is joined by two or 

 three cartilaginous processes, representing the other bones, to the membrana 

 tympani, and rests, by its other extremity, upon the foramen ovale. The 

 tensor tympani is the only muscle present. The three semicircular canals are 

 of large size, in proportion to the cranium ; the vestibule is small. The coch- 

 lea is not convoluted, but forms a slightly curved, conical canal ; in its interior 



