HEARING. 803 



difficulties it has been suggested that the basilar membrane, which is present 

 in birds as well as in mammals, and which, being tense radially but loose 

 longitudinally, i. e., along the spiral of the cochlea, may be considered as 

 consisting of a number of parallel radial strings, each capable of inde- 

 pendent vibrations, is the sought-for organ of analysis ; for it may be shown 

 mathematically that a membrane so stretched in one direction only is capa- 

 ble of vibrating in such a manner. And the radial dimensions of the 

 basilar membrane give a much greater range of difference than do the rods 

 of Corti, diminishing in man downward from 0.495 mm. at the top to 

 0.04125 mm. near the bottom of the spiral, whereas the difference in length 

 of the latter is simply that between 0.048 and 0.085 mm. for the inner and 

 between 0.019 and 0.085 mm. for the outer fibres. According to this view, 

 a particular simple vibration reaching the scala tympani of the cochlea 

 throws into sympathetic vibrations a small portion of the basilar membrane, 

 the vibrations of which in turn so affect the structures overlying it that sen- 

 sory impulses are generated. The sensory impulses reaching the brain give 

 rise to a corresponding sensation of a particular tone. 



The remarkable reticular membrane which has such peculiar relations 

 with the hair-cells, and through them with the basilar membrane, must, one 

 might imagine, have some special function ; but it is impossible at present 

 to assign to it any satisfactory duty. The structural arrangements seem, if 

 anything, to indicate, that when a segment of the basilar membrane is 

 thrown into vibrations, the overlying hair-cells, reticular membrane, and 

 rods of Corti vibrate en masse with it. But this renders the whole matter 

 still more difficult. Indeed the whole subject is in the highest degree ob- 

 scure, and the most we can say is that the organ of Corti as a whole seems 

 to be in some way connected with the appreciation of tones, but that at 

 present it is very hazardous to attempt to explain how it acts, or to assign 

 particular functions to particular parts. The distinction between the inner 

 and outer hair-cells seems to be very parallel to that between the rods and 

 cones of the retina ; but even this analogy may be a fallacious one. 



It has been observed that among the auditory hairs of the Crustacea, 

 some will vibrate to particular notes ; but the auditory hairs of the mammal 

 are far too much of the same length to permit the supposition that they can 

 act as organs of analysis. 



If the organ of Corti is the means by which we appreciate tones, it is 

 evident that by it also we must be able to estimate loudness, for the quality 

 of a musical sound is dependent on the relative intensity, as well as on the 

 nature, of the overtones. And since noise is at best but confused music, the 

 cochlea must be a means of appreciating noises as well as sounds. But this 

 would leave nothing whatever for the rest of the labyrinth to do in respect 

 to the appreciation of sound save so far as the difference in structure between 

 the hair-cells of Corti, with their short, thick rods, and the hair-bearing 

 structures in the maculae and cristse, with their thin, delicate hairs, may 

 possibly indicate a difference of function, the latter being more susceptible 

 to the irregular vibrations of noises. That the vestibule and semicircular 

 canals are, however, concerned in hearing is shown by its being the only 

 auditory organ in the ichthyopsida, unless we suppose that in the higher 

 vertebrates its function has been wholly transferred to the cochlea. That 

 the semicircular canals may have duties apart from hearing we shall show 

 later on. 



717. Concerning the function of the other parts of the internal ear we 

 know very little. The otoliths have been supposed to intensify the vibrations 

 of the endolymph ; but since apparently they are lodged in a quantity of 

 mucus it is probable that they really act as dampers. A similar damping 



