MAMMALIA 303 



membrane. An elastic membranous projection from the lamina 

 spiralis covers them and ends freely over them : this is the 

 membrana tectoria. The waves coming to the perilymph by 

 the fenestra ovalis are distributed as waves of condensation 

 and rarefaction, quadrupled in velocity and increased some 

 thirty times in force in that fluid, and are passed into similar 

 waves of the endolymph. The function of the fenestra rotunda 

 is to act as a disperser of the waves. 



The auditory organ is divided, therefore, into a vestibular 

 portion supplied by the vestibular nerve, and a cochlear 

 portion supplied by the cochlear nerve. The former is the 

 primitive part of the organ and is relegated to equilibration. 

 The semicircular canals are directed mutually at right angles. 

 They are each furnished with an ampulla in which sensory 

 cells are developed with stiff hairs which are affected by changes 

 in the flow of the endolymph, and these are associated with 

 similar areas in the utricle and vestibule. 



The cochlea is the part of the auditory organ associated 

 with hearing, and especially with the fine distinction of 

 sound waves ; and it is evident that the hair cells of 

 the organ of Corti, are the sensory elements developed for 

 'the purpose. They are very numerous along the length of 

 the organ and, as has been said, they are supported upon an 

 elastic membrane the fibres of which radiate from the lamina 

 spiralis to the opposite wall. The cochlea differs from the 

 rest of the labyrinth in that it is firmly attached to the wall 

 of the chamber on each side. The free borders of the vesicle 

 are placed at about an angle of 60 with one another. Both 

 are taut, and the lower one, bearing the organ of Corti, is still 

 further stiffened by the lamina spiralis and by having a thicker 

 wall. The sound waves of condensation and rarefaction are 

 sent down the scala vestibuli and up the scala tympani and 

 will affect both membranes, the one after the other. This 

 means that the wave is reconverted into a wave of vibration 

 which is communicated to the endolymph, and so to the hairs 

 of the sensory cells. If the sound is to be analysed according 

 to the waves it produces, it follows that the cells either have 

 a selective power or are affected according to the position 

 of nodes and internodes. A wave received from the scala 



