304 ESSENTIALS OF ZOOLOGY 



vestibuli and communicated to the endolymph might be sup- 

 posed to affect the membrana tectoria, and by depressing it 

 at a given spot, or spots, produce the result. A wave from the 

 scala tympani might be supposed to have a similar but direct 

 effect by raising a particular region of the organ against the 

 membrana tectoria. As the length of the fibres forming the 

 basis of the organ of Corti gradually increases from the base 

 to the apex, it has been assumed that as these are fixed at 

 both ends they will vibrate only in relationship to waves of a 

 particular pitch, and that they are therefore the mechanical 

 means of communicating to particular cells a given wave. 

 In other words, the drum and the membrane of the fenestra 

 ovalis are aperiodic and communicate all kinds of waves 

 indifferently ; the basilar membrane is periodic in gradually 

 changing degree from one end to the other. This is the basis 

 of Helmholtz's theory of hearing. Wrightson's is founded 

 on the action of the nodes and internodes produced by the 

 resonance of the basilar membrane as a whole. It is possible 

 that the membrana tectoria has the effect of directing the 

 wave of the endolymph across the fibres, and primarily and 

 perhaps even in its highest state of development, in the human 

 ear, the endolymph is the means of communicating the sound* 

 waves to the sensory cells. It is certain, at all events, that the 

 cochlea is the organ of hearing, that injury to it produces 

 deafness, and that the organ of Corti elaborates for the nervous 

 system the sound waves. 



The mammalian nervous system is in a high state of 

 development. But it does not differ from that of the other 

 Craniates except in quantity and quality. Broadly, it may be 

 resolved into somatic receptive constituents, including the 

 distant receptors the sense organs, the nose, eye, and ear 

 the cutaneous nerves and centres associated with these in the 

 central nervous system, and the paths which bring them into 

 relationship ;*" visceral receptive, which bring to the central 

 nervous system impulses from the viscera by VII, IX, X ; 

 somatic effective, the motor nerves of the brain and spinal 

 cord ; visceral effective, afferent fibres of V, VII, IX, X, XI, 

 and visceral motor fibres of the spinal cord distributed by 

 the sympathetic system. 



