174 THE MECHANISM OF LIFE 



have been subjected to operations which destroy certain parts 

 of the central nervous system. But in most animals there is 

 some indetermination and spontaneity of behaviour, and the 

 more highly organised is the central nervous system, the greater 

 seems to be the degree of indetermination that is exhibited. 



The Nature of Sensation. 



We are now about to cross over from the " objective " to the 

 " subjective," and the reader is asked to attach no other mean- 

 ings than those that we specify to the technical terms that we 

 shall employ. By " sensation," then, we mean only the train 

 of physical events which occur when a receptor organ is affected 

 or stimulated by some change in the external medium, and when, 

 in consequence of this stimulation, nervous impulses are pro- 

 pagated along the afferent nerve of the receptor organ into a 

 cerebral or spinal centre. Let us examine this train of events 

 in one case ; the reader can easily make similar analyses for others. 



The Sensation of Hearing. A gramophone begins to play- 

 that is, a disc in which there is a spiral groove of uneven depth 

 revolves, raises and lowers a needle, which presses unequally 

 upon a diaphragm and throws the latter into rapid vibrations. 

 These vibrations set up waves of condensation and rarefaction 

 in the air, and the latter impinge on the drum of the ear, setting 

 this into vibrations which resemble those of the diaphragm of 

 the gramophone. The vibratory movements of the tympanic 

 membrane are communicated to the three little bones of the 

 middle ear, which communicate them to a liquid (the peril ymph) 

 filling the bony labyrinth of the internal ear. Then the vibratory 

 movements of perilymph are transmitted to the endolymph 

 contained in the membranous labyrinth, and the latter vibrations 

 are communicated to the hair cells or other terminations of the 

 cochlear nerve. Nervous impulses are now set up in the fibres 

 of the nerve of hearing, and these are propagated to the auditory 

 centres of the lower brain. 



There is therefore a chain of dependent events : the mechanism 

 of the gramophone, the atmospheric vibrations, the movements 

 of tympanic membrane and auditory ossicles, the vibrations of 

 perilymph, endolymph, and nerve terminations, the nervous 

 impulses, and, finally, the changes produced by the latter in the 

 cells of the auditory centre. Throughout all this series of 

 changes, or motions, there is physical determinism. 



