3^6 THE FACULTY OF HEARING. 



a particular example. For if we begin without knowing for what 

 we are looking, we might end with failing to comprehend what 

 we see. 



But when we have learnt the true significance of any structural 

 element in a given organism of which it forms a constant 

 constituent, then the discovery of the same element re-appearing 

 in the same form and exhibiting the same relation in the unknown 

 structure which it did in the known structure, enables us to 

 determine the character of the former as being homologous with 

 the latter, and to realise its histological value. Thus from the 

 constant occurrence of a structure consisting of a vesicule on 

 whose inner surface nerve filaments are distributed, and which 

 contains a few free molecules of calcic carbonate floating in its 

 fluid content, we infer the morphological equivalent of a sensory 

 organ whose nerve is specifically excited by the contact and 

 movement of these otoliths. When again in our search for facts 

 which serve to prove identity of origin or close relationship of such 

 elements, we find that in every creature in which this morphological 

 type is observed the conditions of its action are the same and 

 invariable, we are warranted in concluding identity of function. 

 Facts and reasonings based upon such principles of interpretation 

 yield far greater certainity of an auditory apparatus proper, than 

 the presence of any drum-like expansion of skin. The doctrine of 

 homologies gives, so to speak, a pedigree to every anatomical fact 

 thus determined, and it becomes a complete item, be it ever so 

 Wmble, of scientific evidence. But an anatomical fact picked up 

 without a pedigree is an unstamped article, which can only be 

 offered for what it may hereafter prove to be worth. 



The discoverers of the ""tympanal structure,'' (Joh: Miiller and 

 Carl V Siebold) were undoutedly first moved to investigate it by 

 the singularity of its position and external appearance. But their 

 attention was mainly drawn to the remarkable dovelopment of 

 nerve ganglia and filaments upon the tracheal sac, which they 

 found adhering to the inner surface of the tympanal membrane. 



