16 TRANSACTIONS LIVEBPOOL BIOLOGICAL SOCIETY. 



Physiologists. Although a most fruitful one it is certainly 

 dangerous and it has often happened that the Physiologist, 

 relying upon it, has leaped into the abyss. 



I need hardly remind you of the innumerable instances 

 in which a structure has been inferred from the observation 

 of a function. Thus, a tissue is ascertained to swiftly 

 alter its form in response to a stimulus and we infer that 

 in this part will be found muscular fibres. The judgment 

 is now urged to deduce the structure from the function. 

 In a vast number of cases the inferred structure is 

 subsequently found to be present, but it may not be found, 

 and then the only safe course is to acknowledge that an 

 inference has been made and to wait. In Biology, more 

 than in any other Science, we must possess our souls 

 in patience. 



One case, in which such an inference wrongly made has 

 bothered the Physiology of one particular organ for years 

 is that of the iris. The iris of the eye is an organ with 

 an aperture, the pupil, which increases and diminishes in 

 size. It decreases when light falls on the eye, when we 

 look at a near object etc. Its size is constantly swaying 

 in response to nerve impulses which reach it. By what 

 mechanism is this movement effected ? The inference is 

 that it is done by muscle. Anatomical examination shews 

 that the iris contains fibres disposed in circular rings 

 round the pupil ; these are seen by microscopic examina- 

 tion to be muscular fibres ; the contraction of these 

 will therefore diminish the size of the pupil, and when 

 the contraction decreases the pupil will expand. Now, no 

 other muscular fibres were definitely found in the iris of 

 mammals, so the dilatation of the pupil was considered to 

 be in all cases due to the cessation of the contraction in the 

 muscular ring or sphincter pupillae, and the iris was said 

 to expand passively. There were however many physio- 



