FUNCTIONS OF SENSORY ORGANS. IO3 



of the lower end of the ascending frontal convolution in 

 man (Broca's convolution), is associated with aphasia, a 

 condition in which the patient, though able to articulate, 

 and possessing the power of forming adequate concep- 

 tions which he remembers, is unable to word them. In 

 the vast majority of the cases in which this happens, the 

 lesion is on the left side of the brain. 



COMMON AND SPECIAL SENSATION. 

 (Functions of Sensory end-organs^) 



Sensations and Perceptions in general. By the word 

 Sensation is meant in physiology the felt effect of an 

 excitation either of a sensory end-organ, or of a sensory 

 nerve. Sensations which originate from end-organs are 

 divided into those of common sensation, vision, hearing, 

 taste, &c., according to the end-organ affected. Sensations 

 which spring from direct irritation of sensory nerves, are 

 usually painful. Very feeble excitation of an end-organ 

 is not felt. As regards those excitations which from their 

 nature admit of measurement, it was found by E. H. 

 Weber that the degree of intensity which must be attained 

 by any excitation in order to be felt the limen (Reiz- 

 schwelle) is constant in the same individual. As regards all 

 excitations of which the intensity exceeds the limen, it is 

 found that the " sensible increment " (i.e. the smallest 

 additional excitation that can be felt), is proportional to 

 the previous excitation. Hence, the sensation produced 

 by any given excitation varies inversely as the intensity of 

 the previous excitation. The ratio of the " sensible incre- 

 ment " to the " previous excitation " differs in different 

 cases. Thus as regards light, it is as I : 100, or there- 

 abouts ; as regards sound as i : 3, and so on. 



