696 THE SENSE OF HEARING. 



by which the eye is accommodated to distance, nor to the 

 perception of the particular part of the nerve affected ; but 

 just as one person sees distinctly only in a bright light, 

 and another only in a moderate light, so in different 

 individuals the sense of hearing is more perfect for sounds 

 of different pitch; and just as 'a person whose vision for 

 the forms of objects, etc., is acute, nevertheless dis- 

 tinguishes colours with difficulty, and has no perception of 

 the harmony and disharmony of colours, so one, whose 

 hearing is good as far as regards the sensibility to feeble 

 sounds, is sometimes deficient in the power of recognising 

 the musical relation of sounds, and in the sense of harmon} T 

 and discord ; while another individual, whose hearing is 

 in other respects imperfect, has these endowments. The 

 causes of these differences are unknown. 



Subjective sounds are the result of a state of irritation or 

 excitement of the auditory nerve produced by other causes 

 than sonorous impulses. A state of excitement of this 

 nerve, however induced, gives rise to the sensation of 

 sound. Hence the ringing and buzzing in the ears heard 

 by persons of irritable and exhausted nervous system, and 

 by patients with cerebral disease, or disease of the audi- 

 tory nerve itself; hence also the noise in the ears heard 

 for some time after a long journey in a rattling noisy 

 vehicle. Hitter found that electricity also excites a sound 

 in the ears. From the above truly subjective sound we 

 must distinguish those dependent, not on a state of the 

 auditory nerve itself merely, but on sonorous vibrations 

 excited in the auditory apparatus. Such are the buzzing 

 sounds attendant on vascular congestion of the head and 

 ear, or on aneurismal dilatation of the vessels. Frequently 

 even the simple pulsatory circulation of the blood in the 

 ear is heard. To the sounds of this class belong also the 

 snapping sound in the ear produced by a voluntary effort, 

 and the buzz or hum heard during the contraction of the 

 palatine muscles in the act of yawning ; during the forcing 



