THE SENSES. 093 



Unequal conducting power of two media through which the same sound 

 is transmitted to the ear, may easily be experienced. If a small bell be 

 sounded in water, while the ears are closed by plugs, and a solid con- 

 ductor be interposed between the water and the ear, two sounds will be 

 heard differing in intensity and tone; one being conveyed to the ear 

 through the medium of the atmosphere, the other through the conduct- 

 ing-rod. 



Subjective Sensations. 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 auditory 

 nerve itself; hence also the noise in the ears heard for some time after a 

 long journey in a rattling, noisy vehicle. Ritter found that electric 

 currents also excite sounds in the ears. From the above truly subjective 

 sound we must distinguish those dependent, not on a state of the audi- 

 tory nerve itself merely, but on sonorous vibrations excited in the audi- 

 tory apparatus. Such are the buzzing sounds attendant on vascular 

 congestion of the head and ear, or on aneurismal dilatation of the ves- 

 sels. Frequently even the simple pulsatory circulation of the blood in 

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

 hum, heard during the contraction of the palatine muscles in the act of 

 yawning, during the forcing of air into the tympanum so as to make 

 tense the membrana tympani, and in the act of blowing the nose, as well 

 as during the forcible depression of the lower jaw. 



Irritation or excitement of the auditory nerve is capable of giving 

 rise to movements in the body, and to sensations in other organs of 

 sense. In both cases it is probable that the laws of reflex action, through 

 the medium of the brain, come into play. An intense and sudden noise 

 excites, in every person, closure of the eyelids, and, in nervous indi- 

 viduals, a start of the whole body or an unpleasant sensation, like that 

 produced by an electric shock, throughout the body, and sometimes a 

 particular feeling in the external ear. Various sounds cause in many 

 people a disagreeable feeling in the teeth, or a sensation of cold tickling 

 through the body, and, in some people, intense sounds are said to make 

 the saliva collect. 



V. Sight. 



Anatomy of the Optical Apparatus. The eyelids consist of two mov- 

 able folds of skin, each of which is kept in shape by a thin plate of 

 yellow elastic tissue. Along their free edges are inserted a number of 

 curved hairs (eyelashes), which, when the lids are half closed, serve 



