584 HANDBOOK OF PHYSIOLOGY. 



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

 long journey in a rattling noisy vehicle. Eitter 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 appa- 

 ratus. 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 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 for- 

 cible 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 individ- 

 uals, a start of the whole body or an unpleasant sensation, like that pro- 

 duced 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 

 movable 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 to 

 protect the eye from dust and other foreign bodies: their tactile sensibil- 

 ity is also very delicate. 



On the inner surface of the elastic tissue are disposed a number of 

 small racemose glands (Meibomian), whose ducts open near the free edge 

 of the lid. 



The orbital surface of each lid is lined by a delicate, highly sensitive 

 mucous membrane (conjunctiva), which is continuous with the skin at 

 the free edge of each lid, and after lining the inner surface of the eyelid^ 

 is reflected on to the eyeball, being somewhat loosely adherent to the* 

 sclerotic coat. The epithelial layer is continued over the cornea at its 

 anterior epithelium. At the inner edge of the. eye the conjunctiva be- 



