300 THE NERVOUS SYSTEM 



Stimulation over a wide area on the occipital lobe in the 

 monkey causes movement of the eyes to the opposite side. 

 It is probable that the visual area is more restricted in 

 man than in the monkey. The view is also held that a 

 small (visuo-sensory) area devoted to the reception of visual 

 impressions is surrounded by a wider (visuo-psychic) area 

 concerned in the higher psychical processes associated with 

 vision. 



3.— HEARING 

 STRUCTURE OF THE EAR 



The External Ear 



This consists of the pinna and external auditory meatus. 

 The pinna in lower animals by its tubular shape serves the 

 purpose of collecting sound-waves, and by its mobility 

 enables the animal to detect the direction from which the 

 sound is coming. 



The meatus is a slightly curved passage, about one inch 

 in length, directed into the skull forwards, inwards and 

 shghtly upwards. Internally it is closed by the tympanum 

 or membrana tympani. The walls of the meatus are hned 

 with skin, which is continued as a thin layer over the 

 tympanum. The meatus by its depth and curvature serves 

 to protect the membrane from damage and cold, and the 

 cerumen secreted by the glands keeps it moist and protects 

 it from insects and bacteria. 



The middle ear is a cavity in the petrous bone. The 

 membrana tympani separates it from the external ear, and 

 two small foramina, the fenestra ovalis oxi^ fenestra rotunda, 

 covered with membranes, separate it from the internal ear. 

 By the Eustachian tube, directed downwards and backwards, 

 it is in communication with the cavity of the pharynx. 

 The opening of the Eustachian tube is normally closed 

 except during the act of swallowing, when it opens. In 

 this way the pressure on the two sides of the membrane is 

 kept equal. When the tube is blocked by disease the air 



