166 VETERINARY PHYSIOLOGY 



to a particular part of the field. The two centres acting 

 together give the whole field of vision. Since the blind spot 

 is not repi'esented in the centre, it is not perceived in the 

 field of vision. 



The centre is said to rectify the inverted image formed 

 on the retina, but this simply means that, as a result of 

 experience, Ave have learned that changes in, say, the lower 

 part of the retin*, and in the corresponding parts of 

 the visual centres, are produced by light from above 

 the head. 



Since the retinal changes vary only with the degree of 

 illumination, i.e. the amplitude of vibration of the ethereal 

 waves, and with the rate of these waves, and since the part 

 of the retina acted on is determined by the direction of the 

 rays, we have the means of getting a flat picture only of what 

 we look at, but no special arrangements for having diiferent 

 sensations according to the distance of an object or according 

 to whether it is flat or in relief. The means of determining 

 the size, distance, and form of objects by the visual mechan- 

 ism is very limited (p. 138). 



III. FOR VIBRATION OF AIR. 

 SENSE OF HEARING. 



1. General Considerations. 



The vibratory changes of pressure in the air, known to 

 physicists as sound waves, stimulate special receptor struc- 

 tures placed in the ear of higher animals. Even simple 

 organisms, devoid of any special organ of hearing, may be 

 affected by vibratory changes, and in fish it is diflicult to be 

 certain how far such vibrations produce their effect through 

 the ear or through the body generally. But in higher 

 vertebrates it is chiefly through the ear that they act. In it 

 there is a special arrangement by which the vibrations of the 

 air are converted into vibrations of a fluid in a sac situated 

 in the side of the head. In this a series of ciliated cells is 

 situated, and round these are the dendritic terminations of 

 the auditory nerve fibres. 



