PRINCIPLES OF PHYSIOLOGY 109 



sensations are located in different spots. Then, by comparing the dis- 

 tribution of the types of sense organs and the types of sensations, it has 

 been possible to identify the sense organ for each stimulus. In lower 

 animals the sensory organs are less differentiated and the identification 

 of a particular nerve ending with a given sensitivity is usually mipossible. 



The sense cells at the base of the bristles of insects are clearly 

 mechanoreceptors, and indeed it has been possible to record impulses in 

 the efferent nerves when the bristle is moved. 



The mammalian ear is a remarkably complex organ which contains 

 the senses of hearing and equilibrium. It can detect the direction of the 

 force of gravity or of linear acceleration, because it contains otoliths, 

 masses of calcium carbonate, attached to slender processes of cells in 

 such a way that the weight of the otolith will pull or push on these 

 processes. Motion of the head about any of its axes is detected by the 

 motion of the fluid in the semicircular canals, which moves clumps of 

 hair-like processes attached to sense cells in the walls of the canals. The 

 detection and analysis of sound waves involves the conversion of the 

 sound waves to mechanical vibrations of the ear drum and middle ear 

 bones, and then to waves of motion in the liquid filling the cochlea of 

 the inner ear. The cochlea contains many sense cells with fibers of differ- 

 ing lengths which respond to sounds of different frequencies. The ear 

 is basically a mechanoreceptor responding to the mechanical displace- 

 ment of sense cells, or their fibers or hairs, produced by sound waves or 

 by changes in position. 



Organs of balance, called otocysts or statocysts, are found in most 

 phyla of animals, even in coelenterates. These are usually hollow spheres 

 of sense cells, in the middle of which is a statolith, a particle of sand or 

 calcium carbonate, pressed by gravity against certain sense cells. As the 

 animal's body changes position, the statolith is pressed against different 

 sense cells and the animal is then stimulated to regain its orientation 

 with respect to gravity. 



Many arthropods, especially insects, have sense organs which respond 

 to sound waves; these organs consist of a fine membrane stretched in 

 such a way that it is free to respond to the vibrations of sound waves. 

 The nerve from the sound-sensitive organ of the locust has been tapped 

 and recordings of the nerve impulses from it show that it can respond 

 to sound waves of between 500 and 10,000 cycles per second. The human 

 ear responds to frequencies between 20 and 20,000 c.p.s., dogs are sensi- 

 tive to sounds as high as 40,000 c.p.s., and the sensitivity of the bat ear 

 extends to high-pitched 80,000 c.p.s. noises. 



Certain insects have balance organs which have evolved from the 

 second pair of wings. These club-shaped structures, called halteres, beat 

 up and down as the wings do, and serve as "gyroscopes." When the 

 direction of the beat is changed, sense organs in the base of the haltere 

 are stimulated and give off nerve impulses. This has been shown by 

 recording the nerve impulses passing through the nerves from the 



halteres. 



Photoreceptors. Almost all animals are sensitive to light and 

 respond to variations in light intensity. Even protozoa which have no 



