96 PHYSIOLOGY OP FARM ANIMALS [CH. 



an impulse started in any one of them is different (though it 

 may be only very shghtly different) from an impulse started 

 in any of the others. 



The sensory end organs, the more important of which are 

 described in the next chapter, may here be tabulated: 



(1) End organs in the skin setting up impulses either on the 

 application of pressure, or on being warmed or cooled. 



(2) End organs in muscles, tendons, and ligaments, starting 

 impulses when these are extended or contracted. 



(3) The eye. 



(4) The ear. 



(5) The semicircular canals of the ear, which serve the pur- 

 pose of balancing organs, owing to their containing water capable 

 of running in three directions in a sj^stem of tubes, according to 

 the position in which the head is held. 



(6) The nasal organ (organ of smell). 



(7) The mouth (taste organs). 



(8) End organs which start imj^ulses when neighbouring 

 tissue is destroyed, the impulses conveying the sensation of pain 

 which must be regarded as a warning of the existence of danger. 



(9) Special end organs starting sensations peculiar to them- 

 selves (e.g. in the rectum, when full of faeces and calling for the 

 act of def aecation) . 



The impulses which enter the central nervous system pass up 

 and down through many neurons, the direction that they take 

 being determined partly by congenital tendency and partly as a 

 result of previous impulses which acted on the nerve cells through 

 which they passed ; that is to say, the direction of a nerve impulse 

 may be affected both by heredity and by education or training, 

 and a nerve reflex may be established in either of these ways. 



An impulse can pass from one neuron to the next in one 

 direction only. This is because the synapse or region where two 

 neurons join has a valve-like action. 



The efferent nerves by which the impulses leave the central 

 nervous system are of two kinds. Firstly, there are axons which 

 innervate voluntary or striated muscle (the movement of which 

 is under the control of the will). Such nerve fibres pass directly 

 to the muscle without interruption, and the nerve cell from 

 which the axons arise is situated within the central nervous 

 system. Secondly, there are efferent nerve channels which. 



