1 82 ANIMALS 



bound together by connective tissue, and the bundles are bound 

 in the same way to form the muscle as a whole. 



430. When a muscle contracts it becomes shorter but pro- 

 portionally thicker, so that its volume is not changed. At 

 the time of contraction it also undergoes electrical and chemical 

 changes, and heat is evolved. These subjects are discussed 

 elsewhere. 



431. The cause of a muscular contraction is in every case a 

 stimulus. Chemical stimuli, like salts and acids, applied to the 

 muscle, will cause a contraction. The electric current will do 

 the same. But the normal stimulus for the body musculature 

 of the higher animals is a nerve impulse originating in some 

 other part of the body. The origin of this impulse is in every 

 case to be traced to some peripheral sense organs. In some 

 cases the impulse may seem to arise in the central nervous 

 system, but a careful analysis will show that even in these 

 cases the impulse can be traced backward to some sense organ. 



432. The glands, luminescent organs and electrical organs 

 are discussed elsewhere. 



433. We will next consider the means by which connection 

 is made between the sense organs and the organs of response. 



THE NERVOUS SYSTEM 



.434. In no respect do the highest animals diverge so. greatly 

 from the lowest as in the way they respond to stimuli. This 

 difference is due, not so much to the differences between the 

 sense organs or the organs of response, as to the way in which 

 the two sets of organs are related. In amoeba the organs of 

 sensation and response are identical, and no system of communi- 

 cation is required, but in mammals the organs of communication, 

 the brain and spinal cord, exceed in complexity all other 

 organs of the body combined. 



435. We have seen that the muscle fibres of hydra are parts 



