2l8 



ELEMENTARY BIOLOGY 



253. Reflexes and tropisms. Reflex actions of animals differ 

 from the tropisms whicii we studied in the young plants, both 

 in the greater speed with which they are executed and in the 

 kind of structure which brings them about. The reflexes all 

 depend upon certain connections of nerves, muscles, and special 

 perceiving organs, such as the eyes, ears, tongue, etc. To 

 understand the mechanism of the reflex we must therefore 

 know something about these three kinds of organs. 



254. The muscle. 

 C^^^ We all know in a gen- 

 eral way that the muscle 

 is the '" thing that 

 makes us move." We 

 also know the appear- 

 ance of a muscle from 

 having handled and 

 eaten the flesh of 

 animals. 



When thousands of 

 millions of such cells 

 contract at the same 

 time, we can see that 

 the whole mass will be 

 considerably shortened. An ordinary muscle of the body, such 

 as draws up the forearm or one of the fingers, is essentially a 

 bundle of several masses of muscle cells, together with connec- 

 tive tissue, blood vessels, and nerve connections (Fig. 82). 



255. Kinds of muscle. The muscles that are most familiar 

 to us are the skeletal muscles (those attached to bones of the 

 skeleton) of such animals as we use for food — chicken, lamb, 

 ox, etc. W'e have already learned that there are other mus- 

 cles, however, such as the muscles of the heart (p. 186) and 

 of the diaphragm (p. 149). The muscles connected with the 

 skin manifest themselves to us in the facial expressions of 

 those we see about us, in the movement of the ears (of which 



Fig. 82. Contraction of a muscle 



The movement of an organ, as the forearm, is 

 brought about by the contraction of a muscle. The 

 mass of muscle cells becomes shorter and thicker, the 

 parts to which its ends are attached being brought 

 closer together 



