VII THE STUDY OF ZOOLOGY 213 



change each fibre becoming shorter and thicker. 

 By this act of contraction, as it is termed, the 

 parts to which the ends of the fibre are attached 

 are, of course, approximated ; and according to the 

 relations of their points of attachment to the 

 centres of motions of the different rings, the 

 bending or the extension of the tail results. Close 

 observation of the newly-opened lobster would 

 soon show that all its movements are due to the 

 same cause the shortening and thickening of 

 these fleshy fibres, which are technically called 

 muscles. 



Here, then, is a capital fact. The movements 

 of the lobster are due to muscular contractility. 

 But why does a muscle contract at one time and 

 not at another ? Why does one whole group of 

 muscles contract when the lobster wishes to ex- 

 tend his tail, and another group when he desires 

 to bend it ? What is it originates, directs, and 

 controls the motive power ? 



Experiment, the great instrument for the ascer- 

 tainment of truth in physical science, answers this 

 question for us. In the head of the lobster 

 there lies a small mass of that peculiar tissue 

 which is known as nervous substance. Cords of 

 similar matter connect this brain of the lobster, 

 directly or indirectly, with the muscles. Now, if 

 these communicating cords are cut, the brain 

 remaining entire, the power of exerting what we 

 call voluntary motion in the parts below the sec- 



