92 LAY SERMONS, ESSAYS, AND REVIEWS. [vi. 



that it is a most beautifully constructed organ of locomotion, by 

 means of which the animal can swiftly propel itself either back 

 wards or forwards. 



But how is this remarkable propulsive machine made to per 

 form its functions ? If I were suddenly to kill one of these 

 animals and to take out all the soft parts, I should find the 

 shell to be perfectly inert, to have no more power of moving 

 itself than is possessed by the machinery of a mill when dis 

 connected from its steam-engine or water-wheel. But if I were 

 to open it, and take out the viscera only, leaving the white flesh, 

 I should perceive that the lobster could bend and extend its tail 

 as well as before. If I were to cut off the tail, I should cease to 

 find any spontaneous motion in it ; but on pinching any portion 

 of the flesh, I should observe that it underwent a very curious 

 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 con 

 tract at one time and not at another ? Why does one whole 

 group of muscles contract when the lobster wishes to extend 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 ascertainment 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 





