But how is this remarkable propulsive machine made to perform 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 disconnected 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 contract 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 his 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 section is destroyed; and, on the other hand, if, the cords remaining entire, the brain mass be destroyed, the same voluntary mobility is equally lost.  Whence the inevitable conclusion is, that the power of originating these motions resides in the brain and is propagated along the nervous cords.

In the higher animals the phenomena which attend this transmission have been investigated, and the exertion of the peculiar energy which resides in the nerves has been found to be accompanied by a disturbance of the electrical state of their molecules.