xti.] THE DIRECTION OF DEVELOPMENT. 139 



some form of vital energy. If so, there is this remarkable 

 parallelism : that the function of the nutritive system is to 

 transform matter, and of the muscular system to transform 

 energy; the function of the vessels is to convey matter, 

 and that of the nerves to transmit energy. 



I must, however, state, in order to avoid a possible mis- Each 

 conception, that the energy put forth by any muscle is not ™suaily 

 brought to it by its nerves, but transformed into motor transforms 

 energy by the muscle itself. There may possibly be excep- itself, 

 tions to this in the constantly-working muscles of the 

 heart and lungs ; but it is certainly true of the voluntary 

 muscles, as is shown by the familiar fact, that fatigue is 

 felt most in those muscles that have been most overworked. 

 The relation of the nerves to the muscles may be compared 

 with the relation of the mind of the master who gives 

 orders, to the muscular force of the workmen who execute 

 the orders. 



It ought also to be observed, that in the well-known 

 experiment of making the cut-off legs of a frog kick by 

 means of an electric stimulus, the electricity is a mere 

 stimulus, an excitor to action, but does not supply the 

 motive power ; the motive power is obtained by the trans- 

 formation of vital energy, as in the living frog. The proof 

 of this is, that the cut-off legs become fatigued with repeated 

 stimulation, and respond less forcibly, but recover their 

 power when laid by for a time, which would not be the 

 case if the motive power were supplied by the transforma- 

 tion of the electricity. Of course the same applies to the 

 muscle and nerve in Helmholtz's experiment. 



Where there are no nerves, as in the hydrozoa, the Hydrozoa 

 muscular tissue has itself the power of transmitting sti- ^oa. ^^°' 

 muli : thus, if one of the tentacles of a hydrozoon be 

 irritated, not only that tentacle but the others will contract. 

 In this class, however, the propagation of the stimulus is 

 relatively slow, and, consequently, if one tentacle is irritated 

 the rest will contract but slowly. The hryozoa are a mol- 

 luscan or moUuscoidan class, adapted to the same kind of 

 life as the hydrozoa, but with a totally different anatomy, 

 and a discernible, though rudimentary, nervous system; 



