138 



HABIT AND INTELLIGENCE. 



[chap. 



of nutri- 

 tive and 

 nervo- 

 muscular 

 systems. 



Nerves are 

 to muscles 

 what ves- 

 sels are to 

 nutritive 

 system. 



Intemun- 

 cial func- 

 tion of 

 nerves. 



Helm- 

 holtz's ex- 

 periment. 



Velocity 

 of nervous 

 stimulus 

 measured. 



Nerves, 

 probably, 

 transmit 

 energy. 



distinct apparatus for the transformation of energy. This 

 is the nervo-niuscular system, which constitutes the organs 

 of animal life, though none of these organs are to be dis- 

 tinguished in the very lowest animals. 



A muscular system is found in some animals — among 

 the hyclrozoa, for instance — which have no rudiment of 

 nerves ; but the converse is not the case : there are never 

 any nerves except where there are distinct muscles, l^erves 

 are to the muscular system what sap- or blood-vessels are 

 to the nutritive system. The analogy is not close, but it is 

 real. The function of the vascular or circulatory system is 

 to increase the efficiency of the nutritive system by con- 

 veying nutritive material from one part to another ; and 

 the primary function of the nerves is to increase the rapidity 

 and precision of comhined action among the muscles, and 

 thereby to increase the efficiency of the muscular system 

 by transmitting stimuli from one part to another. It was 

 said long ago by John Hunter, that the primary function of 

 the nerves was internuncial ; and Helmholtz has shown by 

 a most ingenious experiment that some influence — in some 

 respects like electricity, but not identical with it — runs 

 along a stimulated nerve with a great but measurable 

 velocity. On stimulating by electricity a piece of muscle 

 taken from a newly-killed frog, the muscle contracted. On 

 applying the electric stimulus, not to the muscle itself, but 

 to the loose end of a nerve in connexion with the muscle, 

 the muscle again contracted ; but between the application 

 of the stimulus to the one end of the nerve and the con- 

 traction of the muscle at the other end a time elapsed 

 which, though quite too short to be perceptible to the eye, 

 was measured by the delicate apparatus employed by 

 Helmholtz, and enabled him to estimate the rapidity of 

 the transmission of impulses along the motor nerves of the 

 frog at from 81 to 126 feet per second.^ 



But what is it that is transmitted along the nervous 

 fibres ? I think it is scarcely possible to doubt that it is 



1 Carpenter's Human Physiology, pp. 473, 474. It would be nearly 

 impossible to explain the apparatus by which the time is measured, without 

 a diagram. Of com'se it is electrical. 



