490 Dynamic Theory. 



ergy, when current, can only be conjectured. The term current itself 

 is used in a rather vague way in this connection, and is applied indiffer- 

 ently to the movement in all sorts of nerves, as it is to the movement 

 through metallic conductors ; when the fact may be that the movement 

 up one nerve may be by a progression of vibrations, like those of light, 

 while up another it may consist of a succession of pulsations, like those 

 of sound. It may even consist of a rush of the ether itself along the 

 conductor. If it should turn out to be either a succession of pulses or 

 a succession of undulations, we can readily comprehend how it might 

 occur in different tones or wave-lengths, and how, like sound, light and 

 heat, it is retarded in its passage through its conductors ; and how it 

 gets through some of its nervous conductors more rapidly than through 

 others. The nerve current cannot be propagated over a nerve that has 

 been divided by a transverse cut, even when the ends are placed in as 

 close contact as possible, while magnetism will traverse such a break. 

 A divided nerve connected by a piece of wire, will convey a current of 

 electricity but not a nervous current. Moreover, the neurilemma, or en- 

 veloping sheath of the nerve fibres, is a non-conductor of nervous en- 

 ergy and serves to insulate the fibres as against that, while it is a con- 

 ductor of electricity. At the same time, however, nervous energy is in- 

 terchangeable with the other forms of polar and molecular energy. 



The connection between nerves and muscles is extremely intimate, 

 and what is called the power of contractility of the muscles is never 

 brought into exercise in the body except upon a stimulation by a nerve 

 current. The nerves are divided into two general classes. Those which 

 convey stimulations from the organs of sense to the general nervous 

 centers, viz. , the spinal cord and brain, are called afferent or sensory 

 nerves; while those which convey stimulations from these centers to the 

 muscles, causing their contraction, are called efferent or motor nerves. 

 Sometimes the former are called centripetal, and the latter peripheral 

 nerves. 



Assuming that nervous energy is a form of polar energy of which 

 electricity is the type, I shall speak of it as electricity and current, be- 

 cause there are no more convenient terms. To this form of energy, 

 then, is every other form reduced before it can have any effect upon an 

 organism through its natural organs. The change is always from a 

 mode of motion external to the body, to a mode of motion internal. 

 The external motion, called light, agitates the rods and cones of the 

 retina, and is reduced by them to the nervous current of the optic 

 nerve. Those called heat, touch, and pressure, agitate the epithelial 

 and tactile cells, and are reduced by them to the nervous current of the 

 skin nerves and epithelial nerves. Those vibrations called sound agi- 

 tate the auditory organs and are changed by them to the nervous current 



