13 



The doctrine of vibrations here spoken of is in reality a new 

 theory of the structure of the nerves. Instead of being filled 

 with 'animal spirits', a fluid which was supposed to move with 

 inconceivable rapidity, the nerves are supposed to be com- 

 posed of particles which move in ether. The impulses are 

 passed along them in the form of a vibration of these particles. 

 Consequently when a vibration comes to the end of a nerve, 

 and enters the brain, it begins ' 'to be propagated freely every 

 way over the whole medullary substance, being diminished in 

 strength in proportion to the quantity of matter agitated". 

 So also ' 'we must suppose that the vibrations, which ascend 

 along any sensory nerve, affect the region of the brain which 

 corresponds to this sensory nerve more, and the other regions 

 less". 1 



To gather these various points into one statement, we may 

 quote again from Hartley: "Let it be remarked also, that, if 

 the performance of sensation by vibratory motions of the 

 medullary particles be admitted, the existence of a subtle 

 elastic fluid must be admitted in consequence thereof, as the 

 only means that can be conceived for their rise and free propaga- 

 tion, so as to answer to the phenomena of sense, motion, and 

 ideas; and reciprocally, if the existence of so subtle and elastic 

 a fluid, as the ether described by Sir Isaac Newton, can be 

 established upon independent principles, it may reasonably 

 be supposed to penetrate the pores of the medullary substance, 

 how small soever they be, in the same manner as air penetrates 

 grosser cavities and pores, and, like air, both be itself agitated 

 by vibrations from a variety of causes, and also communicate 

 these to the medullary particles. We may therefore either 

 deduce the doctrine of vibrations here proposed from the con- 

 sideration of the ether, or the existence of the ether from the 

 doctrine of vibrations, according as either of these can be first 

 established." 2 



The application of this doctrine of vibrations to^the explana- 

 tion of the mental processes is worked out in detail in chapters 

 following dealing with the various classes of sensations, but it 

 would be unfair to Hartley to suppose that he regards what in 

 all his statements is obviously a mere correlation of facts^as a 

 completely satisfactory explanation. The doctrine of vibra- 

 tions seems to fit the physiological facts better than the theory 

 formerly held ; the doctrine of association, in exactly the same 

 way, seems to suit the mental facts better than any which he 

 knows; and the doctrine of vibrations is exactly suited to the 



'O.C. p. 24. 

 'O.C. p. 25. 



