THE BUILDING OF REFLEX ARCS 247 



as the transmitting and receiving centres between which they pass. 

 We may then regard the appearance of neuro-fibrils within the 

 protoplasmic rudiment of the nerve-trunk as the coming into view 

 of tracks, along which, owing to their high conductivity, nerve- 

 impulses are repeatedly passing. It may be that as each successive 

 passer-by causes a. jungle -pathway to become more clearly defined 

 so each passing impulse makes the way easier for its successors and 

 makes it less likely for them to stray into the surrounding substance 

 (p. H2). 



Professor Macdonald, in the Portsmouth address referred to, 

 speaking of the states of the cells under excitation, rest, and inhibi- 

 tion, says " excitation is associated with an increase in pressure of 

 certain particles within the cells ; in rest these particles are in their 

 normal quantity and have their normal number. During inhibition 

 they are decreased in number or have a retarded motion. Thus 

 it happens that the excited cell tends to grow in size, on the other 

 hand the inhibited cell tends to diminish, and the resting cell to 

 remain unaltered in the nervous system. Structure is everywhere 

 the outcome of function." Speaking of the relationship of parts 

 within the nervous system, " In so far as it is fixed, it is a sign of 

 the orderly action of circumstance upon the structures of the body, 

 and the result rather than the cause of the monotony of existence. 

 I hold it as probable that all the individual structures of the nervous 

 system, and so in the brain, have just so much difference from one 

 another in size and shape and in function as is the outcome of that 

 measure of physical experience to which each one of them has been 

 subjected ; and that the physiological function of each one of them 

 is of the simplest kind. The magnificent utility of the whole system, 

 where the individual units have such simplicity, is due to the 

 physically developed peculiarities of their arrangement in relation 

 to one another, and to the receptive surfaces and motor-organs of 

 the body." As to the lens-system of the eyeball he remarks, 

 " Surely there is no escape from the statement that either external 

 agency cognisant of light, or light itself has formed and developed 

 to such a state of perfection this purely optical mechanism, and that 

 natural selection can have done no more than assist in this process." 

 He applies the same conclusion to the formation of the sound- 

 conducting and resonant portion of the ear as well as the semi- 

 circular canals and to the cerebellum. These statements are not 

 strictly associated with this chapter but bear by analogy very 

 strongly on the matter at issue. Indeed the whole of this address 

 might be utilised by a junior counsel for Lamarck if he rested alone 

 on the authority of a leading physiologist. The same may be said 



