164 PHYSIOLOGY 



The voluntary energy of the brain is not in proportion to its size, 

 and seems to bear no kind of proportion [thereto]. The power of the 

 brain to stimulate a nerve to action, and the effect or power of that 

 action of the nerve upon a muscle, is as strong in the insect as in the 

 human subject ; therefore, whatever properties size of brain may have 

 in an animal, they are not, in the brain, employed upon the body, but 

 employed about its own actions, as in a greater effort of the mind, and 

 a greater scope of reasoning. Nor does a large brain require larger 

 nerves to make the impression of sense 1 ; I believe, rather less. 



A nerve is a sensitive organ, but has no business with the mind ; 

 for if a nerve has informed the mind of anything, that nerve may be 

 totally lost, and yet the remembrance of the thing will continue ; so 

 that the nerve has done its whole business in communicating its im- 

 pressions to the mind. We are too apt to take effects for causes ; and 

 it is natural for us to do so, because the effect comes first, makes the first 

 impression, and in most cases it is not necessary to look out for a cause. 

 Impressions or sensations are effects : — the causes of the impressions 

 are external bodies. 



Nerves have nothing to do with muscular motion [in itself]; a 

 muscle has all the powers of action without nerves ; but muscles must 

 have a stimulus. 



Muscles are divided into two kinds, one having a constant stimulus, 

 and which never tire, [others having the stimulus of the will, and 

 which do tire.] There is also a mixed kind ; where muscles act by the 

 natural stimulus and do not tire, and where they are exerted beyond 

 that stimulus by the will, when they soon tire. Therefore it is the 

 stimulus of the will that tires. 



It is impossible for the mind to form just ideas of causes and modes 

 of action where neither cause nor mode of action is known ; nor, pro- 

 bably, within the reach of human sagacity. These reflections are 

 immediately applicable to the causes and actions of an animal body. 

 We see the body move. We go further ; we see the parts that have 

 within themselves motion, which is the immediate cause of the motion 

 of the whole ; and we see how that motion can be excited. But all this 

 does not give us the first cause of motion in those parts, nor does it 

 explain the mode of action of the parts themselves. 



The only thing, probably, left for us to do, is to observe, as much as 

 possible, all the visible causes of motion in those parts of motion ; which 

 of course will give us all the visible effects : carrying these researches 

 into eveiy class of animals ; seeing how far they vary, so as to be able 



1 [The organs of sense are generally inversely to the brain as to size : compare 

 the eves and internal ears of fishes with those of birds and mammals.! 



