LETTERS TO BROTHER JOHN. 13 



Being, thus endowed, discriminates between that 

 which is conducive, and that which is detrimental to 

 his welfare ; enabling' him to seek the former, and 

 shun the latter ; and establishing a due connexion 

 between him and the objects which surround him : 

 and fifthly, the nutritive organs, nourishing and 

 sustaining life in the whole. 



I have spoken of the motive power of the mus- 

 cles ; both the solid muscles, moving bones ; and the 

 hollow muscles, as the heart and arteries, moving 

 the fluids;- 1 - and I have said that each of these 

 derive their motive power from the brain and 

 spinal marrow (which is but an elongation of the 

 brain), and those little separate brains which are 

 placed within the cavities of the chest and abdomen. 



The functions of the brain are not well under- 

 stood : but I believe it to be a gland which elabo- 

 rates out of the blood sent to it by the heart 

 a fluid somewhat analogous, perhaps, in its 

 nature, to the fluid called electric ; and that this 

 fluid is carried out of the brain, and distributed 

 where it is wanted by the nerves. But the brain 

 is itself derived from the blood, which is circulated 

 through it by the heart. Whence, then, does the 

 heart derive its power of propelling the blood 

 through the brain its motive power? We have 



