XII.] THE DIEECTION OF DEVELOPMENT. 143 



supplies the various parts of the body with energy as well Blood 

 as with matter. The nerves, of course, transmit energy ene^g^^L 



only. ^«V/' 



'' matter. 



What has been said on the direction of animal develop- 

 ment may be thus summed up : — 



Every organism transforms both matter and energy, and Summary, 

 it is probable that every living part of an organism always 

 continues to transform them both. Eut the characteristic 

 of animal development is, that one set of organs — the 

 nutritive — is specially appropriated for the transformation 

 of matter; and another — the nervo-muscular — for the trans- 

 formation of energy. The nutritive, or vegetative, system 

 of organs is for the most part internal, and the charac- 

 teristically animal organs — those of the limbs and mouth, 

 with their muscles — are external to the nutritive. In both 

 vegetables and animals, a vascular, or circulatory, system 

 is developed, the function of which is to minister to the 

 nutritive life by the circulation of nutritive fluid ; and in 

 animals a nervous system is developed, the primary function 

 of which is to ensure the harmonious and efficient action 

 of the muscular system, by the transmission of stimuli from 

 one part to another. 



The foregoing account of the relation of nerve to muscle. Ganglia, 

 however, would be most imperfect were it not mentioned 

 that nerve-fibres never exist without ganglia, which are 

 situated at the junctions of the fibres, and from which the 

 fibres radiate throughout the body. The action of ganglionic 

 tissue is even more mysterious than that of nerve-fibre. It 

 appears, however, to be a universal law that one fibre can 

 communicate a stimulus to another only through a gan- 

 glion. In the higher animals, sensation and thought are Sensation, 

 produced in some altogether inscrutable way by the mutual 

 action of the ganglia and the nerve-fibres : sensation can 

 be produced neither in a ganglion unless it is acted on by 

 a nerve-fibre, nor in a nerve-fibre unless it is in communi- 

 cation with its gangbon. 



