ECONOMY OF THE NERVOUS SYSTEM. 479 



will. Such are the essential and appropriate func- 

 tions of the organ termed the brain. Thirdly, the 

 efferent nerves, which, like the first, are composed 

 of white neurine, but take their rise from the cen- 

 tral grey neurine, and are distributed to every 

 other part of the body, where they terminate. 

 Those efferent nerves which are distributed to 

 muscular structures, and are instrumental in ex- 

 citing them to contract, whether belonging to the 

 ganglionic or to the sensorial system, have been 

 specially termed motor nerves. Those which belong 

 to the ganglionic system either excite involuntary 

 contractions of the muscles, or influence the actions 

 of the blood-vessels and secreting organs. 



There is still another class of white nervous 

 fibres, constituting the fourth set of parts of the 

 nervous system. These, which are termed commis- 

 sural Jibres, have both their origin and their termi- 

 nation in the grey neurine, and pass transversely 

 from one portion of this substance to another ; 

 serving apparently the purpose of establishing 

 direct communications between them. The gan- 

 glia, generally, are connected together by nervous 

 fibrils of this description. Between the two an- 

 terior columns of the spinal cord in vertebrated 

 animals, there passes transversely a band of me- 

 dullary substance, which connects these parts. In 

 the brain, these commissures exist on a large scale, 

 and unite more especially the several portions of 

 grey neurine, which are interspersed in various 

 situations, and compose a considerable portion of its 

 whole mass. The purposes they serve, are, in all 

 probability, to establish an unity of consciousness 

 in the individual percipient being, and to secure 



