ACTIVITIES OF LIVING BEINGS 35 



(4) Nervous activities. All the various vital 

 activities are more or less controlled and 

 regulated by the nervous system, which may 

 be regarded as the master system of the body. 

 This consists of the brain, spinal cord, ganglia, 

 and nerves, but it may be more shortly de- 

 scribed as formed of nerve centres and nerves. 

 The nerves pass to and from the centres, 

 connecting up all parts of the body with those 

 centres. Nervous energy may originate in the 

 centres, brain, or cord, or ganglia, and it 

 may stream out along the fibres in the nerves 

 to various organs, such as muscles, glands, 

 blood vessels, and, in some animals (such as 

 the electric fishes), electric organs. This 

 energy, as to the nature of which we know 

 little, awakens the activities of those organs. 

 Under its influence, muscular fibres will 

 contract, the cells of a gland will secrete, 

 the walls of small vessels will alter their 

 calibre, and an electric organ will be the seat 

 of electrical changes. The centres, in their 

 turn, receive from the periphery of the body, 

 from the sense organs, and from all other 

 organs, nervous impulses that awaken activi- 



