THE NERVOUS SYSTEM 219 



about -g-^Tj- of an inch in diameter, termed the nerve-fibres ; 

 and (2) the gray substance, composed of grayish-red, or ashen- 

 colored cells, of various sizes, generally possessing one or more 

 off-shoots, which are continuous with the nerve-fibres just men- 

 tioned. 



5. Each nerve-fibre is continuous from the nerve-centre from 

 which it springs to that part of the body in which it ends. 

 It consists of a white sheath and a gray axis cylinder within. 

 The axis cylinder serves to convey the nerve-impulse, while 

 the white sheath seems to serve as a protection, very similar 

 to the insulation on electric wires. Many nerve-fibres unite 

 into a bundle and form nerves. These nerve-bundles may be 

 compared to electric cables containing many separate wires. 

 The function of nerve-fibres is to convey nerve-impulses from 

 the organs and periphery of the body to the nerve-centres, or 

 from the nerve-centres to the organs and the periphery. 



6. The gray, cellular substance constitutes the larger por- 

 tion of those important masses which bear the name of nervous 

 centres and ganglia (from ganglion, a knot), in which all the 

 nerve-fibres unite. The function of nerve-centres is three- 

 fold:" to receive nerve-impulses; to originate and impart 

 nerve-impulses ; and to transmit nerve-impulses from one 

 centre to another. That part of the nervous system which 

 is concerned in the animal functions comprises the brain, the 

 spinal cord, and the nerves which are derived therefrom; 

 these are, together, called the cerebro-spinal system (Fig. 54) ; 

 while that other set of organs, which presides over and regu- 

 lates the vegetative functions, is called the sympathetic system 

 of nerves. 



7. The Brain. The brain is the great volume of nervous 

 tissue that is lodged within the skull. It is the largest and 

 most complex of the nervous centres ; its weight, in the adult, 



5. Of what does a nerve-fibre consist? Purpose of the axis cylinder? Of the white 

 sheath ? What is the function of nerve-fibres ? 



6. Nervous centres and ganglia? Nerves? What do they serve? Cerebro-spinal 

 system ? 



7. Location of the brain ? Its weight? Itsflhkpe? Of what does it consist? What 

 organs at the base ? x 



