62 PRINCIPLES OF PHYSIOLOGY 



be movement of a limb ; or it may, when 

 it reaches the non-striated muscle in the 

 bowel, cause slow ring-like movements of 

 the tube (peristaltic) ; or it may affect the 

 number and strength of the heart beats, or it 

 may cause the walls of small arteries to con- 

 tract and thus regulate the amount of blood 

 flowing through them ; or it may stimulate 

 the activity of a secretion, as seen, for example, 

 in the increased flow of saliva when a sapid 

 substance is in the mouth, or even when 

 the sight or thought of delicious food has 

 the same effect. The electric discharge of an 

 electric fish is also under the control of the 

 nervous system, which also appears to affect 

 the light of the firefly, glowworm, and even 

 the light of the luminous organs found in some 

 fishes that live in the profound darkness of 

 the depths of the ocean. Finally, in the nerve 

 cells that form the main part of the nerve 

 centres, such as the brain and spinal cord, 

 there are molecular changes of which we know 

 next to nothing, and yet on these all nervous 

 activities depend, even those associated with 

 mental processes. 



