THE LIVING BIRD 13 



nerve cells is termed the brain. Its substance is 

 continued backward to the root of the tail as the 

 spinal cord inside a protective canal perforating the 

 chain of vertebrae which form the axis of the bony 

 skeleton. The 7ierves that supply all organs and 

 parts of the body — the peripheral nervous system — 

 take origin either from the brain or the cord, twelve 

 springing from the brain as in reptiles and mam- 

 mals. Distribution of all but the last three of 

 these is confined to the head whose muscles and 

 sense organs they supply, as well as the teeth in 

 reptiles and mammals. The spinal nerves supply 

 the skin and muscles of the trunk and limbs. In 

 addition to these elements there exists what is 

 termed the sympathetic or involuntary nervous 

 system, innervating the viscera. The sense organs, 

 since they keep the central nervous system in touch 

 with the outside world, the animal's environment, 

 are obviously as important as the brain itself and 

 we shall have to give them particular attention. 



Nerve fibres are of two kinds, sensory and motor. 

 The former carry impulses from the outside in, i.e. 

 from the external centres of perception to the 

 internal central nervous system; the latter send 

 impulses outwards from within, inducing responses 

 in the musculature which bring about appropriate 

 movements in reply to the sensory stimuli pre- 



