ECONOMY OF THE BODY 119 



(i) a peripheral nerve-ending (s.E.), 



(2) a sensory or afferent nerve-fibre (s.F.), 



(3) a sensory cell-body in a spinal ganglion (s.N.), 



(4) a sensory nerve-fibre branching in the spinal cord 



(BR.). 



(5) an associative neurone with its branches (as.n,), 



(6) a motor neurone (m.n.), 



(7) a motor or efferent nerve-fibre (m.f,), and 



(8) a nerve-ending (m.pl.) on a muscle (mu.). 



The mother bird touches with food the bill of its half- 

 asleep nestling and instantly, before we can say " reflex 

 action," the mouth is open wide. Then follow other 

 reflex actions of swallowing and the like. 



A sensory message entering the spinal cord may affect an 

 associative neurone which passes on the stimulus to a motor 

 neurone, but it may affect more than one, so that another 

 motor neurone is called into action, or so that the most direct 

 answer-back is inhibited by a counteractive influence. For 

 reflexes are not quite obligatory, as we know when we 

 smother a cough or inhibit a sneeze on a solemn occasion. 



Reflex actions, such as opening the mouth at the touch 

 of food, or gripping the food with the pharynx, or violently 

 squirting out nauseous stuff when startled, or shutting the 

 eye when a blow is threatened, play an important part in 

 the life of birds. They are inborn ; they do not require 

 to be learned ; they are due to pre-arranged linkages of 

 certain nerve-cells with certain muscles ; they are often 

 bound together in a sequence so that one pulls the trigger 

 of another ; in their simplest forms they do not require the 

 activity of the brain, only of the spinal cord. 



The diagram of the bird's nervous system shows the 

 relatively large brain sheltered by the skull, the long spinal 

 cord sheltered by the neural arches of the vertebras, and the 

 numerous cranial and spinal nerves. Some of the cranial 

 nerves are purely sensory, such as the optic which carries 

 impressions from the retina to the brain ; others are purely 

 motor, such as the oculomotor which is in command of four 

 of the six muscles that^move the eye-ball ; and others are 



