112 ANIMAL BEHAVIOUR 



42. ON THE EFFECTOR ORGANS 



The parts of the animal body that are visibly and immediately 

 active in behaviour are limbs, jaws, teeth, etc. — all those instru- 

 ments that we have considered in Sections 23-4. Subservient 

 to them, in the sense of supplying energy, are the organs of 

 assimilation, respiration, circulation, etc. — these also we have 

 considered (Sections 12, h-c). The elementary agencies of 

 behaviour are not apparent on mere inspection of the active 

 and living animal — they are the muscles that actuate limbs, 

 wings, jaws, etc. ; that cause the heart to contract and relax ; 

 that regulate the calibres of the blood-vessels ; that actuate the 

 respiratory organs, etc. Along with them are the glands — those 

 that secrete saliva and other digestive fluids ; that elaborate the 

 internal secretions ; that are concerned in general metabolism ; 

 that make poisons, etc. The muscles and glands are the effector 

 organs. 



Effectors do not function by themselves but must be stimu- 

 lated to activity by nerves — they are innervated. Thus there is 

 another side to the sensori-motor system : stimuli originating 

 in the receptor organs set up nervous impulses which enter the 

 ganglia via the afferent nerves. In the ganglia these impulses 

 traverse synapses and then set up other impulses in the efferent 

 nerves. These then stimulate the effector organs to activity. 

 Thus the whole process of an element of behaviour involves 



-> Receptors -> Afferent -> Synapses in -> Efferent -> Effectors. 



nerves the ganglia nerves 



\2a. The Anatomical Conception of the Reflex Arc. A 

 reflex action may be illustrated by the experience of " blinking." 

 Something is thrown against a man's face when he immediately 

 closes his eyes momentarily. The stimulus is the moving 

 object, the receptors are the eyes, the afferent nerves are the 

 optic ones, the ganglia are in the brain, the afferent nerves are 

 those that go to the muscles of the eyelids, which are the 

 effectors. 



The " simple " reflex arc is an anatomical fiction cherished 

 because of its value in exposition : it represents the simplest 

 conceivable mechanism capable of carrying out an elementary 

 act of behaviour. Fig. 20, 5, shows that at least two neurones 



