146 ANIMAL BEHAVIOUR 



(as in " pithing " a frog), or the head may be severed from the 

 body (as in frogs, insects, etc.) and the animals will continue for 

 a time to live and function. The " spinal " or headless frog 

 will swim, preserve normal posture, wipe off irritants from its 

 body, etc. All such activities are reflexes. They only occur 

 when the receptors of the skin are stimulated. If there is no 

 such external stimulation the frog will exhibit few or no move- 

 ments. It has no *' spontaneity " of behaviour. 



The decerebrate animal. The lower mammal (even a dog) can 

 be made " decerebrate " by the gradual removal, by operation, 

 of the entire cortex cerebri, and even much of the underlying, 

 more primitive basal ganglia (corpora striata, etc). Even with 

 such mutilation the animal may live, feed, reproduce, etc. (But, 

 of course, there are limitations to its bodily activities and there 

 are extraordinary emotional modifications.) Here the functional 

 centres are the deep ganglia of the brain, the cerebellum, pons, 

 medulla, cord, etc. These are adequate for a great number of 

 reflexes and the receptors of the latter are the skin and the great 

 sense-organs of the head. The afferent nerves from the cephalic 

 sense-organs do not directly go to the cortex (except in the 

 case of the olfactory receptors) but to the medullary and basal 

 ganglia. 



Thus there are numerous nerve-centres in the animal body 

 that are stimulated, via afferent nerves coming from receptor 

 organs. These centres then initiate stimuli which are trans- 

 mitted to the muscles and other effector organs and the latter 

 then move, or secrete, performing reflex activities. Some of 

 these systems of reflex arcs, the intero-ceptors (stimulated by 

 food substances), the nerve-nets and the muscles of the alimentary 

 canal, are practically independent of the rest of the nervous 

 system ; others, such as the arcs pivoting in the heart ganglia, 

 those centering round the respiratory ganglion in the medulla, 

 some of the sympathetic ganglionic arcs, etc., are nearly inde- 

 pendent. They carry on regularly automatic reflex-activities, 

 but there is always some control of the latter exerted by the 

 higher brain-centres. Even the movements of the limbs in 

 walking, swimming and other habitual activities are the results 

 of reflexes centering in the ganglionic spinal cord and they may 

 work largely independently of the brain. But there is always 

 potential, or actual, control of them by the brain. 



