THE LEVELS OF BEHAVIOUR 145 



(2) The nuclei, or ganglionic centres, in the medulla. 



(3) The ganglia of the pons and cerebellum. 



(4) The great, primitive basal ganglia of the mid-, and fore- 

 brain. 



(5) The grey matter of the cortex cerebri, which covers the 

 cerebral hemispheres. 



These nerve-centres form a hierarchy of increasing importance 

 in the order (i) to (5). They are connected by tracts of nerve- 

 fibres and the arrangements of centres and tracts has great 

 anatomical complexity. There are also the following centres in 

 the vertebrate animal : 



(6) The diffuse " nerve-net " which exists as the plexuses in 

 the walls of the alimentary canal. (This is very primitive 

 according to a certain morphological hypothesis, not entirely 

 accepted by anatomists, but never confuted. These nerve-nets 

 are what remains, in the higher vertebrates, of the original, or 

 primitive pre-chordate nervous system.) 



(7) Ganglionic centres in the heart substance. 



(8) The ganglia of the sympathetic nervous system. 



53^. The Centres in Reflex Activities. To some extent 

 each centre is autonomous, that is, it can be operative, of itself, 

 in reflex activity. 



The nerve-nets. The most finely adjusted reflexes that can be 

 imagined are carried out by the plexuses in the alimentary canal 

 walls (intestine). These are the wave-like movements of peri- 

 stalsis effected by exactly co-ordinated contractions and relaxations 

 of the transverse and longitudinal muscles of those walls. The 

 centre is the plexus itself. 



The heart-centres. The heart muscles contract and relax in 

 most complex and nicely co-ordinated ways under the control 

 of their own ganglia. Heart-movements are controlled from 

 higher (cerebral) centres, but the isolated heart in lower verte- 

 brates will continue, of itself, to beat. 



The sympathetic ganglia. These control the working of many 

 glands and the blood-vessels, by themselves (though like the 

 ganglia of the heart, they are also controlled by the brain). 



The spinal cord. A very great number and variety of reflexes 

 can be carried out by the spinal cord ganglia. In many lower 

 vertebrates the brain can be completely cut off from the cord 



