THE LEVELS OF BEHAVIOUR 147 



536. The " Integrative Action of the Central Nervous 

 System." Every nerve-centre is in indirect connection with all 

 the receptors, on the one hand, and with all the effector organs, 

 on the other. But each centre is predominantly associated with 

 some limited system of receptors and effectors, that is, certain 

 reflex activities are its characteristic province. Thus there are 

 ganglia, or regions of grey matter in the cord, roughly dehmited 

 by the repetitional spinal nerve-roots, and each such region, or 

 segment, has associated with it some region of skin with its 

 receptors, and some group of muscles. (But the demarcation 

 is not precise and the segments overlap.) Similarly the cere- 

 bellum is associated with the receptors of the otic labyrinth, 

 with the mechanism of tonus of the skeletal muscles, etc. 

 Nuclei in the brain are associated with the afferent nerves coming 

 from the great receptors of the head. And so on. But while 

 these ordinarily working delimitations exist it is nevertheless the 

 case that any system of receptors can be put in connection with 

 any system of effectors and this is because connecting tracts of 

 nerve-fibres potentially or actually join up all the centres with 

 each other. This is the only general statement we can make 

 as to the extraordinary complexity of the brain and spinal cord. 

 The analogy with a telephonic exchange has often been suggested : 

 it is useful, but must not be laboured. Above all is the cortex 

 cerebri in the higher vertebrates such a mechanism for joining 

 up systems of receptors and effectors via itself and the subordinate 

 centres. This is the key to the almost indefinitely great com- 

 plexity of the mammalian brain. 



53^. Characteristics of Reflexes. We approximate to the 

 conception of the '' simple reflex " by partially mutilating the 

 experimental animal. Thus the brain of the frog is destroyed 

 so that activities pivot on the spinal cord, or the cerebral hemi- 

 spheres are wholly or partially removed so that it is the cord 

 and the lower brain that are the centres, or the cord may be cut 

 through so that segments, or groups of such, are the centres 

 for the reflexes. Mostly the latter are studied in mutilated 

 animals so as to simplify the phenomena to be observed. 



(i) In the most simple cases, as in the spinal frog, the reflex 

 is nearly " inevitable." That is, given the stimuli and freedom 

 from control by the higher centres, the effect nearly always 

 occurs and nearly always in the same way. (2) It is " all or 



