468 PRINCIPLES OF GENERAL PHYSIOLOGY 



m 3 



s 



FIG. 143. DIAGRAMS OF THE EVOLUTION OF THE CENTRAL NERVOUS SYSTEM. 



S, Sensory neurone. A, Association neurone. M, Motor neurone. e, Epithelial cell. m, Muscle cell. 

 The dotted lines indicate the boundaries of the nerve centres. 



1. Sponge. 



2. Sea anemone. 



8. Simplest form in the earthworm. 



4. Intercalation of association neurones in the earthworm. 



5. Exceptional, simple, reflex arc in vertebrates. Possibly exiting in the case of the knee jerk. 



0. Usual type in vertebrates. The cell bodies of the sensory neurones are in the dorsal root ganglia, instead 



of in the receptor organs, except in the olfactory organ. 

 7. Addition of higher centres, consisting only of association neurones, some of which are inhibitory. They 



form, <w it were, longer and longer parallel or alternative loops between the receptor and effector 



organs. These loops may be followed in Fig. 147. 



