nerve-tissue. The nerve-track is found to enter a cell of 

 very mobile protoplasm, and then it does not traverse the 

 organism, but usually bends on itself and ends in a muscle. 

 This is the key to nervous organisation. From so simple 

 an arrangement is it that the most complex of systems, the 

 nervous system — brain, and cord, and nerves — is built up. 

 Everywhere we can reduce the most comj)licated arrange- 

 ment to its simplest expression — ingoing nerve-track, 

 mobile nerve-cell, and outgoing nerve-track usually ending 

 in a muscle. 



In the jelly-fish we find ingoing nerves which end in 

 cells whence issue outgoing nerves. But the cells are 

 collected together into nerve-centres, and each nerve-centre 

 is brought into relation with many ingoing as well as out- 

 going nerves, and not merely so, but also, by means of 

 connecting nerves, with other centres. A stimulus is thus 

 capable of being conveyed throughout the organism. But 

 if you cut the animal so as to isolate a nerve-centre, with 

 its ingoing and outgoing nerves, the part so isolated will 

 respond to stimuE, but the effect of the stimulus is localised 

 to that part. It is no longer felt or responded to by the 

 organism as a whole. This is a so-called reflex action in 

 its simplest expression. So that the association of nerve- 

 centres effects a co-ordination of movement, implies an 

 analogous associated response of the entire organism. 



In the higher forms of jelly-fish we get a definite 

 response to stimulation according to the seat of its appli- 

 cation. If you prick a polypite it moves over towards 

 the needle. The movements are co-ordinated so as to 

 make appropriate responses to definite stimulations. But 

 directly you sever the connections of the nerve-centres, a 

 stimulus ia only responded to by that part of the animal 

 in which the nerve-centres still remain intact. It is easy 

 to understand how the proximity and association of nerve- 

 centres facilitates the adjustment of the response to the 

 stimulus. It favours the transmission of messages along 

 an increasing complexity of nerve-routes. Particular 

 stimuli produce a response at a distance, having traversed 

 particular nerve-paths. Each advance forms a base of 

 operations for future advances. Just as in learning to 

 walk each adjustment of the body makes the next step 

 easier; or, as in learning a language, each acquirement 

 makes progress possible, each simple sentence helping the 

 leai-ner to construct sentences more complex, so does each 



