ECONOMY OF THE BODY 117 



compared to the wires of a telephonic system. The nerve- 

 cells, which compose the centres or ganglia, may be com- 

 pared to individual telephones which receive news (sensory 

 stimuli) from far and near and likewise issue orders to 

 executive parts such as the muscles. A comparison may 

 be made between the chief nerve-centres, say in the brain, 

 and the " central office " of the telephone system, where 

 messages may be shunted and connections established. 

 But the comparison must not be pressed far, else it breaks 

 down badly. The central nervous system stores messages 

 in its ner^^e-cells, but this is not allowed in a telephonic 

 exchange ; and the central nervous system co-ordinates 

 messages, combining them and allowing them to influence 

 one another, which is also quite unpermissible in the 

 telephonic " central." 



It must also be noted that nerve-fibres are outgrowths 

 of nerve-cells or neurones. There are not two units — the 

 nerve-fibre and the nerve-cell ; there is but one — the 

 neurone. This gives off in typical cases (i) a number of 

 short protoplasmic processes or dendrites which link up 

 with similar processes from another neurone, and (2) an 

 axis-cylinder or main fibre which is often long and carries 

 commands to muscle or gland or brings in messages from a 

 distant external or internal part. 



A ganglion or nerve-centre is a collection of neurones in 

 intimate linkage with each other. A nerve is a bundle of 

 nerve-fibres within a sheath, and sensory nerve-fibres which 

 carry messages to the centre must be distinguished from 

 motor nerve-fibres which carry commands outwards. The 

 nature of the message or of the command that " travels " 

 along a nerve-fibre remains an enigma. 



A Reflex Action. — Not even an elementary under- 

 standing of the activity of the nervous system is possible 

 without knowing what is meant by a reflex action. In 

 backboned animals, such as birds, its typical expression — 

 the outcome of a long evolution — is as follows. A nerve 

 issuing from the spinal cord is made up of dorsal and ventral 

 roots. The dorsal roots (otherwise called posterior or 



