SENSES AND NERVES I3I 



which are speciaHzed in their structure and function as 

 nerve cells. Broadly speaking, the function of nerve cells is 

 to transmit messages from one part of the body to another. 

 Like all cells, a nerve cell consists of a minute unit of proto- 

 plasm surrounded by a very thin membrane; in the proto- 

 plasm there lies a central body, the nucleus, which differs 

 chemically from the rest of the protoplasm and is in effect 

 in control of all its activities, for without it the cell cannot 

 live. But the nerve cell differs from other cells in having its 

 body drawn out into a number of threads, some long and 

 some short, radiating in different directions. The longer 

 threads are the nerve fibres which when gathered together 

 like the wires in a telephone cable form a nerve. In higher 

 animals there is a single long fibre which serves for carrying 

 messages to the nerve cell from a sense organ, or from the 

 nerve cell to a muscle or other executive structure. There 

 are generally several shorter fibres, and their function is to 

 make contact with the fibres of other nerve cells. 



In the sea-anemone the nerve cells and their fibres make 

 a diffuse network, but in higher animals the network is infi- 

 nitely more complicated because the nerve cell-bodies are 

 concentrated in a central nervous system, such as the brain 

 and spinal cord in the animals with backbones, or the nerve 

 chain in the under part of the body in those without. The 

 long fibres are bunched together and run to all parts of the 

 body as nerves. The network, whether simple or complicated, 

 is not strictly like a man-made net, although it may be 

 roughly likened to one, the knots representing the nerve cells 

 and the joining strings the fibres. It differs fundamentally 

 from an artificial net because the threads radiating from 

 each nerve cell are not actually joined to the threads of the 

 other cells ; the ends of the threads lie very close to each 

 other but they are not in complete contact, though they are 

 often branched so that the twigs interdigitate like the fingers 

 of two interlocked hands. 



The end of the long fibre of a sensory cell lies in similar 

 near-contact with an end-organ, the nature of which we shall 

 examine later on ; when it receives a stimulus from the end- 

 organ it sends a message back to the body of the cell. The 

 stimuluscauses a physico-chemical change — a re-arrangement 



