The Body-Machine and Its Work 345 



a postal system and a telegraphic system in the body. Certain 

 organs discharge certain chemicals (hormones) into the blood, 

 and the blood delivers them to distant organs which are thus set 

 to work. Obviously, this postal system would be too slow for 

 the purposes of ordinary life, and so the telegraphic system is 

 richly developed. Suppose that in bathing you tread upon a 

 sharp stone. In a fraction of a second a nerve-thrill flashes from 

 that part of your foot to a certain centre in the spinal cord, and 

 a return thrill causes the muscles of the leg to contract, thereby 

 jerking back the foot. In an animal far down in the scale like 

 the octopus this nerve-message goes at about eighty inches a 

 second; in the frog the speed has worked up to ninety feet a 

 second; in man it reaches about four hundred feet a second. 



In the case of man the nerve-message often goes on to ring 

 a bell in the brain, as it were to announce itself in consciousness 

 but the greater part of the body-machine is run, as we have 

 seen, by automatic action, and we will first master this. We 

 have spoken repeatedly of "reflex action." We mean by this 

 nervous action without conscious effort. The message that goes 

 to the brain or spine is automatically "reflected," along a different 

 "wire," to the muscles or glands. When a piece of dust blows 

 against your eyeball, one nerve sends some sort of thrill to a 

 centre in the brain. Within a very small fraction of a second 

 this message passes through a nerve-centre in the brain, and 

 another thrill comes back to the muscles of the eyelids. Nearly 

 the whole body is connected up, generally through the spinal cord, 

 by an automatic nervous machinery of this kind. For the muscles 

 of the head and face the nerve-centres are in the brain. 



The nerve-cells (or neurons, as they are often called)' 

 have a cell body and outgrowing fibres or "wires." Each cell 

 has two or more fibres running out to it, and these in most cases 

 end in brushes of still finer threads. The nerve-cells are, there- 

 fore, particularly suitable for communicating with each other, 

 In the brain and spinal cord especially each cell runs into a little 



VOL. II 4 



