256 THE ENGINES OF THE HUMAN BODY 



for the dispatch of messages. The form shown in 

 fig. 46, B, is the kind used near joints for transmitting 

 pressure messages. Messages are dispatched whenever 

 pressure is applied to the transmitter. It is not sensitive 

 to sound waves. There is a wealth of transmitter patterns 

 in the human body. The retina of the eye is studded 

 with one type ; the ear with another ; the nose and 

 tongue have each their peculiar design. Muscles and 

 tendons, as well as joints, are closely set with transmitters 

 of the kind shown in fig. 46, B. They serve only for the 

 dispatch of pressure messages. Every movement we 

 make sends thousands of messages through these pressure 

 transmitters. (2) A nerve filament or wire to carry 

 messages from the transmitter to an exchange. Man 

 has drawn copper into fine wire to serv^e for the 

 carriage of messages from one point to another. For 

 the same purpose Nature has drawn out the substance of 

 nerve corpuscles or cells to form microscopic filaments or 

 fibres. She leaves, as is shown in fig. 46, the filament 

 attached to the parent cell, which, in the instance shown 

 in fig. 46, B, is a cell stationed in the ganglion of the 

 posterior root of a spinal nerve. (3) But what of the 

 battery which causes a current of electricity to flow along 

 the copper wire and thus makes it alive and fit to carry 

 messages ? What corresponds to a battery in the nerve 

 system and makes the nerve filament alive and fit to carry 

 messages ? The nerve unit or cell maintains the filament 

 alive and thus corresponds to a battery. But a nerve 

 unit differs from a battery in this sense ; it sends out no 

 flowing current ; the unit and filament are pulsating and 

 throbbing with life from end to end, and it is because of 

 its vital molecular movements that a. nerve filament is 

 capable of transmitting messages. (4) Then we come to 

 the fourth element of a nerve unit system — the exchange 

 terminal. It consists of a branching system of most 

 delicate filament, quite different from the simple exchange 

 terminal of a telephone unit. As to the fifth element, 

 the receiver^ it is not represented because it is unnecessary, 

 and for this reason. Messages travel over telephone wires 



