INTRODUCTION 9 



which wires pass to every part. The wires are the nerves, the 

 head office is the central nervous system ; in ourselves the 

 brain and spinal marrow. There are wires for bringing 

 messages in (afferent nerves) and wires for taking messages 

 out (efferent nerves), and messages are continually travelling 

 along them. News comes to the central office of some change 

 or occurrence without or in some part of the body, and 

 forthwith messages are flashed out to the appropriate parts, 

 ordering muscles to contract, glands to secrete, etc. But how 

 are the messages sent ? All living substance is to a certain 

 extent irritable that is to say, it responds to a shock or 

 stimulus of any kind, and this response is not the passive 

 result of pull or push, but an active manifestation of energy, 

 often disproportionate to the amount of the disturbing in- 

 fluence. Nervous tissue is the irritable tissue par excellence, 

 and stimuli are conveyed to it by means of the organs of 

 special sense viz. touch, sight, hearing, smell, and taste. 

 Touch organs are distributed all over the skin, in the form of 

 minute bulbs, each connected with a fine nerve fibril. The 

 eye is the organ of sight, and it consists essentially of end 

 organs of extreme minuteness, which are capable of being 

 affected by luminiferous undulations of ether. The bulk of 

 the eye in ourselves is made up of arrangements for focussing 

 the light on the actual end organs. The ear similarly has 

 end organs, which are affected only by sound waves, and in 

 addition, there is an apparatus for communicating the vibra- 

 tions of the air to the end organs. Smell and taste are 

 localised patches of end organs of similar kind. Beyond these 

 we may speak of a muscular sense, and a temperature sense. 



Through these channels comes all the information that we 

 have of the outside world, and, in addition, a great deal of 

 information comes to our brains about the working operations 

 of our own bodies, but this information is not recorded in our 

 consciousness and we are not aware of its being sent. The 

 brain, however, does the needful, and sends out the messages 

 appropriate to keeping up the working of the machine. 



The various organs must not only work together, but must 

 also be held together, and they are bound up and connected 

 by a special form of tissue appropriately called connective 

 tissue. Moreover, every free surface of the body and its 

 organs is covered by a protecting membrane known as an 



