A POSTAL SYSTEM 225 



do not make their effect evident immediately because 

 they have merely to do with the growth of parts — parts 

 which are combined together when carrying out a 

 common action. The missives sent out by the glands 

 of internal ^ secretion, then, are of two kinds — functional 

 and developmental. The microscopic units of every 

 organ of the body, be it a muscular engine or a laboratory 

 for the preparation of a digestive juice, have always a 

 double duty. They have not only to contract or secrete ; 

 they have also to take a share in the development, growth, 

 upbuilding, and maintenance of the engine or laboratory 

 of which they form a part. Hence every unit should 

 receive two classes of hormones : one to tune them for 

 their share in the active work of the body, and the other 

 to sensitise and regulate them in what may be called their 

 household duties. 



We should be led too far afield were we to follow out 

 the multiple activities carried on in the hormone factories. 

 We shall confine ourselves to one example from the 

 adrenal factory to illustrate the use served by hormones 

 in regulating a functional act and two others, one from 

 the pituitary gland and another from the genital glands, 

 to show how they control the functional growth of the 

 body. Professor W. B. Cannon of Harvard University 

 was the first to piece together the evidence we are now 

 to examine relating to the manner in which the adrenal 

 missives mobilise the systems of the body when a great 

 physical effort has to be made. When our brain perceives 

 that a great effort is required of us to overcome a diffi- 

 culty or perhaps to avoid a danger which is imminent, 

 it immediately braces up all the muscular engines under 

 its control. At the same moment as the brain's commands 

 are dispatched to the muscles, messages are automatically 

 sent out from nerve centres in communication with the 

 adrenal bodies — telegraphic messages. The result is 

 that the adrenals at once begin to dispatch with alacrity 

 a hormone — adrenalin — by the usual postal system — the 

 blood circulation. The action of adrenalin upon the 

 stopcock mechanism of the body is instantaneous ; those 



15 



