226 THE ENGINES OF THE HUMAN BODY 



supplying the alimentary tract are closed to a minimum, 

 but those of the muscles of the heart and lungs and of 

 the nerve centres immediately concerned in the effort 

 are opened to their widest. There is a nerve system — 

 the vasomotor system — for controlling the stopcock 

 mechanism of the arterial system, but adrenalin comes 

 into play and relieves or assists the system in tiding over 

 a prolonged effort. Adrenalin has also another effect. 

 When muscles are working hard they consume fuel 

 quickly and need an increased as well as a steady supply. 

 That, too, is provided by the adrenal hormone. As it 

 circulates through the liver it stimulates the sugar-pro- 

 ducing laboratories of that great gland to set free its 

 stores in the blood which, by means of the circulation, 

 are carried to the muscular engines, thus furnishing them 

 with an ample and steady supply of fuel. At the same 

 time as the adrenal laboratories begin work, nerve 

 messages reach the thyroid gland in the neck, which also 

 passes into a state of activity. The thyroid hormone 

 thus thrown into the circulation has the same effect on 

 the tissues of the body as the impregnation of paper with 

 a nitrate solution. The addition of nitrates makes a 

 paper scintillate and smoulder rapidly, as in match paper. 

 That is the effect which one of the thyroid hormones has 

 on the living tissues of the body ; it sensitises them and 

 makes them greedy for oxygen ; it thus aids in combustion 

 and in the performance of work. It reinforces and increases 

 the action of adrenalin. Dr George Crile has shown that 

 both of them, particularly the hormone of the adrenal 

 bodies, stimulate the nerve corpuscles of the brain to 

 their full activity. There is no postal mechanism corre- 

 sponding to the hormone system in the organisation of 

 a modern army. A general order which confers special 

 rations on the men in the firing line, with extra allowances 

 of rum, tobacco, and coffee, would represent a poor 

 substitute for the stimulating and controlling messages 

 sent out to the body by the adrenal and thyroid glands. 



We now pass on to consider the effect of one of the 

 developmental or growth-regulating hormones which is 



