228 THE ENGINES OF THE HUMAN BODY 



have to answer is : How does overwork cause a muscle 

 to grow big and strong ? You may wear an engine out 

 by hard work, but you cannot make it stronger by such 

 means. Somehow, in a manner we do not yet understand, 

 the brain, when it is about to set muscles into motion, can 

 make a demand on the pituitary gland. It can unlock the 

 flood-gates of the pituitary and set free its hormones 

 which, circulating in the blood, sensitise the living units 

 which are concerned in the upbuilding and growth of 

 muscles, so that they respond to work by becoming larger. 

 If the pituitary hormone is thrown out too abundantly, 

 as in giants and in the acromegalic, the muscles are over- 

 sensitised and respond too readily, even when worked 

 to a slight degree. If the muscles are increased in power 

 then the levers on which they work must also be 

 strengthened ; the bone-builders have to be sensitised 

 at the same time as the muscles if a useful effect is 

 to be obtained. It is also necessary that other systems 

 of the body be strengthened. A great muscular system 

 needs a great heart or pump to feed it and great 

 lungs and respiratory bellows to supply it with an 

 adequate amount of oxygen. The alimentary system has 

 to be increased to supply the machine with sufficient 

 fuel. These changes are exactly what we find overtaking 

 the human body in the first outburst of acromegaly and 

 giantism. The heart and arteries become hypertrophied ; 

 the chest becomes enormous, the jaws, the muscles of 

 mastication, markedly overgrown ; the muscles at first 

 develop great strength, then sink into impotency ; all the 

 bones, particularly the areas giving attachment to muscles, 

 become thick, clumsy, and their muscular attachments 

 exaggerated. Soon they too begin to atrophy. Clearly, 

 then, one of the harmones posted in the general circulation 

 of the blood by the pituitary gland has to do with the 

 sensitising of all the systems of the body concerned in 

 muscular work. The growth hormone of the pituitary 

 gives to the systems of the body the power to respond 

 to the burdens which are cast on them. There is no 

 mechanism of this description in the organisation of any 



