224 THE ENGINES OF THE HUMAN BODY 



in the control or management of the human body. Several 

 instances have been alluded to in former chapters. 



The most remarkable advance of recent years was the 

 discovery that there are certain laboratories or glands in 

 the body which are entirely devoted to the manufacture 

 and dispatch of these key-missives. These laboratories 

 are usually called glands of internal secretion, but such a 

 name gives no indication of the important control they 

 exercise on the growth and working of the human machine. 

 These glands, laboratories, or control offices are placed in 

 various parts of the body and all are of small size. Two 

 of them, the right and left adrenal glands, each of the 

 size of a segment of a small orange, are placed over the 

 right and left kidney. They receive an abundant supply 

 of blood and also a leash of nerves which places them in 

 telegraphic communication with nerve exchange centres 

 in the spinal cord. The largest of the series, the thyroid 

 gland, is placed in the front of the neck, astride the wind- 

 pipe and just above the breast bone. It, too, has a nerve 

 supply from the involuntary system as well as a rich 

 blood supply. Then, within the head, cradled in the 

 floor of the skull and attached to the brain by a stalk, 

 is the pituitary gland about the size of a ripe cherry. 

 There is also a minute one, little larger than a barley- 

 grain, the pineal gland, buried deeply between the lobes 

 of the brain. Lastly, in the genital glands, besides the 

 main parts which have to do with the formation of human 

 seed, there is a second element which is concerned in the 

 production and issue of controlling missives. So small 

 is the total mass, that if all the glands of internal secretion 

 were rolled together they would form a parcel small 

 enough to go in a waistcoat pocket, yet such a small mass 

 can influence the working and growth of the whole body. 



In each of these small but busy laboratories, hormones 

 or missives of two kinds are prepared. There are those 

 which are made up and dispatched for immediate use in 

 controlling or tuning parts of the body concerned in the 

 performance of definite movements or operations. Then 

 there is a second class which, although posted daily, yet 



