356 The Outline of Science 



regarded as ultra-microscopic Yale keys sent out to search for the 

 locks of letter-boxes which they can fit and enter." They circu- 

 late round the body until they find their destination. "What is 

 still more wonderful in this system is that the letter-boxes, or we 

 may call them locks, have a positive attraction for the key mis- 

 sives which are destined for them." It was Professor Starling 

 who named these messengers Hormones. 



The thyroid glands two little lobes on either side of the 

 windpipe are bodies of this nature which have attracted a good 

 deal of popular attention of late years. The secretion formed is 

 discharged straight into the blood stream, and for that reason they 

 are called ductless glands or glands of internal secretion. They 

 will be discussed at greater length in a later section of this work. 

 Here it is enough to say that the chemical stuff, or "hormone," 

 which they secrete increases the vitality of the tissues; it makes 

 the tissues "greedy for oxygen," and the work goes on more 

 briskly. Hence it is that decay or imperfect development of the 

 thyroid glands leads to that state of bodily and mental feebleness 

 which is called "cretinism," while the extract from the glands can 

 be used for the purpose of "rejuvenation." This small organ, 

 the thyroid gland, is necessary to the health and normal develop- 

 ment of both body and mind, and this knowledge has been 

 put to practical application in some cases with astounding 

 results. 



Near the thyroid glands are four small bodies the "para- 

 thyroids." The function of these is not clear, but there is serious 

 nervous trouble if they are removed. Then there is a "thymus 

 gland," which seems in some way to prevent the sex -organs from 

 developing too early. It is situated in front of the breast-bone, 

 and must act by "postal service." The internal sex-organs them- 

 selves post a good many of these "hormones" in the blood. 

 Everybody knows the striking difference between a normal and a 

 castrated animal. The development of secondary sexual charac- 

 teristics, such as antlers, seems to be largely stimulated by 



