378 HUMAN PHYSIOLOGY 



still more are to be anticipated. But down to the present 

 time the observations have constantly increased the 

 difficulty of the whole matter. 



It may fairly be claimed that every kind of tissue may 

 give rise to hormones. That is, it may be argued that 

 the tissues differ in their chemical nature and metabolism 

 so distinctly that each one must have products which 

 no other can evolve. The normal composition of the 

 blood must depend on the blending of all these innumer- 

 able contributions. It is curious to reflect that the Greek 

 masters in medical science taught that health was founded 

 on the right combination of chemical principles, the 

 "four humors," in the body. We are coming to believe 

 in our own age that many disturbances of health are 

 actually due to excesses or deficiencies of internal secre- 

 tions, a view that obviously recalls the ancient one. 

 While the formation of hormones may be theoretically 

 universal, there are some organs which exhibit it most 

 conspicuously and of these we have now to give brief 

 accounts. 



The Thyroid. The thyroid gland, or thyroid body, is 

 in the neck between the larynx above and the breast 

 bone below. There are two lobes united by an isthmus 

 which crosses in front of the trachea. The normal thy- 

 roid is small and not easily noticeable from the surface, 

 but there are many thin subjects in whom its form is 

 quite apparent. It may be greatly enlarged and when this 

 is so it forms the disfiguring swelling known as a goiter. 

 The enlargement may not be attended by other than local 

 symptoms, but in many cases it is the central element in 

 a serious disease. 



We have said that extracts of the thyroid produce 

 nervousness, emaciation, and palpitation of the heart 

 when given to human subjects. The enlarged gland may 

 deal out to the circulation abnormal quantities of an 

 internal secretion capable of just these effects. The result 

 is much as though the extracts were given by the mouth. 

 In bad cases an odd symptom is a bulging of the eyes. 



