226 The Vertebrate Organ Systems 



reason for this is easily understood when the problems of experimenta- 

 tion with these glands are considered. One very obvious technique is 

 the actual surgical removal of the gland with a study of the resultant 

 physiological changes. For example, a gland is removed and shortly 

 after that the blood pressure is lowered. It might then be assumed 

 that the gland probably had some control of blood pressure. Even 

 this initial experimentation, however, has many inherent difficulties. 

 Many glands are located so deeply within other organs that surgical 

 removal may injure other tissues. It is not always possible to be sure 

 that the effect of gland removal is being studied or only some post- 

 operative effect. Also many glands are actually double ; for example, 

 the adrenal is composed of two parts each with an entirely separate 

 secretion and function. Another difficulty that may be encountered 

 is that some glands are very closely associated with one another. To 

 illustrate this, the first operations on the thyroid were fatal in some 

 animals. It was not until much later that it was learned that buried in 

 the thyroid tissue was another smaller gland, the parathyroid. It 

 was the removal of this latter gland that caused the death of the ex- 

 perimental animal. Again, the secretions of a gland may be so com- 

 plex that it is difficult to isolate single effects. And last, the secre- 

 tions may stimulate other glands and give secondary effects. 



Once the effects of extirpation of the gland are established, the 

 next experimental procedure is to administer extracts of the gland 

 to observe if the changes resulting from removal are alleviated. This 

 is a check which helps to determine whether it is the gland or some 

 secondary effect that is being observed. As a final step, chemical analy- 

 sis and possible synthesis of the hormone give a more nearly complete 

 knowledge of its function and nature. These latter steps have been 

 accomplished for only a few glands ; however, this information is of 

 great medical importance. Thus the knowledge of the endocrines, 

 limited as it is, has contributed much to medical practice. 



In a healthy animal, the endocrine system aids the nervous system 

 in keeping all parts in close coordination. The endocrines must secrete 

 definite amounts of their products. Even a slight over- or undersecretion 

 may produce drastic effects. Much of the knowledge of these glands has 

 been gathered from observations of these naturally occurring abnormal- 

 ities. If a gland fails to produce an adequate amount of its products, 

 it is said to be hypofunctioning ( hypo = below) ; if it secretes too much, 

 it is hyperfunctioning ( hyper = above). Both of these conditions can 



