6 INTERNAL SECRETION 



(organs of influence), which might be contracted into B-organs. 

 It is, however, more expedient to call them internal secretory 

 organs. It must be clearly understood, however, that not every 

 change in the blood-plasma due to organic intervention, and not 

 all metakerastic organic activity, is to be included under the 

 heading " internal secretory." The term can apply to the form- 

 ation of chemical substances only. Those organs, for instance, 

 which are known to be the formative agents of the morphological 

 elements of the blood (as the spleen, the lymphatics and the 

 bone marrow) in so far as the changes they effect in the character 

 of the blood are due to the contribution of new form-elements 

 are not to be regarded as internal secretory. 



Very little is known about the specific form in which the 

 true internal secretions, or hormones, find their way into the blood. 

 It may be that they enter the circulation as morphological con- 

 stituents of the blood, or perhaps in combination with such. 

 But, in the present state of our knowledge, we must bear in mind 

 that, though the cellular elements of the blood may possess 

 internal secretions, they are not internal secretions in themselves. 



The internal secretions include the chemical products of 

 organic metabolism only, though this limitation of the term is 

 not accepted without question by morphologists. 



Assuming always that the elaboration of substances from 

 materials withdrawn from the blood and the returning of the 

 changed materials back to the blood again, is a property common 

 to all living tissue, yet, from the point of view of the morphologist, 

 certain tissues only possess a true secretory value. According 

 to this view, those tissues only are able to secrete which possess his- 

 tologically differentiated epithelial cells, whether occurring singly 

 or in groups. An excretory gland is no longer to be recognized 

 in virtue of its possession of an excretory duct, but by its consti- A 

 tution in epithelial elements. Should the duct be absent but the 

 epithelial structure present, then, and then only, should the 

 term " internal secretory gland " be employed. Following out 

 this idea, the internal secretory glands would include : The 

 thyroid, the parathyroids, the epithelial layer of the suprarenals, 

 the glandular portion of the hypophysis cerebri (pituitary gland), 

 and the inter-tubular cell-agglomerations of the pancreas. Other 

 tissues, as, for instance, the so-called carotid gland, the medullary 

 substance of the suprarenals, and the chromaffine tissue in 

 general, would be excluded from this group. 



The last-named example clearly shows, however, that there 

 is not sufficient grounds for adopting a morphological and genetic 

 limitation of the term " internal secretion," and that to do so 

 would not in any way conduce to a solution of the question. We 

 have unequivocal proof of the internal secretory activity of the 

 chromaffine tissue, for we are able to obtain its secreted substance 

 in a state of chemical purity. Yet the chromaffine tissue cannot be 



