312 HANDBOOK OF PHYSIOLOGY. 



The Ductless Glands 

 AND INTERNAL SECRETIONS. 



The discovery of the remarkable and sometimes fatal effects of the 

 removal of certain of the ductless glands has given a marked impetus to 

 the study of these organs, so that at the present time they occupy a place 

 of importance in physiology formerly unthought of. The converse ef- 

 fects of removing certain of these glands and of injections of extracts 

 (aqueous and others) of them into healthy animals or those operated upon, 

 have led to the belief that they elaborate in the course of their metabolic 

 activity some substance or substances which are of use to the body. 

 Since the parenchyma cells of these glands belong morphologically to 

 the secretory type, and since active constituents may be extracted from 

 the glands, it is assumed that they produce a secretion. But this secre- 

 tion, whatever its quantity may be, passes either into the blood stream 

 directly (supra-renal) or indirectly by way of the lymphatics (thyroid), 

 instead of discharging through a duct upon a free surface, as in the 

 case of the salivary glands and others. Hence the term internal secretion 

 has come into popular use by way of distinction. 



It must be borne in mind, however, that both anabolic and katabolic 

 products are formed by all tissues and are absorbed to a greater or less 

 extent into the circulation. But the term internal secretion does not 

 apply to these. It is confined to such products as are formed by organs 

 of a distinctly glandular type. 



The glands which are known certainly to form internal secretions are 

 the thyroid, the supra-renal capsules, the pancreas, and possibly the pitu- 

 itary body. And Howell has called attention to the fact that to be 

 consistent the glycogen formed by the liver from dextrose (and proteid) 

 should be regarded as an internal secretion. Thus the liver forms both 

 an internal and external secretion, as in the case of the pancreas. 



The spleen has been included in this chapter for convenience. It 

 has not been proved to form an internal secretion. 



The Thyroid. The thyroid gland is situated in the neck. It con- 

 sists of two lobes, one on each side of the trachea, extending upward to 

 the thyroid cartilage, covering its inferior cornu and part of its body; 

 these lobes are connected across the middle line by a middle lobe or 

 isthmus. The thyroid is covered by the muscles of the neck. It is 

 highly vascular, and varies in size in different individuals. 



Structures. The gland is encased in a thin transparent layer of dense 

 areolar tissue, free from fat, containing elastic fibres. This capsule sends 

 in strong fibrous trabeculaB, which inclose the thyroid vesicles which are 

 rounded or oblong irregular sacs, consisting of a wall of thin hvaline 



