INTRODUCTION 875 



A. Introduction 



The endocrine glands are those glands which produce hormonal secretions. 



The term hormone is derived from a Greek word meaning to stimulate or to 

 stir up. Selye in 1948 (p. 11) defined hormones as "physiologic, organic com- 

 pounds produced by certain cells for the sole purpose of directing the activities 

 of distant parts of the same organism." 



The endocrine organs may be separated into two main groups: 



( 1 ) purely endocrine glands, and 



(2) mixed endo-exocrine glands. 



Purely endocrine glands have as their sole function the production of hor- 

 mones. Under this heading are included the pituitary (hypophysis), thyroid, 

 parathyroid, pineal, adrenal (suprarenal), and thymus glands. 



Mixed endo-exocrine glands are exemplified by the pancreas, liver, duo- 

 denum, and reproductive organs. Parts of these organs are purely exocrine, 

 e.g., the pancreas where pancreatic juice is produced by the acinous cells 

 but which elaborates, at the same time, insulin from the islets of Langerhans. 

 The liver elaborates the exocrine secretion, bile, which is discharged through 

 the bile ducts and, concurrently, manufactures the antipernicious-anemia factor 

 which is dispensed into the blood stream directly. The duodenum produces 

 digestive substances and also secretin. Secretin is elaborated by the epithelial 

 lining cells of this area, and it stimulates the pancreas to secrete its pancreatic 

 juice. 



Relative to their secretory activities all endocrine glands have this physio- 

 morphological feature in common: They discharge the hormonal or endocrine 

 substance directly into the blood stream without the mediation of a duct 

 system. Endocrine glands, therefore, are distinguished by this process from 

 exocrine glands, which exude the secretory product into a duct system from 

 whence the secretion passes to the site of activity. 



B. Morphological Features and Embryological Origin of the 

 Endocrine Glands 



1. Pancreas 



The islets of Langerhans are small masses of cells or islands scattered 

 among the acini (alveoli) of the general pancreatic tissue. The pancreatic 

 islets appear to arise as specialized buds from the same entodermal cords 

 which give origin to the alveoli. The islets separate early from the entodermal 

 cords and produce isolated cellular cords. Blood capillaries form a meshwork 

 within these cords of cells (figs. 295G; 365A). Their secretion, insulin, is 

 concerned with sugar metabolism and prevents the malfunction known as 

 diabetes. 



Pancreatic islets are found extensively in the vertebrates and generally are 



