CHAPTER XIX 

 INTERNAL SECRETION 



An internal secretion may be defined as a more or less complex material 

 or agent, produced by the secretor activities of epithelial cells of organs and 

 tissues, and which are discharged into the blood and distributed to organs 

 more or less remote, the activities of which they influence in varying ways 

 and degrees. Some increase, some inhibit physiologic processes while others 

 stimulate growth and in different ways modify metabolism. The internal 

 secretion in many, if not all instances belongs to a class of agents known as 

 hormones, agents which according to Starling, are of known or unknown 

 composition, characterized by a relatively simple chemic or molecular com- 

 position, an easy diffusibility across the walls of the capillary blood-vessels, 

 a ready susceptibility to oxidation and a rapid elimination, as a result of 

 which, their action does not continue indefinitely. 1 



The internal secretions and the organs by which they are produced, 

 constitute a secondary coordinating mechanism, which supplement the pri- 

 mary coordinating nerve mechanisms of the body; the cooperation of both, 

 however, being necessary for the maintenance of the activities of individual 

 organs as well as of the activities of the body as a whole. 



Though the term internal secretion has been applied to various sub- 

 stances such as urea and carbon dioxid, which arise in consequence of tissue 

 metabolism and which, after being discharged into the blood, influence in 

 many ^ ways physiologic processes, yet the term is more applicable to the 

 secretions of organs possessing epithelial cells in anatomic relation to blood- 

 vessels. The glands by which these specific materials or hormones are 

 secreted are known as 



Glands of Internal Secretion or Endocrinous Glands. The glands 

 consist mainly of epithelial cells in close relation to the walls of capillary 

 blood-vessels and lymphatics, and in some instances, if not all, under the 

 control of the central nerve system. By reason of the absence of ducts and 

 their relation to blood-vessels they have also been termed ductless glands 

 and vascular glands and inasmuch as the secretion is discharged internally 

 (into the blood) they have been designated endocrinous glands. 



The glands which fall into this category are the thyroid, the parathyroids, 

 the hypophysis cerebri or pituitary, the adrenals, the pancreas, the ovaries, 

 and testicles. 



THE THYROID 



The thyroid gland or body consists of two lobes situated on the lateral 

 aspect of the upper part of the trachea (Fig. 221). Each lobe is pyriform 



1 By reason of the fact that some of the active principles of the internal secretions instead 

 of exciting, relax or inhibit activity, the term hormone for the class is inapplicable according to 



schafer, and he therefore suggests for the latter class, the term 'chalone' from \a\&w, to relax 

 PJz. m s *ack. Because they act like certain drugs he also proposes the term 'acoid' from 



akos, a remedy, to denote this quality, and to denote that they arise within the body, the prefix 

 aut can be employed. We thus get the complete expression autocoid substance to denote any 



irug-like principle which is produced in or can be separated from the internally secreting organs 

 and we may divide these autocoid substances into hormones or excitatory agents, and chalones 

 or inhibitory agents. 



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