388 THE SOLIDS. 



ART. XXI. GLANDS. 



MUCH uncertainty has until recently prevailed as to what 

 really constitutes a gland : some have considered those only 

 as true glands which are furnished with distinct openings or 

 excretory ducts ; others again have regarded those organs 

 only as glands which give forth a secretion : the former view 

 of a gland would exclude the vascular glands as well as some 

 others, and the latter, although it would include these, and, 

 doubtless, also every other glandular structure, is not a suffi- 

 ciently obvious or structural character, the secreted product 

 of glands in many cases being furnished in small and scarcely 

 appreciable quantities, and which again are immediately on 

 their formation, absorbed, in some cases, into the circulation. 

 Again, secretions are supplied by parts and extended surfaces 

 to which, although they are essentially glandular, the term 

 gland would scarcely be applicable ; of this nature are the 

 free surfaces of all membranes covered by epithelium, as the 

 mucous, serous, synovial, &c. 



We have still then to inquire, first, what are the essential 

 structural elements of which all glandular organs or parts are 

 constituted ? and, second, what is the exact definition to be 

 given of a gland ? 



The researches of modern microscopists have indisputably 

 proved the fact, that, the only essential elements of a secreting 

 structure are granular cells and a circulating fluid., the se- 

 creted product being formed out of the latter by the vital 

 endowments resident in the former. 



According to this definition, the blood itself is to a con- 

 siderable extent glandular, inasmuch as it contains a vast 

 number of granular cells to which the elaboration of the 

 fibrin is, in all probability, due. 



Again, the free surfaces of all membranes are glandular ; 

 the epithelium covering them constituting the one essential 

 glandular element : the varieties in the size and form of the 



