CHAP.XXXII.] STRUCTURE OF GLANDS. 453 



of parts, which we have before designated the mucous system, in- 

 cluding, under one common title, the skin, mucous membranes, 

 and true glands ; the term, true glands, meaning those which, by a 

 duct, or otherwise, pour their secretion on the external surface, and 

 not into the blood. For a summary account of the tissues forming 

 this order of parts, the note at p. 162 of the present volume should 

 be referred to. The skin, and most of the great regions of the 

 mucous membranes, together with some of the glands, have been 

 already minutely described in different parts of the preceding 

 pages ; and it now only remains, before passing to a description, 

 in detail, of the great remaining glands, to offer some observations 

 on the general plan or scheme of structure, discoverable in these 

 special organs of secretion. 



Between skin, mucous membranes, and glands, there exists every 

 gradation of structure, by which one can be conceived to pass into 

 the other. They are modifications of a common type. The two 

 former are secreting organs, in an expanded form, sometimes pre- 

 senting glandular involutions in their thickness. But, in proportion 

 as the glands differ from mere membranes, we find them assuming 

 a more solid form, gathered up, as it were, into a more compact 

 mass, in which are to be still recognised all the elements of the 

 simple membrane in their true relative positions the free surface 

 being still the free surface, though now forming, it may be, the 

 lining of ducts, and composing the internal tubular, or follicular 

 recesses of the solid organ, and the deep, or vascular surface 

 preserving the same relation as before to blood-vessels, lymphatics, 

 nerves, or areolar tissue, under its various modifications. In par- 

 ticular, the epithelium, or proper glandular tissue, remains capable 

 by its anatomical position in regard to the external surface, to 

 discharge its product on that surface. Thus the epithelium of the 

 parotid gland, the liver, or the kidney, has such a relation to 

 the remote parts of the excretory ducts of those organs, as that 

 the secretion, resulting from the mutual action of that tissue, 

 and the blood, is set free into those channels, which are, in reality, 

 continuations of the integument, and, therefore, in one sense, 

 portions of the external surface of the body. 



It will be readily seen how close and intimate must be, in all 

 cases, the proximity of the glandular epithelium to the ducts. In 

 fact, in many instances, the epithelium of the ducts is beyond doubt 

 the secreting tissue, and the gland is a congeries of finely divided 

 passages, or ducts, whose epithelium (though in a modified form) is 



