22 GENERAL CONCEPTIONS. 



thelial gland. In such structures the secretion can escape only by transfusion into 

 the blood or lymph. 



The epithelial glands with ducts exhibit two main sorts of modification, for 

 they are either small structures which occur in great numbers, or they are larger 

 structures, each constituting a separate organ. Hence, we divide the glands into 

 simple glands and compound or organic glands. The simple glands are always 

 small and have one or several centers of growth, according as they are simple 

 tubes or slightly branching. Those of each kind are always very numerous, and 

 occur more or less together over considerable areas. Good illustrations of simple 

 glands are offered by the sweat glands and the intestinal glands. The compound 

 glands are of greater bulk. They are provided with a single main duct which is 

 more or less branched, each branch connecting finally with the secretory portion 

 proper of the organ, which portion may itself also be branched. Each gland fall- 

 ing in this division is a more or less complete organ by itself, receiving its special 

 blood supply and its special innervation it is, in short, a clearly marked physio- 

 logical entity. A specially striking morphological modification in the structure of 

 compound glands is offered by the liver. In this organ, branches of the glands 

 unite and form an anastomosing gland structure, in connection with which we 

 observe that the branches of the gland are not associated with a development of 

 connective tissue and of blood capillaries between the epithelial elements of the 

 organ, as in other compound glands; but are associated, on the contrary, with the 

 presence of a sinusoidal circulation. We must therefore regard the liver as a type 

 by itself. 



Another class of secreting organs may be termed false glands, as they never 

 have ducts at any stage of their development. Their chemical product is termed 

 an internal secretion, and is removed by transfusion into the blood. One division 

 of the false glands is derived from the growth of epithelium, while another division 

 arises by modifications in mesenchymal cells. As regards the glands of the cyto- 

 genic class, we have to deal with those which produce the free wandering cells, of 

 which the most familiar example is the white corpuscle of the blood; those which 

 produce the red corpuscles of the blood; and, finally, those which produce the 

 genital elements. 



As the student proceeds in his study of embryology, he will have clear illustra- 

 tions of the development and morphology of all the various sorts of glands. He 

 will find it advantageous, as his acquaintance with glands increases, to consult the 

 classification of glands as presented in the following table, based upon the very 

 important morphological distinctions pointed out in the preceding paragraph. 

 Formerly the classification of glands was based upon relatively unimportant details 

 of their microscopic form, and not upon true morphological differences. Hence 

 the classification here proposed differs radically from those in vogue up to the 

 present time. 



