SECRETION. 



153 



if we neglect the existence of the villi and crypts, as representing a secreting 

 surface constructed on this type. If we suppose such a membrane to become 





\fe^ 



FIG. 64.-To illustrate the simplest form of a tubular and a racemose or acinous gland. 



invaginated to form a tube or a sac possessing a definite lumen (see Fig. 64), 

 we have then what may be designated technically as a gland. 



It is obvious that in this, case the gland may be a simple pouch, tubular or 

 saccular in shape (Fig. 65), or it may attain a varying degree of complexity by 

 the elongation of the involuted portion and the development of side branches 



FIG. 65. Simple alveolar gland of the 

 amphibian skin (after Flemming). 



FIG. 66. Schematic representation of a lobe of a 

 compound tubular gland (after Flemming). 



(Fig. 66). The more complex structures of this character are known sometimes 

 as compound glands, and are further described as tubular, or racemose (saccular), 

 or tubulo-racemose, according as the terminations of the invaginations are 

 tubular, or saccular, or intermediate in shape. 1 As a matter of fact we find 

 the greatest variety in the structure of the glands imbedded in the cutaneous 

 and mucous surfaces, a variety extending from the simplest form of crypts or 

 tubes to very complicated organs possessing an anatomical independence and 

 definite vascular and nerve-supplies as in the case of the salivary glands 

 or the kidney. In compound glands it is generally assumed that the terminal 

 portions of the tubes alone form the secretions, and these are designated as the 

 the acini or alveoli, while the tubes connecting the alveoli with the exterior are 

 known as the ducts, and it is supposed that their lining epithelium is devoid 

 of secretory activity. 



The secretions formed by these glands are as varied in composition as the 

 glands are in structure. If we neglect the case of the so-called reproductive 



1 Flemming has called attention to the fact that most of the so-called compound racemose 

 glands, salivary glands, pancreas, etc., do not contain terminal sacs or acini at the ends of the 

 system of ducts ; on the contrary, the final secreting portions are cylindrical tubes, and such 

 glands are better designated as compound tubular glands. 



