SECRETING GLANDS. 99 



the primary one. as in Fig. 71, D and E, the gland is then termed a compound 

 one. These secondary invaginations may assume either a saccular or tabular form, 

 and so constitute the two subdivisions the compound saccularor racemose gland, 

 and the compound tubular. The racemose gland in its simplest form consists of a 

 primary invagination which forms a sort of duct, upon the extremity of which are 

 found a number of secondary imaginations called saccules or alveoli, as in Brun- 

 ner's glands (Fig. 71. D). But, again, in other instances, the duct, instead of being 

 simple, may divide into branches, and these again into other branches, and so on ; 

 each ultimate ramification terminating in a dilated cluster of saccules, and thus we 

 may have the secreting surface almost indefinitely extended, as in the salivary 

 glands (Fig. 71, E). In the compound tubular glands the division of the pri- 

 mary duct takes place in the same way as in the racemose glands, but the branches 

 retain their tubular form, and do not terminate in saccular recesses, but become 

 greatly lengthened out (Fig. 71, r). The best example of this form of gland is to 

 be found in the kidney. All these varieties of glands are produced by a more or 

 less complicated invagination of a secreting membrane, and they are all identical 

 in structure ; that is to say, the saccules or tubes, as the case may be, are lined 

 with cells, generally spheroidal or columnar in figure, and on their outer surface 

 is an intimate plexus of capillary vessels. The secretion, whatever it may be, is 

 eliminated by the cells from the blood, and is poured into the saccule or tube, and 

 so finds it way out through the primary invagination on to the free surface of the 

 secreting membrane. In addition, however, to these glands, which are formed by 

 an invagination of the secreting membrane, there are some few others which are 

 formed by an evayination or protrusion of the same structure, as in the vascular 

 fringes of synovial membranes. This form of secreting structure is not nearly so 

 frequently met with. 



