284 THE HUMAN BODY. 



of secreting surfaces are found on the serous membranes, but 

 are not common; in most cases an extended area is required 

 to form the necessary amount of secretion, and if this were 

 attained simply by spreading out plane surfaces, these from 

 their number and extent would be hard to pack conveniently 

 in the Body. Accordingly in most cases, the greater area is 

 attained by folding the secreting surface in various ways so 

 that a large area can be packed in a small bulk, just as a 

 Chinese lantern when shut up occupies much less space than 

 when extended, although its actual surface remains of the 

 same extent. In a few cases the folding takes the form of 

 protrusions into the cavity of the secreting organ as indicated 

 at (7, Fig. 104, and found on some synovial membranes; but 

 much more commonly the surface extension is attained in 

 another way, the basement membrane, covered by its epithe- 

 lium, being pitted in or involuted as at B. Such a secreting 

 organ is known as a gland. 



Forms of Glands. In some cases the surface involutions 

 are uniform in diameter, or nearly so, throughout (B, Fig. 

 104). Such glands are known as tubular; examples are found 

 in the lining coat of the stomach (Fig. 113); also in the skin 

 (Fig. 135), where they form the siveat-glands. In other cases 

 the involution swells out at its deeper end and becomes more or 

 less sacculated (E) ; such glands are racemose or acinous. The 

 small glands which form the oily matter poured out on the 

 hairs are of this type. In both kinds the lining cells near the 

 deeper end are commonly different in character from the rest; 

 and around that part of the gland the blood-vessels form a 

 closer network. These deeper cells form the true secreting 

 elements of the gland, and the passage, lined with different 

 cells, leading from them to the surface, and serving merely to 

 carry off the secretion, is known as the gland-duct. When 

 the duct is undivided the gland is simple ; but when, as is 

 more usual, it is branched and each branch has a true secret- 

 ing part at its end, we get a compound gland, tubular (&) or 

 racemose (F, If) as the case may be. In such cases the main 

 duct, into which the rest open, is often of considerable length, 

 ,so that the secretion is poured out at some distance from the 

 main mass of the gland. 



A fully formed gland, H, thus comes to be a complex 

 structure, consisting primarily of a duct, c, ductules, dd, and 

 secreting recesses, ee. The ducts and ductules are lined witli 



