SECRETING GLANDS. ccxxi 



tain biliary matter, and others contain fat ; and in the recent soft part of 

 the epidermis and its appendages, it ia quite common to see cells filled 

 with pigment mixed with others which are colourless. 



A secreting apparatus, effectual for the purpose which it is essentially 

 destined to fulfil, may thus be said substantially to consist of a simple 

 membrane, named the membrana propria or basement-membrane (marked a 

 in the plan, fig. cxxvi. ), supporting a layer of secreting cells on one of its 

 surfaces (indicated by the dotted line 6, in the figure), whilst finely ramified 

 blood-vessels are spread over the other (c). But whilst the structure may 

 remain essentially the same, the configuration of the secreting surface, or 

 (what amounts to the same thing) of the supporting basement-membrane, 

 presents various modifications iii different secreting organs. In some cases, 

 the secreting surface is plain, or, at least, expanded, as in various parts of 

 the serous, syuovial, and mucous membranes, which may be looked on as 



Fig. CXXVI. 



Fig. CXXVL PLAN OF A SECRETING MEMBRANE. 



a, membrana propria or basement-membrane ; 6, epithelium, composed of secreting 

 nucleated cells ; c, layer of capillary blood-vessels. 



examples of comparatively simple forms of secreting apparatus ; but, in 

 other instances, and particularly in the special secretory organs named 

 glands, the surface of the secreting membrane is variously involved and 

 complicated. An obvious, and no doubt a principal, purpose of this com- 

 plication is to increase the extent of the secreting surface in a secreting 

 organ, and thus augment the quantity of secretion yielded by it. No 

 connection has been clearly shown to exist between the quality of the 

 secretion and the particular configuration, either internal or external, 

 of the organ ; on the other hand, we know that the same kind of secretion 

 that is derived from a complex organ in one animal, may be produced by an 

 apparatus of most simple form in another. 



The more immediate purpose of the complication of the secreting mem- 

 brane being to augment its surface within a comparatively circumscribed 

 space, two principal modes are found by which the membrane is so in- 

 creased in extent, namely, by rising or protruding, in form of a prominent 

 fold or some otherwise shaped projection (fig. cxxvii., d, e), or by retiring, 

 in form of a recess (fig. cxxvm., g, h). 



The first mentioned mode of increase, or that by protrusion, is not what is 

 most generally followed in nature, still it is not without example, and, as in- 

 stances, we may cite the Haversian fringes of the syuovial membranes', the 

 urinary organ of the snail, which is formed of membranous lamellae, and 

 perhaps, also, the choroid plexuses in the brain, and the ciliary processes in 

 the eye-ball, although secretion may not be the primary office of the last- 

 rneutioned structures. In most of these cases, the membrane assumes the 

 form of projecting folds, which, for the sake of further increase of sur- 

 face, may be again plaited and complicated, or cleft and fringed, at their 

 borders (fig. cxxvir., e, /). 



The plan of augmenting the secreting surface by recession or inversion of 

 the membrane, in form of a cavity, is, with few exceptions, that generally 



