AUGMENTATIOX OF SECRETIXG SUEFACE. 233 



of young cel^s, spermatic filaments, starch-granules, oil, various colouring 

 matters, and other new compounds, in tlie interior of cells. 



Both in animals and plants, the individual cells which are associated 

 together on the same secreting surface may differ from each other in the 

 nature of their contents. Thus, in the liver of mollusca some cells con- 

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

 of the epidermis and its appendages, it is 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 



Fix. 160. 



Fig. 160. — Plan of a- Secreting Membrane. 



a, membrana propria or basement-membi-ane ; b, epithelium, composed of secreting 

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



(marked a in the plan, fig. 160), supporting a layer of secreting cells 

 on one of its surfaces (indicated by the dotted line b, in the figure), 

 whilst finely ramified blood-vessels are spread over the other (c). Buc 

 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 in 

 different secreting organs. In some cases, the secreting surface is plain, 

 or, at least, expanded, as in various parts of the serous, synovial, and 

 mucous membranes, which may be looked on as examples of compara- 

 tively 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 complication 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 qualitij 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 

 membrane being to augment its surface within a comparatively circum- 

 scribed space, there are two principal modes by which the membrane 

 is so increased in extent, namely, by rising or protruding in form of a 

 prominent fold or some otherwise shaped projection (fig. 161, d, e), or 

 by retiring, in form of a recess (fig. 162, //, 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 instances, we may cite the Haversian fringes of the 

 synovial membranes, the urinary organ of the snail, which is formed of 

 membranous lamellae, and perhaps, also, the choroid plexuses in the 



