TISSUES OF THE BODY. 359 



of these structures known as the prostrate. The lungs might also be added 

 here on account of their structure and development. The gland vesicles, 

 almost always formed of a delicate membrana propria, vary in size from 

 (H128 to O0451 mm., with extremes in both directions. The contents 

 consists either of rounded, or more or less cubical cells. Some of them are 

 filled with a fatty secretion. We have already considered their efferent 

 ducts in the foregoing section. 



3. Turning, finally, to those glands consisting of entirely dosed roundish 

 cavities, the thyroid (fig. 352) may be taken as the type. . Here we 

 find a number of short glandular spaces of roundish form in a ground- 

 work of connective-tissue, having a diameter of 0'1128-0'0564 mm. and 

 less, and consisting of a fibrous wall (without any distinct membrana pro- 

 2)ria) with a coating of small round cells. In the Graafian vesicle of the 

 ovary, which is opened by rupture, and destroyed after expulsion of the 

 ovum and remaining contents, we have another more complicated cap- 

 sule, also imbedded in abundant, dense fibrous tissue. The interior is 

 lined by minute, round, nucleated cells, in the midst of which lies the 

 primitive ovum. 



199. 



As to the composition of glandular-tissue, to which we will now devote 

 a few lines, it is one of the most neglected subjects in histology. Even 

 of the nature of the membrana propria we know but little : its substance, 

 however, is no albuminous one. It consists rather of some material 

 difficult of solution, and offering a tolerably prolonged resistance to the 

 action of weak acids and alkalies, reminding us of the bearing of the 

 transparent membranes of the eye. Its power of resisting concentrated 

 alkalies is sometimes also considerable, in which cases this gland-enve- 

 lope may consist of elastin, an important point when we take into 

 account its indifferent nature and stability, and the great secretory energy 

 of the organ. In other cases this membrane is not so durable, and we have 

 not the slightest clue as to its composition. It need hardly be remarked 

 that at those points where, instead of a transparent homogeneous mem- 

 brane, a layer of connective-tissue presents itself, bounding the sub- 

 divisions of the organ, we have to deal with a glutin-yielding substance. 



The gland cells, the most important parts of the organ in question, 

 those, in fact, which constitute them, glands, have but little remarkable 

 about them excepting the contents of their bodies. Their membranes 

 consist, for the most part, of a matter which gives way even to the weaker 

 acids, but sometimes of a material possessing much greater power of resist- 

 ance, thus reminding us of many of the so closely allied epithelia. The 

 nuclei present the same peculiarities here as elsewhere. 



The matters, however, contained in these gland cells vary with the 

 species of secretion to be produced. Thus, for instance, we meet with 

 materials in the cells of the liver which are subsequently found free in 

 the bile, such as fats, pigments, and glycogen, which leads to the forma- 

 tion of sugar, and is carried off with the blood of the hepatic vein. In 

 the cells of the mammary gland, further, we have the butter fats of the 

 milk ; in those of the sebaceous glands, the fatty matters observed on the 

 skin ', in the gastric cells, the pepsin found in the juices of the stomach, 

 and so on. Muciri also is contained, together with other substances, in 

 those cells held to be the generators of mucus. 



Now, although the components of the secretions present themselves 

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