280 THE MICROSCOPE AND ITS REVELATIONS. 



the saliva, bile, urine, etc.), are formed by the instrumentality of organs 

 termed Glands; which are, for the most part, constructed on one funda- 

 mental type, whatever be the nature of their product. The simplest idea 

 of a gland is that which we gain from an examination of the * follicles ' or 

 little bags imbedded in the wall of the stomach; some of which secrete 

 mucus for the protection of its surface, and others gastric juice. These 

 little bags are filled with cells of a spheroidal form, which may be con- 

 sidered as constituting their epithelial lining; these cells, in the progress 

 of their development, draw into themselves from the blood the constitu- 

 ents of the particular product they are to secrete; and they then seem to 

 deliver it up, either by the bursting or by the melting-away of their walls, 

 so that this product may be poured-forth from the mouth of the bag into 

 the cavity in which it is wanted. The Liver itself, in the lowest animals 

 wherein it is found, presents this condition. Some of the cells that form 

 the lining of the stomach in the Hydra and Actinia, seem to be distin- 

 guished from the rest by their power of secreting bile, which gives them 

 a brownish-yellow tinge; in many Polyzoa, Compound Tunicata, and 

 Annelida, these biliary cells can be seen to occupy follicles in the walls of 

 the stomach; in Insecta these follicles are few in number, but are im- 

 mensely elongated so as to form biliary tubes, which lie loosely within 

 the abdominal cavity, frequently making many convolutions within it, 

 and discharge their contents into the commencement of the intestinal 

 canal; whilst in the higher Mollusca, and in Crustacea, the follicles are 

 vastly multiplied in number, and are connected with the ramifications of 

 gland-ducts, like grapes upon the stalks of their bunch, so as to form a 

 distinct mass which now becomes known as the Liver. The examination 

 of the biliary tubes of the Insect, or of the biliary follicles of the Crab, 

 which may be accomplished with the utmost facility, is well adapted to 

 give an idea of the essential nature of glandular structure. Among Ver- 

 tebrated animals the Salivary glands, the Pancreas (sweet-bread), and 

 the Mammary glands, are well adapted to display the follicular structure 

 (Fig. 471); nothing more being necessary than to make sections of these 

 organs, thin enough to be viewed as transparent objects. The Liver of 

 Vertebrata, however, presents certain peculiarities of structure, which 

 are not yet fully understood; for although it is essentially composed, like 

 other glands, of secreting cells, yet it has not been determined beyond 

 doubt whether these cells are contained within any kind of membranous 

 investment. The Kidneys of Vertebrated animals are made-up of elon- 

 gated tubes, which are straight, and are lined with a pavement-epithelium 

 in the inner or ' medullary' portion of the kidney, whilst they are con- 

 voluted and filled with a spheroidal epithelium in the outer or ' corti- 

 cal.' Certain flask-shaped dilatations of these tubes include curious little 

 knots of blood-vessels, which are known as the ' Malpighian bodies ' of 

 the kidney; these are well displayed in injected preparations. For such 

 a full and complete investigation of the structure of these organs as the 

 Anatomist and Phosiologist require, various methods must be put in prac- 

 tice which this is not the place to detail. It is perfectly easy to demon- 

 strate the cellular nature of the substance of the Liver, by simply scraping 

 a portion of its cut surface; since a number of its cells will be then de- 

 tached. The general arrangement of the cells in the lobules may be dis- 

 played by means of sections thin enough to be transparent; whilst the 

 arrangement of the blood-vessels can only be shown by means of Injections 

 ( 687). Fragments of the tubules of the Kidney, sometimes having the 

 Malpighian capsules in connection with them, may also be detached by 



