610 VERTEBRATED ANIMALS. 



dered as constituting their epithelial lining ; these cells, in the 

 progress of their development, draw into themselves from the 

 blood the constituents of the particular product they are to 

 secrete; and they then seem to deliver it up, either by the- burst- 

 ing 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 distinguished 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 Insects, 

 these follicles are few in number, but are immensely elongated 

 so as to form biliary tubes, which lie loosely within the abdomi- 

 nal 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 folli- 

 cles 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 Vertebrated 

 animals, the salivary glands, the pancreas (sweetbread), and the 

 mammary glands, are well adapted to display the follicular struc- 

 ture; 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 yet been determined beyond doubt, whether these cells 

 are contained within any kind of membranous investment. The 

 kidneys of Vertebrated animals are made up of elongated tubes, 

 which are straight, and 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 

 " cortical." Certain flask-shaped dilatations of these tubes in- 

 clude curious little knots of bloodvessels, which are known as the 

 "Malpighian bodies" of the kidney; these are well displayed in 

 injected preparations. For such a full and complete investiga- 

 tion of the structure of these organs as the Anatomist and Phy- 

 siologist require, various methods must be put in practice, which 

 this is not the place to detail. It is perfectly easy to demonstrate 

 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 detached. The general arrangement of the cells in the 

 lobules, may be shown by means of sections thin enough to be 



