OF SIMPLE ISOLATED CELLS FORMING SOLID TISSUES. 31 



greater part of their thickness ; and is so distinct from the subjacent 

 layers as to be readily separated from them. The elasticity of these 

 membranes is dependent upon the presence of the yellow fibrous 

 tissue in the areolar tissue. 



Mucous membranes are very speedily regenerated whenever they 

 have been destroyed either by injury or disease. They constitute 

 the medium through which all the changes are effected that take 

 place between the living organism and the external world. 



The character of the secretions of mucous membranes varies in 

 almost every part, and is dependent upon the properties of the 

 epithelial cells which cover them. In the ultimate tubuli of glands, 

 these cells are found to contain the peculiar substances which cha- 

 racterize the secretion. They are not mere protective agents, as 

 the epidermic cells are, but, in the case of the glands, they are con- 

 cerned in elaborating their peculiar secretions, and in the mucous 

 membrane of the small intestine, in effecting the absorption of nu- 

 trient materials. 



In the bronchio-pulmonary and gastro-intestinal mucous membrane, 

 we meet with the peculiar secretion called mucus^ which is intended 

 to lubricate the parts on which it is thrown out, and to protect them 

 from the action of irritating substances. It is also found in the 

 ducts of glands, and in the urinary and gall-bladders, but is gene- 

 rally mixed with the secretions of these parts. 



Mucus is a viscid, colourless, or slightly yellow fluid, not miscible 

 with water, and containing a substance called 'mucin, upon which its 

 characteristic properties depend. This appears to be an albuminous 

 compound, altered by the presence of an alkali. Mucus contains, 

 also, a small proportion of solid matter, and some salts, resembling 

 those of the blood, and epithelial scales, together with peculiar cor- 

 puscles called mucus corpuscles. 



OF SIMPLE ISOLATED CELLS, FARMING SOLID TISSUES BY THEIR 

 AGGREGATION. 



TBfe cells of this class begin and end their lives as such without 

 undergoing any transformation, but instead of lying upon free sur- 

 faces, or being cast off from them, they form part of the substance 

 of the fabric. Two examples of this kind are found in the cells de- 

 veloped at the extremities of the intestinal villi, and those which 

 exist at the terminal extremities of the hepatic ducts. Both these 

 sets of cells seem to have the power of selecting from the fluids pre- 

 sented to them certain materials which they deliver up, to the 

 absorbents in one case, and the hepatic duct in the other. An ana- 

 logous set of cells are found at the extremities of the foetal tufts 

 in the placenta, which have a like power of selecting nutritious 

 materials from the maternal blood for the nourishment of the foetus. 



The fat-cells, of which adipose tissue is composed, also perma- 

 nently exhibit the original type of structure in its simplest form. 



