150 ON THE ELEMENTARY PARTS OF THE HUMAN FABRIC. 



interior of the various glands, are the instruments by which their respective 

 products are eliminated from the blood. And lastly, the Skin is concerned 

 in two great classes of changes; the excretion of various matters from its 

 surface, and from the glandulse in its substance; and the reception of impres- 

 sions upon the nerves, with which it is so copiously supplied. 



179. The character of the secretions formed by the Mucous Membranes, 

 is different in almost every part; and is dependent, as will be shown here- 

 after, upon the properties of the Epithelium-cells which cover them. These 

 cells, instead of forming a comparatively permanent stratum, like that which 

 covers the surface of serous membranes, are in a state of continual change 

 and renewal; the older layers falling off, whilst new ones are produced in 

 immediate contact with the subjacent membrane, and this, not merely on its 

 simple plane surfaces, but on its prolongations, whether these form the cover- 

 ings of villi, or the lining of follicles. The purpose of the cells which form 

 the Epidermis, is simply to protect the sensitive surface of the true skin; and 

 these cells have the power of drawing a horny matter into their interior. On 

 the other hand, the Epithelium cells of the ultimate tubuli or vesicles of glands, 

 contain the substances which characterize the secretions of those glands. It 

 is chiefly on the bronchio-pulmonary and gastro-intestinal mucous membranes, 

 that we meet with the peculiar secretion termed Mucus ; which appears to be 

 expressly formed to shield them from the irritation they would suffer through 

 the contact of air, or of solids or liquids. This secretion is also found on 

 the lining membrane of the larger excretory ducts of most of the glands; and 

 it is mixed, in greater or less amount, with most of the secretions discharged 

 by them. It is found also upon the lining membrane of the gall-bladder, 

 and of the urinary bladder. When these membranes are in a state of unusual 

 irritation, the amount of mucus which they discharge is very considerable ; 

 but it ordinarily forms an extremely thin layer. The characters of Mucus, 

 obtained from various sources, are by no means invariable. In general, how- 

 ever, it may be described as a fluid of peculiar viscidity, either colourless or 

 slightly yellow, transparent or nearly so, incapable of mixing with water, and 

 sinking in it, except when buoyed up by bubbles entangled in its mass, 

 which is commonly the case with the bronchial and nasal mucus. This 

 fluid contains from 4 to 65 per cent, of solid matter, of which a small part 

 consists of salts resembling those of the blood : whilst the chief organic con- 

 stituent is a substance termed Mil tin, to which the characteristic properties of 

 the secretion are due. This appears to be an albuminous compound, altered 

 by the action of an alkali ; for as Dr. Babington has shown, any albuminous 

 fluid may be made to present the peculiar viscidity of mucus, by treating it 

 Avith liquor potassae. That the mucin of Mucus is held in solution by an 

 alkali, appears from this, that it is readily precipitated by acids, which neu- 

 tralize the base ; and that a sort of faint coagulation may be induced even by 

 water, which withdraws the base from it. When Mucus is examined with 

 the Microscope, it is found to contain numerous epithelium-scales (or flattened 

 cells) ; together with round granular corpuscles, considerably larger than 

 those of the blood, and closely resembling the nuclei of the epithelium-cells, 

 which are commonly termed mucus-corpuscles. In the more opaque mucus, 

 discharged from membranes in a state of irritation or inflammation, these cor- 

 puscles are present in greatly-increased amount; and cells are often developed 

 around them. 



8. Of Simple Isolated Cells, forming Solid Tissues by their Aggregation. 



180. We now proceed to a class of Cells, which are equally independent 

 of each other, which begin and end their lives as cells without undergoing any 



