282 SECRETION 



action of mucus is alkaline ; the only exception to this being the vaginal 

 mucus, which is very liquid and is distinctly acid. 



It is difficult to get an exact idea of the composition of normal 

 mucus, from the fact that the quantity secreted by the membranes in 

 their natural condition is small, being just sufficient to lubricate their 

 surface. All varieties, however, contain an organic matter called 

 mucin, which gives to the liquid its viscidity. They likewise present 

 a considerable variety of inorganic salts, as sodium chloride, potassium 

 chloride, alkaline lactates, sodium carbonate, calcium phosphate, a small 

 proportion of the sulphates, and in some varieties, traces of iron and 

 silica. Of all these constituents, mucin is the most important, as it 

 gives to the secretion its characteristic properties. Like other nitroge- 

 nous substances, mucin is coagulable by various reagents. It is im- 

 perfectly coagulated by heat ; and after desiccation it can be made to 

 assume its peculiar consistence by the addition of a small quantity of 

 water. It is coagulated by acetic acid and by a small quantity of the 

 strong mineral acids, being redissolved in an excess of the latter. It 

 also is coagulated by strong alcohol, forming a fibrinous clot soluble in 

 hot and cold water. Mucin may be readily isolated by adding water 

 to a specimen of normal mucus, filtering, and precipitating with an 

 excess of alcohol. If this precipitate, after having been dried, is ex- 

 posed to water, it assumes the viscid consistence peculiar to mucin. 

 This property serves to distinguish it from albumin and other nitroge- 

 nous matters. 



General Uses of Mucus. The smooth, viscid and adhesive character 

 of mucus, forming, as it does, a coating for the mucous membranes, 

 serves to protect these parts, enables their surfaces to move freely 

 one upon the other, and modifies to a certain extent the process of 

 absorption. Aside from these mechanical uses, it has been shown that 

 mucus, in connection with the epithelial covering of the mucous mem- 

 branes, is capable of preventing the absorption of certain substances. 

 It is well known, for example, that venoms may be applied with im- 

 punity to certain mucous surfaces, while they produce poisonous effects 

 if introduced into the circulation. These agents are not neutralized by 

 the secretions of the parts, for they will produce their characteristic 

 effects when removed from the mucous surfaces and introduced into 

 the circulation ; and it is reasonable to suppose that the mucous mem- 

 branes are capable of resisting their absorption. 



Physiological Anatomy of the Sebaceous, Ceruminous and Meibomian 

 Glands. The true sebaceous glands are found in all parts of the 

 skin that are provided with hair; and as nearly every part of the 

 general surface presents either the long, the short or the downy hairs, 



