This section is from the book "Human Physiology For The Use Of Elementary Schools", by Charles Alfred Lee. Also available from Amazon: Human Physiology, for the Use of Elementary Schools.
16. The mucous coat is the seat of some of the most important functions of the economy ; in the lungs, of respiration ; in the stomach, of digestion; and in the mouth and nose, of taste and smell, etc, and forms, with scarcely an exception, a continuous whole. That portion which lines the eyes and eye lids, is connected with that which lines the nostrils, by means of the nasal canal; while that which lines the mouth, meets in the throat with that which comes down from the nose. In the fauces it divides, and while one portion goes down the windpipe into the lungs, the other passes down the esophagus into the stomach, forming a lining for the whole tract of the intestinal canal.
17. Mucous membranes are of a loose, spongy texture, of a reddish colour, and are largely supplied with blood vessels and nerves. They are also numerously supplied with small glandular bodies called mucous glands or folicles. The chief use of these membranes is, to sheath and protect the inner surfaces of the body, as the skin does the outer, and by means of the mucus, with which they are always covered in a state of health, to guard them against the contact of irritating substances.
18. We find a remarkable sympathy existing between all the mucous membranes ; accordingly their diseases, particularly the catarrhal affections, to which they are often subject, are very apt to spread in them. By reason of this sympathy, the state of one part of these membranes may be ascertained by examining another part ; the state of the tongue for example, indicates the condition of that of the stomach and intestinal canal. There is also an extraordinary sympathy between the mucous membranes and the respiratory organs.
This membrane, called also cutis or skin, is not only directly continuous with the mucous membranes that line the internal passages, but is also analagous in its structure. We see them passing into each other at the orifices of the internal canals, as in the lips, nostrils, eye lids, external ear, etc. We find it varying in thickness according to its situation ; in the face and body being thin and delicate, and on the palms of the hands and soles of the feet, considerably firmer and thicker. The skin is abundantly supplied with blood vessels and nerves; so numerous indeed, that the finest needle cannot enter the skin without piercing many of both, which is proved by the bleeding, as well as the pain which follow.
20. The skin, in man, is made up of four parts, or layers, the cuticle, rete mucosum, corpus papillare, and corium. It is covered externally by the cuticle or epidermis, which is destitute of nerves and blood vessels, insensible, and probably inorganic. It is the cuticle which is raised by the application of a blister, or by a burn or scald. It is supposed to be a secretion from the true skin which concretes on the surface becomes dry, and thus offers protection to the parts beneath, and also serves to prevent excessive absorption or the escape of the fluids by evaporation. It is pierced by little pores, for the passage of hairs, sweat, and the fluids, taken up by absorption, although Humboldt asserts that he could not discover them with a miscroscope which magnified 312,000 times.
21. The next layer of the skin is the rete mucosum, or mucous web. It has been supposed to consist chiefly of mucus, as it is of a soft pulpy texture. It is the seat of the coloring matter in the different races. In the European, it is white, in the African black, and in the Mulatto and Malay, copper coloured. In the Ethiopian race, it is much thicker than in the light coloured varieties of the human species, and may easily be separated both from the cutis and cuticle, and made to appear as a distinct membrane.
22. The dark color of the skin in the inhabitants of the torrid zone, is ascribed by most physiologists, to the influence of the sun upon the surface of the body ; but the tinge produced on the skin by exposure to a bright light, appears to have no connection with the permanent colour of the negro. Dr. Bostock states, that the blackest complexions are not found in the hottest regions, and that there are some tribes nearly under the equator, whose skin is whiter than that of many Europeans. Besides, the tinge produced by the sun is not transmitted from parents to their offspring, whereas the children of negroes are equally black in whatever climate they are born, and their com. plexion is not altered by any number of generations. When a person is tanned, as it is called, by the influence of the sun, it is the cuticle only which is affected ; if this be removed by a blister, in a few days new cuticle will be formed, and the skin in that place will appear as white as it ever did.
In the Caucasian race, from which Europeans and Americans have descended, there is usually a mixture of red or brown, with white in the complexion ; but where the skin is of a uniform, clear, pearly whiteness, the individuals are called Albinos, from albus, white. In these persons, the hair is generally white, corresponding with the colour of the skin, while the eye is without that substance which gives the various colour's to the iris. The iris, in such cases, is generally of a bright rose colour, and the eyes are so sensible to light, that they cannot be kept open in the sun shine, although in the shade, or dusk of the evening, the vision is perfect. It is now well ascertained that the redness of the eye and the whiteness of the skin in albinos depends on the same physical defect in their organization, and that it is owing to the absence of the colouring matter in the rete mucosum. In the eye, this matter, spread over the retina, is called Pigmentum nigrum, or the black paint.
24. What is called the Corpus papillare, is merely a collection of small 'papillae, formed by the extremities of nerves and blood vessels, and lying immediately under the mucous web, etc. It is in this layer that the sense of touch resides ; the papillae can easily be seen when the cuticle has been removed by a blister.