STRUCTURE OF THE HUMAN BODY. 33 



18. We find a remarkable sympathy existing between all 

 the mucous membranes ; accordingly their diseases, particu- 

 larly the catarrhal affections, to which they are often sub- 

 ject, are very apt to spread in -them. By reason of this sym. 

 pathy, the state of one part of these membranes may be as- 

 certained 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 sym- 

 pathy between the mucous membranes and the respiratory 

 organs. 



19. Dermoid Membraite. This membrane, called also 

 cutis or skin, is not only directly Continuous with the mu- 

 cous membranesTJthat 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, nos- 

 trils, eye-lids, external ear, &c. We find it varying in thick- 

 ness 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 nume- 

 rous indeed, that the finest needle cannot enter the skin 

 without piercing many of both, which is proved by the bleed- 

 ing, 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;fcuticleSor epidermis, which is 

 destitute of nerves and blood-vessels, insensible, and probably 

 inorganic. It is the cuticle which is raised by the applica- 

 tion of a blister, or by a burn or scald, fit 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, -&nd the fluids, taken up by ab- 

 sorption, although Humboldt asserts that he could not discover 

 them with a miscroscope which magnified 312,000 times. 



