STRUCTURE OF HAIR. EPITHELIUM. 143 



solvent power of chemical re-agents, except when these are employed in un- 

 usual strength. The Hair is constantly undergoing elongation, by the addi- 

 tion of new substance at its base ; and the part which has been once fully 

 formed, and which has emerged from the follicle, usually undergoes no sub- 

 sequent alteration. There is evidence, however, that it may be affected by 

 changes at its base, the effect of which is propagated along its whole extent : 

 thus, it is well known that cases are not unfrequent, in which, under the in- 

 fluence of strong mental emotion, the whole of the hair has been turned to 

 gray, or even to a silvery white, in the course of a single night; a change 

 which can scarcely be accounted for in any other way thlin by supposing that 

 a fluid, capable of chemically affecting the colour, is secreted at the base of 

 the hair, and transmitted by imbibition through the medullary substance to 

 the opposite extremity. Another evidence of their retention of a degree of 

 vitality, is found in the fact of Hairs having a tendency to become pointed, 

 after having been cut short off. In the hairs of some animals (particularly 

 the whiskers of the Seal and other Carnivora) the base is hollow, and con- 

 tains a true papilla, or elevation of the cutis, furnished with nerves and blood- 

 vessels ; this is separated by a layer of basement-membrane from the proper 

 tissue of the Hair. In such cases, there is bleeding from the stumps of the 

 hairs, when they are shaved off close to the skin. There is an approach to 

 this papillary structure in Man; and it may perhaps be an abnormal develop- 

 ment of it, which occasions the hair to bleed in the disease termed Plica Po- 

 lonica. The hair of individuals affected with it is further disposed to split 

 into fibres, often at a considerable distance from the roots, and to exude a 

 glutinous substance ; these two causes unite in occasioning that peculiar mat- 

 ting of the hair, which has given origin to the name of the disease. 



169. The layer of cells covering the internal free surfaces of the body, is 

 known under the name of Epithelium. In some instances it appears to serve 

 to the subjacent membranes, like the Epidermis to the Cutis, merely as a pro- 

 tection; whilst in other cases, as we shall presently find, it answers purposes 

 of far greater importance. It has long been known that the epidermic layer 

 might be traced continuously from the lips to the mucous membrane of the 

 mouth, and thence down the o?sophagus into the stomach ; and that, in the 

 strong muscular stomach or gizzard of the granivorous birds, it becomes quite 

 a firm horny lining. But it has been only since the application of the Mi- 

 croscope to this investigation, that a continuous layer of cells has been traced, 

 not merely along the whole surface of the mucous membrane lining the ali- 

 mentary canal, but likewise along the free surfaces of all other Mucous Mem- 

 branes, with their prolongations into follicles and glands ; as well as on the 

 Serous and Synovial membranes, and the lining membrane of the heart, blood- 

 vessels, and absorbents. 



170. The forms presented by the Epithelium cells are various. The two 

 chief, however, are the tesselated, forming the pavement-epithelium; and the 

 cylindrical, forming the cylinder-epithelium. The Tesselated Epithelium 

 covers the serous and synovial membranes, the lining membrane of the blood- 

 vessels, and the ultimate follicles or tubuli of most glandular structures con- 

 nected with the skin or mucous membranes, as also the mucous membranes 

 themselves, where the cylinder-epithelium does not exist. The cells compos- 

 ing it are usually flattened and polygonal (Fig. 38, A.) so as to come into con- 

 tact with each other at their edges, like the pieces of a tesselated pavement 

 (Fig. 30) ; but they sometimes retain their rounded or oval form, and are se- 

 parated from each other by considerable interstices. (Fig. 38, B.) This last 

 form seems to be the commonest, where the cells are most actively renewed, 

 so that they have not time (so to speak) to be developed into a continuous 

 stratum. The number of layers is commonly small; and sometimes there is 



