476 



OF NUTRITION. 



[Fig. 110. 



[Fig. 111. 



A small portion of the Follicle of a Ha : r of the 

 Beard, with the Arteries supplying it very 

 highly m;ignified; 1, its follicle; 2, its pulp; 3, 

 the trunk of the hair without the follicle ; 4, 4, two 

 arteries going to the base of the follicle; 5,5, 

 their distribution; G. 6, the renticalated tissue of 

 the follicle.] 



The Root of one of the Hairs of the Beard, 

 with its Pulp and Follicle cons : derably magni- 

 fied; 1, a small portion of its trunk; 2, the corti- 

 cal substance ; 3, the medullary ; 4, the root of 

 the hair; 5, the bifid portion of the root, called the 

 bulb; 6, its excavated base, in which the pulp, 

 7, is inserted; 8, a small artery; 9, its distribution 

 to the pulp; 10, the membrane of the follicle of 

 the hair; 11, its base placed in the pulp of the 

 hair; 12, the cuticle or epidernrs of the follicle, 

 which joins with the bulb of the hair.] 



brane, distinct pigment-cells may often be seen. In several Mammalia, the 

 medullary substance is deficient at intervals ; giving to the hair a peculiar 

 appearance, often very characteristic of the species. A better notion of the 

 relations of these substances may be obtained, by the examination of the quill 

 of the Porcupine, or of the spine of the Hedgehog, which are nothing else 

 than Hair on a large scale. The interior of these is composed of a sort of 

 medulla, which, when examined with the Microscope, is found to consist entirely 

 of distinct cells, resembling those of the pith of Plants ; whilst the exterior is 

 formed by a cylinder of horny fibres, exactly resembling that of the Human 

 hair. The Chemical constitution of Hair has been already noticed, as being 

 the same as that of Nails, Horn, &c. ; that of the colouring matter, however, 

 is not precisely known. This substance is bleached by Chlorine ; and its hue 

 seems in part to be influenced by the presence of iron, w r hich is found in 

 larger proportion in dark than in light hair. It is probably nearly related to 

 Haematosine. 



624. It is quite evident that the active Growth of the Hair can only take 

 place at its base, where alone it is in connection with the vascular system ; but 

 the knowledge of its organized structure enables us to explain many pheno- 

 mena which were previously obscure. Thus, in the disease termed Plica Po- 

 lonica, a change takes place in the Hair, which often occurs at a distance from 

 its roots ; this change consists in the splitting of the hair into fibres, and the 

 exudation from it of a glutinous substance ; and these two causes unite in 

 occasioning that peculiar matting of the hair, which has given origin to the 

 name of the disease. It is said that bleeding takes place, in this disease, from 

 the stumps of hairs which are cut off close to the skin ; and this may be easily 

 credited, since the increased activity in the formative power of the cells of the 

 hair must require an increase in their supply of blood. It is very easy to un- 

 derstand, from the analogy of the Cellular Plants in which no vessels exist, how 



