TEGUMENTARY ORGANS. (J47 



the shaft of the hair becomes much more narrow than the 

 base from which it originates, and the fibrous structure is 

 most apparent on the peripheral or cortical portion of 

 the shaft. The fibrous composition of hair was described 

 and figured by Leuwenhoek. The nuclei of the cytoblasts 

 almost disappear, in the elongated cells forming, by their 

 lineal aggregation, the ultimate filaments of hair; and the 

 artificial separation of the constituent filaments, is ren- 

 dered much more easy, by macerating the hairs in dilute 

 muriatic acid, when they are seen to be disposed in a longi- 

 tudinal, rectilineal and parallel order, from the bulb to the 

 point of each hair. The filamentous structure and fibrous 

 decomposition of hairs were familiar also to Hooke in 1667. 

 The diameter of the ultimate fibrils of a hair is about the 

 two thousandth of a line, and a human hair of one tenth of 

 a line in thickness, has about two hundred and fifty fibrils in 

 its mere diameter, and about fifty thousand in its entire 

 calibre : so that these ultimate fibrils are finer than those of 

 almost any other known tissue, from the great elongation 

 and narrowing of their constituent cells, as they are drawn 

 out into the shaft of the hair during growth ; and hence the 

 expanded bulb of the hair, where the cells are yet spherical 

 and soft. 



In the larger hairs, bristles, and spines there is generally 

 a more compact, thin, dense cortical part, inclosing a loose 

 cellular medullary portion, not perceptible in the human 

 hairs ; so that they more resemble the shafts of feathers des- 

 titute of lateral barbs. The highly vascular and sensitive 

 hair-pulp in the long whiskers of carnivora, extends through 

 the bulb into the shaft, and increases the sensibility of these 

 parts, while it adds to their strength of attachment, and to 

 their surface of increment. The nails and claws of mam- 

 malia and other vertebrata are composed of the same epider- 

 mic cells, %nd grow in reduplications of the skin forming 

 compressed, curved follicles, like the cylindrical hairs and 

 spines in their circular cavities. The successive strata of 

 polygonal cells, which are most easily separable in the embryo 

 and foetus, are added, in lineal aggregations, to their covered 

 base, and their compact, dense, free portion is gradually pro- 

 truded from the compressed enveloping follicle. The con- 

 stituent cytoblasts with their nuclei and fluid contents, are 

 most distinct at the soft, white base, and at the inferior sur- 



