256 



OP THE PRIMARY TISSUES OF THE HUMAN BODY. 



trades or lengthens out the lower end of the hair-follicle into a process, at the 

 bottom of which is found the generative papilla (c) now, by the interposition 

 of the new cell-growth, withdrawn from the root of the hair. The newly-formed 

 mass of cells occupying the lower or prolonged part of the follicle, and rest- 

 ing on the papilla (B, m), is gradually converted into a new hair (c, f, g, h) with 

 its root-sheath (6), just as happens in the primitive process of formation in the 

 embryo j and as the new hair lengthens and emerges from the follicle (D, </), 

 the old one (d, e), detached from its matrix, is gradually pushed nearer to the 

 opening, and at last falls out, its root-sheath having previously undergone a partial 



absorption. A similar death of the old 



Fig. 47. hairs, and replacement by new ones gene- 



rated within the same follicles, seem to 

 take place at intervals through the whole 

 of life (Fig. 47) ; and it is obvious, as 

 Mr. Paget has pointed out, that the 

 death of the old hair is not the conse- 

 quence of absorption at its root, caused 

 by the development of a new one beneath 

 it, but is simply the termination of a 

 series of degenerative changes that have 

 been for sometime in progress. 1 This is 

 one illustration, out of many that might 

 be cited, of the general fact of the 

 limited duration of the individual parts 

 of the living organism ( 114) ; the in- 

 tegrity of which is maintained by the 

 continual development of new structure, 

 in place of that which has become effete. 

 The regeneration of Hairs which have 

 been plucked out of their follicles 

 is very complete ; provided the follicles 

 themselves, and their papillae, have not 

 been injured. The cavity of the follicle 

 (according to the observations of Heu- 

 singer) is at first filled with blood, which 

 is gradually absorbed ; a dark spot, con- 

 sisting of a cluster of newly-formed epi- 

 dermic cells containing pigmentary mat- 

 ter, is seen upon the summit of the pap- 

 illa; and this gradually elongates itself, 

 and undergoes development into the 

 several parts of the new hair and of its 

 sheath, just as in the case of the first 

 evolution. 



Intended to represent the changes undergone 

 by a hair towards the close of its period of exist- 

 ence. At A its activity of growth is diminishing) 

 as shown by the small quantity of pigment con- 

 tained in the cells of the pulp, and by the inter- 

 rupted line of dark medullary substance. At B 

 provision is being made for the formation of a 

 new hair, by the growth of a new pulp connected 

 with the pulp or capsule of the old hair. c. A 

 hair at the end of its period of life, deprived of its 

 sheath and of the mass of cells composing the pulp 

 of a living hair. 



3. Of the purely Cellular Tissues; Fat and Cartilage. 



248. The Adipose tissue, which is only second to the Areolar in the extent 

 of its diffusion through the Human body, continues throughout life, to present 

 the primitive cellular type in its purest form ; this tissue, wherever it occurs, 

 being composed of an aggregation of cells, which never depart widely from the 

 spheroidal form (Fig. 48), the chief alteration in shape which they undergo 

 being the flattening of their walls from mutual pressure (Fig. 49). Fat-cells 



1 See Kirkes and Paget's " Manual of Physiology," 224-6, Am. Ed. 



