494 REGENERATION OF TISSUES, 



while the deepest layer of cells forms large multi-nucleated cells which 

 reproduce by division polygonal, flat nucleated cells (Klebs, Heller). 

 The nails grow from the root forwards ; those of the fingers in 4-5 

 months, and that of the great toe in about 1 2 months, although growth is 

 slower in the case of fracture of the bones. The matrix is co-extensive 

 with the lunule, and if it be destroyed the nail is not reproduced 

 (see vol. ii.). The eyelashes are changed in 100-150 days 

 (Donders), the other hairs of the body somewhat more slowly. 

 If the papilla of the hair follicle be destroyed, the hair is not 

 reproduced. Cutting the hair favours its growth, but hair which has 

 been cut does not grow longer than uncut hair. After hair has grown 

 to a certain length it falls out. The hair never grows at its apex 

 (Aristotle). The epithelial cells of mucous membranes and secretory 

 glands seem to undergo a regular series of changes and renewal. The 

 presence of secretory cells in the milk ( 231) and in the sebaceous 

 secretion (vol. ii.) proves this ; the spermatozoa are replaced by the 

 action of spermatoblasts. In catarrhal conditions of mucous membranes, 

 there is a great increase in the formation and excretion of new epithe- 

 lium, while many cells are but indifferently formed and constitute mucous 

 corpuscles. The crystalline lens, which is just modified epithelium, is 

 reorganised just like epithelium ; its matrix is the anterior wall of its 

 capsule, with the single layer of cells covering it. If the lens be 

 removed and this layer of cells retained, these cells proliferate and 

 elongate to form lens fibres, so that the whole cavity of the empty lens 

 capsule is refilled. If much -water be withdrawn from the body, the 

 lens fibres become turbid (Kunde, Koehnhorn). [A turbid or opaque 

 condition of the lens may occur in diabetes, or after the transfusion of 

 strong common salt or sugar solution into a frog.] 



3. The blood-vessels undergo extensive regeneration, and they 

 are regenerated in the same way as they are formed (p. 13). 

 Capillaries are always the first stage, and around them the characteristic 

 coats are added to form an artery or a vein. When an artery is injured 

 and permanently occluded, as a general rule the part of the vessel up to 

 the nearest collateral branch becomes obliterated, whereby the deriva- 

 tives of the endothelial lining, the connective tissue-corpuscles of the 

 wall and the leucocytes change into spindle-shaped cells and form a 

 kind of cicatricial tissue. Blind and solid outshoots are always found 

 on the blood-vessels of young and adult animals, and are a sign of the 

 continual degeneration and regeneration of these vessels (Sigm. Mayer). 



4. The contractile substance of muscle may undergo regeneration 

 after it has become partially degenerated. This takes place after amy- 

 loid or wax-like degeneration, such as occurs not unfrequently after 

 typhus and other severe fevers. This is chiefly accomplished by an 



