72 



DISCOVEKY 



flucncc is closely related td the origin and growth of 

 cancer. 



The second peculiarity of living things is one which 

 is of the greatest importance to the well-being, and in 

 fact the existence, of man and animals. It is wthin 

 the experience of everyone that the tiny patch of skin 

 which is knocked off by accidental contact with a sharp 

 table corner is quickly replaced. As soon as bleeding 

 stops, either by the application of pressure or, as 

 we say, of itself, a red glazed surface forms. The edges 

 of the little wound, which were at first irregular and 

 steep, become shelving, and the opaque white sur- 

 rounding healthy skin thins off to a translucent bluish 

 edge. This creeps in over the red, raw surface from 

 every side till the whole defect is repaired. It is worth 

 while to go into the details of the process, so far as 

 the outer layer of the skin is concerned, with the aid 

 of the accompanying diagrams. Let the three black 



(2) 



(3) 



Diagrams to illustrate the healing of a small wound of the skia. The 

 three black circles in the upper row represent the raw surface un- 

 covered by sldn, as seen from the surface. The second row shows 

 the wound, on the same scale, as if cut across and viewed from the 

 cut surface. The third row shows the appearances of the margins 

 in the second row, more highly magnified to show the single cellS- 

 Dottcd lines join the corresixjnding parts of the three sets of ligiu-es. 



circles i, 2, 3, represent the raw surface as soon as 

 bleeding has stopped (i), next day (2), and on the day 

 after (3), when healing is nearly complete. Below, 

 on the same scale, are the appearances that would be 

 seen if the wound were cut across and viewed from the 

 cut surface — that is, in section. The third row of 

 figures shows the appearances seen when a thin slice 

 from this cut surface, suitably prepared, is examined 

 under the microscope. The outer layer of the skin 

 consists of several layers of cells, the outermost of 

 which are flattened, dried, and horny. These are 

 dead and are being constantly nibbed off, insensibly 

 for the most part, in Nasiblc shreds when a Turkish 

 bath is taken. They are replaced by division and 

 growth of the cells of the deepest layer, which look 

 like a row of bricks stood on end. This basement 

 layer is the only one capable of cell-division, and 

 alone takes part in the process of repair. The cell 

 of this layer next the defect divides into two, and the 

 new cell next the raw surface is pushed out over it. 



Still retaining contact with its parent, it flattens out 

 and itself divides. The mother-cell divides again, 

 and the process goes on till a considerable part of the 

 edge of the wound is covered by a single layer of 

 flattened growing cells. Those nearest the undamaged 

 skin soon acquire the elongated form of the deepest 

 layer (3), and when they divide, one of the daughter 

 cells is pushed upwards and repeats the appearances 

 first described. The new skin thus increases in thick- 

 ness till the outer surface is flush with its surroundings. 

 When this stage is reached, the rate of cell-di\'isioii 

 slows down again to the original spaced. 



The important points for us to notice in this process! 

 are, first, that the cells of the skin can be wakened upl 

 from the ordinary rate of reproduction and excited to 

 more rapid division and growth by removal of some of jj 

 their neighbours; and, second, that when the gap ha 

 been mended, the temf>orary spurt is checked, and tb 

 cells return to their regular workaday jogtrot. It is very 

 probable that reciprocal influences of cells of the same 

 kind on each other are in part responsible for this op- 

 portune halt. A complete explanation, which does 

 not attribute semi-intelligent action to the cells, is 

 yet to seek. It has an unmistakable resemblance, 

 however, to the regulating power, already mentioned, 

 which maintains the just proportion of the different 

 parts of the body. Repair processes, essentially of 

 the same kind, take place in the deeper parts of the 

 skin after such a slight wound as has been described. 

 In other organs, also, destruction and damage are 

 followed by a similar response. 



The description of the various agents which cause 

 damage to the tissues, and the peculiarities of the 

 alterations they produce, is the province of the science 

 of pathology. It will be sufficient to draw attention 

 to a few examples of agencies which produce rather 

 less obvious forms of damage than the simple abrasion 

 just described. Workers in paraffin-oil refineries, 

 after long-continued action of the oil on the hands and 

 arms, frequently suffer from a slow inflammatory 

 condition of the skin. When the irritated condition 

 of the skin has persisted for some time, numbers of 

 warts make their appearance. Warts are circum- 

 scribed overgrowths of the covering layer of the skin. 

 We may assume, either that some kind of damage 

 to the cells of the skin releases them from the regulating 

 power which restricts their rate of multiplication, or 

 that substances in the oil excite the cells to di\ision 

 in spite of the restraining influence. The majority 

 of such warts after attaining a certain size cease to 

 grow. If the special irritant which caused them be 

 removed, they usually diminish in size and may dis- 

 appear. If the irritation be continued, however, 

 further changes take place in some of them indicating 

 a profound alteration in character. Instead of re- 



