22 4 AGE, GROWTH, AND DEATH 



cells of the nervous system are separated into these two 

 fundamental classes they cannot change. A cell form- 

 ing a part of the supporting framework of the brain 

 cannot become a nerve cell ; and a nerve cell cannot 

 become a supporting cell. The destiny of them be- 

 comes more and more fixed, their future possibili- 

 ties more and more limited, as their cytomorphosis 

 goes on. 



The law of genetic restriction has a very important 

 bearing upon questions of disease. When disease 

 occurs, the cells of the body offer to us two kinds of 

 spectacles. Sometimes we see that the cells causing 

 the diseased condition are more or less of the sort 

 which naturally belong in the body ; that they are 

 present where they do not belong, or they are present 

 where they ought to be, but in excessive quantity. 

 There is a kind of tumor which we call a bony 

 tumor. It consists of bone cells such as are natur- 

 ally present in the body, but that which makes this 

 growth of bone a tumor is its abnormal dimensions, 

 or perhaps its being altogether in the wrong place. 

 The second sort of pathological alteration, which I 

 had in mind, is that in which the cells really change 

 their character. Now, the young cells are those which 

 can change most ; in which the genetic restriction has 

 least come into play ; and accordingly we find that a 

 large number of dangerous, morbid growths, tumors, 

 arise from cells of the young type, and these cells, 

 having an extreme power of multiplication, grow 

 rapidly, and they may assume a special character of 



