82 AGE, GROWTH, AND DEATH 



birth a well-developed gland known as the thymus. 

 After childhood this undergoes a retrograde develop- 

 ment ; it becomes gradually absorbed and persists only 

 in a rudimentary condition. With the loss of the teeth 

 occurring during infancy, you are familiar, and know 

 that the first set of teeth are but for a short period, 

 and are to be replaced by the permanent set. In very 

 old persons we see a great deal of the bony material 

 absorbed, and this absorption of the bone is a phe- 

 nomenon which occurs at almost every period of the 

 development. Portions of the epidermis or outer skin 

 are constantly shed, as is well known, and the loss of 

 hair and the loss of portions of our nails are so famil- 

 iar to us that we hardly heed them. Of the constant 

 destruction of the cells which are found in the lining 

 of the intestine, I have already spoken. At all times 

 in the body there is a vast amount of destruction of 

 blood corpuscles going on, a destruction which is phys- 

 iologically indispensable, for the material which the 

 blood corpuscles furnish is used in many ways. For 

 instance, the pigment which occurs in the hair is sup- 

 posed to be derived from the chemical substances the 

 use of which the body obtains by destroying blood 



mated debates, one might almost say of constant controversy. This need not 

 be wondered at for the changes involved are very complicated, owing to the 

 fact that the formation and destruction of cartilage, the formation of new and 

 the removal of old bone, and the development of a new tissue (marrow) all go 

 along together, and often may all be seen at once, each in various phases, within 

 the limits of a single microscopic field of view. Our present knowledge renders 

 it certain that the cartilage degenerates, dies, and disappears and takes no 

 share in the production of bone. That certain rare exceptions to this rule occur 

 has been maintained, but the evidence is, in my opinion, unconvincing. 



