THE CELLULAR CHANGES OF AGE 39 



problem of age without including in it also the con- 

 sideration of the problems of growth and the prob- 

 lems of death. I hope to so entice you along in the 

 consideration of the facts which I have to present, 

 as to lead you gently but perceptibly to the conclu- 

 sion that we can with the microscope now recognise 

 in the living parts of the body some of those charac- 

 teristics which result in old age. Old age has for its 

 foundation a condition which we can actually make 

 visible to the human eye. As a step towards this 

 conclusion, I desire to show you this evening some- 

 thing in regard to the microscopic structure of the 

 human body. 



We now know that the bodies of all animals and 

 plants are constituted of minute units so small that 

 they cannot be distinguished by the naked eye, 

 although they can be readily demonstrated by the 

 microscope. 1 These units have long been known to 

 naturalists by the name of cells. The discovery of 

 the cellular constitution of living bodies marks one 

 of the great epochs in science, and every teacher 

 who has occasion to deal in his lectures with the 

 history of the biological sciences finds it necessary to 

 dwell upon this great discovery. It was first shown 

 to be true of plants, and shortly after likewise of ani- 

 mals. The date of the latter discovery was 1839. 

 We owe it to Theodor Schwann, whose name will 



1 I have estimated the average diameter of the cells in the human adult as 

 fifteen thousandths of a millimetre (0.015 mm.). One millimetre is approxi- 

 mately one twenty-fifth of an inch. This estimate is probably not exact, but 

 may serve to indicate the order of cell dimensions. 



