THE CAUSES OF DEATH 111 



both physical and chemical, than is the skin with its pro- 

 tecting layers of stratified and cornified epithelium. 



The organs concerned with the blood an.d its circula- 

 tion — the heart, arteries and veins, etc. — stand third in 

 importance in the mortality list. Biologically the blood, 

 through its immunological mechanism, constitutes the 

 second line of defense which the body has against noxious 

 invaders. The first line is the resistance of the outer 

 cells of the skin and the lining epithelium of alimentary 

 tract, lungs, and sexual and excretory organs. When 

 invading organisms pass or break down these first two 

 lines of defense, the battle* is then with the home guard, the 

 cells of the organ system itself, which, like the industrial 

 workers of a commonwealth, keep the body going as a 

 whole functioning mechanism. Naturally it would be ex- 

 pected that the casualties would be far heavier in the first 

 two defense lines (respiratory and alimentary systems 

 and the blood and circulation) than in the home guard. 

 Death rates, when biologically classified, bear out this 

 expectation. 



In the United States the kidneys and related excre- 

 tory organs are responsible for more deaths than the sex 

 organs. This relation is reversed in England and Wales, 

 and in Sao Paulo. This difference is mainly due in both 

 countries to premature birth. The higher premature 

 birth rate for these two localities as compared with the 

 United States might conceivably be explained in any one 

 of several ways. It might mean better obstetrics here 

 than in the other localities, or it might mean that the 

 women of this country, as a class, are somewhat superior 

 physiologically in the matter of reproduction, when they 

 do reproduce, or it might be in some manner connected 

 with differences in birth rates. 



