270 HUMAN PHYSIOLOGY 



well-conditioned person has a vasomotor system which 

 can be relied on to abate this congestion in a prompt 

 and decisive manner when the external conditions are 

 changed. Another, less highly endowed in this re- 

 spect, may not terminate the congestion in the nose 

 with such success. It may linger 'until the tissue is 

 definitely injured or inflamed, when it will tend to 

 become passive rather than active in character. 



A passive congestion is always deleterious to the 

 local cell-life. The mucous membrane beneath which 

 the circulation is thus interrupted becomes fallow 

 ground for the growth of microorganisms. There are 

 ordinarily enough of these present to furnish seed for 

 an infection. So a cold may develop which does not 

 appear to have been " taken" from another person. 

 The symptoms may be due to the activity of bacteria, 

 but the occasion for this ebullition of germ life has 

 been furnished by the lowered resistance of the con- 

 gested tissue. 



When we attribute freedom from colds to " vital re- 

 sistance" our reference may be to either of two condi- 

 tions. We may have in mind a chemical peculiarity 

 of the blood and other fluids which confers on them 

 the power to destroy bacteria. We are quite as likely 

 to be picturing an organization which is resistant be- 

 cause of its vasomotor efficiency. The hardiness gained 

 by athletic training and cold bathing is probably not 

 a chemical superiority but a mechanical one. The 

 merit lies less in the composition of the blood than in 

 the positive and consistent government of the circula- 

 tion. Colds are not contracted because incipient con- 

 gestions are overcome by the vigorous constriction of 

 the arteries immediately after the period of exposure. 



The Cranial Blood-supply. It might seem most 

 difficult to learn anything about the changes taking 

 place in the blood-vessels of the brain. The surface 

 of the cerebrum has sometimes been observed after 

 extensive fractures of the skull, and it hu,s been reported 



