THE REGULATION OF THE CIRCULATION 271 



lanes, while there is no hindrance to its free escape 

 through the veins. Second, there is the passive type in 

 which the outstanding feature is the narrowing or the 

 obstruction of the veins. The result is an increased 

 quantity of blood in the capillaries, but it is more or 

 less completely arrested or dammed up. This is the 

 case when cold has had its usual effect on the skin. 



We have emphasized the part played by the small 

 arteries in vasomotor regulation. The small veins have 

 a less marked but still noteworthy muscular develop- 

 ment and there is no doubt that they have a share in the 

 adaptations. But this is seldom given much prominence 

 by writers on the subject and there is much uncertainty 

 in regard to its extent. The passive congestions of path- 

 ology are not due to muscular constriction of the veins so 

 much as to internal accumulations of cells, chiefly white 

 corpuscles. Complete closure of the veins in any region 

 will throw the full arterial pressure into the capillaries, 

 distending them severely and probably increasing the 

 formation of lymph as the fluid niters out into the tissue 

 spaces. 



Colds. What has just been said of congestion is 

 related to our susceptibility to common colds. We 

 speak of taking or catching cold, and it has become 

 probable that there are two senses in which we may 

 do just this thing. A cold is almost certainly an in- 

 fectious disease; as such one may take it from a previous 

 victim. But we also speak of catching cold when we 

 fix the blame on circumstances under which we have 

 been chilled by drafts or by getting wet. We shall 

 find that the recognition of the infectious nature of 

 colds does not lead to the denial of the other mode of 

 acquiring them. .. 



When the surface of the body is cooled it is entirely 

 normal for the mucous membranes to become engorged 

 with blood. An active congestion, more or less marked, 

 is to be expected. This may result in increased secre- 

 tion on the part of the nasal lining. But a hardy and 



