270 HUMAN PHYSIOLOGY 



The object is to promote the dissipation of the extra 

 heat which is being produced.) 



In a later chapter we shall discuss the regulation of 

 body temperature. At this time we ought to point out 

 briefly that the vasomotor system plays an important 

 part in this necessary work. When the surface of the 

 body is warmed a familiar vasomotor reflex leads to the 

 enlargement of the cutaneous vessels and the quantity 

 of blood near the cooling atmosphere is augmented. 

 Under the influence of cold we find that the circulation 

 in the skin is hindered. Moderate cooling of the sur- 

 face leads to paling. But we must use a little space to 

 comment on the effect of more decided chilling. Every- 

 one knows that this produces a reddening, and it might 

 be supposed that the underlying condition would be 

 the same as that of the skin when flushed with heat. 

 The actual state of affairs seems to be quite different. 



Suppose that one hand is held for a while in ice water 

 while the other is in water as hot as can be borne. Both 

 will probably be red, but we can discover signs that the 

 existing conditions are not identical. It will be seen 

 that the veins in the heated hand are swollen while in 

 the chilled hand they are small. The redness induced 

 by cold does not seem to be due to a copious flow of 

 blood through the skin but to a gathering of relatively 

 stagnant blood in the capillaries. The venous outlets 

 appear to be contracted. This is in agreement with 

 the fact that the skin which has been reddened by cold 

 may easily become bluish. The bluish cast is always 

 the sign of an impeded rather than an accelerated 

 circulation. 



This illustration makes it convenient to speak briefly 

 of congestions. When there is an excess of blood in 

 a part we say that there is a local congestion. But 

 congestions are of two types and fairly exemplified by 

 the states produced by heating and cooling the skin. 

 First, we have the active type in which the arteries are 

 dilated, admitting more blood than usual to the capil^ 



