444 THE NERVOUS REGULATION OF THE BLOOD-VESSELS 



necessitates the transfer of a certain quantity of blood from the portal 

 organs and the cutaneous tissues, whenever it is called upon to do 

 extra work. At all events, it is certain that these systems bear a 

 reciprocal relation to one another, so that, for example, the processes 

 of digestion and mental activity should never be closely associated. 



Changes in the blood supply of the brain may also be effected in 

 an indirect way by various sensory impressions derived from the skin 

 and subcutaneous tissues. Thus, the immersion of the body in 

 moderately cold or warm water, or its exposure to cold or warm air, 

 produces, on the one hand, a vasoconstriction and, on the other, a 

 vasorelaxation. In the former case, the blood flow through the cere- 

 brum is augmented and, in the latter, diminished. These changes are 

 generally associated either with a greater or a lesser mental and bodily 

 alertness. Account should also be taken of the fact that these vaso- 

 motor reactions are usually accompanied by changes in the energy 

 of the heart and in the frequency and amplitude of the respiratory 

 movements. 



