274 HUMAN PHYSIOLOGY 



graph is a cylinder of glass open at one end and provided 

 with one or two necks with which tubes may be con- 

 nected. The hand and forearm are introduced into the 

 open end and a water-tight closure is effected around 

 the arm. The cylinder is then filled with warm water. 

 One of the side branches is put in connection with a 

 gage of a sensitive character. 



Now if the arm gains in volume, water will be dis- 

 placed from the plethysmograph and the gage will 

 indicate the occurrence. If the arm shrinks, water 

 will be drawn back and the gage will show the counter- 

 movement. Any quick changes in the volume of the 

 hand and arm may be ascribed to vasomotor phenomena. 

 More gradual ones are less easy to explain changes in 

 the amount of lymph would have to .be considered as 

 possible factors. The gage of the plethysmograph may 

 be made to write a record on smoked paper and when 

 all is going well the results are of extraordinary interest. 



A person who is seated comfortably with his hand 

 enclosed in the instrument may have fallen into a' 

 drowsy condition. He is abruptly called upon to 

 answer a question, perhaps to perform a sum in mental 

 arithmetic. As he rallies his faculties and fixes his 

 attention upon the task the record shows that the 

 volume of the limb is notably reduced. The inference 

 is that the nervous system has sent out impulses over 

 the constrictor paths to the vessels of the extremity 

 and so caused them to be contracted. We cannot 

 exactly determine how generally the vessels of the 

 skin participate in this narrowing. Still less can we 

 say whether there is a similar heightening of tone in 

 the internal organs. 



If we assume that the act of concentrating the at- 

 tention is accompanied by vasoconstriction in many 

 parts of the body, we can see that the result may be 

 serviceable to the brain. The contraction of numerous 

 arteries will elevate the pressure in the large trunks and 

 there will be a swifter flow of blood through any vessels 



