PHILOSOPHICAL SOCIETY OF WASHINGTON. 77 



demonstrate experimentally that this is true. Mosso of Turin sup- 

 posed that he had succeeded in doing this with his plethysmo- 

 graph. 55 The instrument is essentially a cylinder of water, into 

 which the arm is introduced and so fastened in place by a caoutchouc 

 membrane that the slightest increase or diminution in the volume 

 of the arm will cause the rise or fall of the water, through a tube 

 connected at one end with the interior of the cylinder and at the 

 other with a suitable recording apparatus. The pen or pencil of 

 this apparatus inscribes a curve that rises or falls with the fluid in 

 the tube. Among the curious observations made with this instru- 

 ment, Mosso reports that the mental operations and emotions of 

 the persons he experimented on were accompanied by a fall of the 

 curve, which he regarded as proof that more blood goes to the 

 brain and less to the arm during emotion, or mental action, than 

 at other times. But the following year these observations were re- 

 peated with great care, and with an improved plethysmograph by 

 Basch, of Vienna, 56 who failed to verify them. Most of the phleg- 

 matic Germans on whom he experimented did sums in their heads, 

 and otherwise exerted their minds, without producing the slightest 

 modification of the curve, and none of them appear to have been 

 as emotional as Dr. Pagliani, of whom Mosso relates that, his arm 

 being in the plethysmograph, when the revered Prof. Ludwig en- 

 tered the room the curve fell as if he had received an electric 

 shock. Basch has cautiously investigated the causes of the varying 

 quantity of blood in the arm in these experiments, and has clearly 

 shown how many general and local conditions concur in producing 

 the result. Especially has he emphasized the effect of variations 

 in the abdominal circulation, which appear to exercise a much more 

 considerable influence upon the size of the arm than any changes 

 that occur in the brain. 



In subsequent works Mosso has stated that during mental effort, 

 such, for example, as is required to multiply small numbers in the 

 head, the radial pulse, as recorded by the sphygmograph, is shown 

 to become somewhat more frequent, and the recording lever does 

 not rise so high as at other times. 57 Thanhoffer, who has pointed 

 out that in these observations the influence of respiration on the 

 pulse was neglected, concluded, nevertheless, from his own sphyg- 

 mographic observations, that after due allowance is made for this 

 complicating influence, it must be conceded that cerebral activity 

 does exercise a certain effect upon the pulse, and in the direction 



