76 BULLETIN OF THE 



trivance, he observed during mental effort a rise "of the surface 

 temperature of the head, which sometimes amounted to as much as 

 one-twentieth of a degree centigrade. 51 Subsequent and more elab- 

 orate investigations confirmed him in this conclusion, which has 

 also beeu supported by observations made with thermo-piles by 

 Schiff and Bert, as well as by the use of surface thermometers in 

 the hands of Broca and L. C. Gray of Brooklyn. 52 Gray claimed 

 to have observed a maximum rise of as much as two and a half 

 degrees Fahi-enheit. These physicians and some others have also 

 investigated the relative temperature of the two sides of the head, 

 of different regions on each side, the variations produced in certain 

 regions by voluntary muscular movements, and those resulting from 

 localized brain diseases. 53 



To attempt any discussion of these interesting studies, and their 

 conflicting results, would lead me altogether beyond my prescribed 

 limits. It is enough for my present purpose to point out that the 

 recent investigations of Francois Frank 54 would seem to indicate 

 that the variations of temperature actually observed are chiefly due 

 to changes in the cerebral circulation. Plunging suitable sounds, 

 connected with a thermo-electric apparatus, into the brains of ani- 

 mals to different depths, Fi'ank found that the deeper pafts of the 

 brain are always warmer than its superficial layers. The super- 

 ficial layers are continually cooled by radiation, and their temper- 

 ature is a degree, or more than a degree centigrade, lower than that 

 of the deeper parts. Even these, however, are .1° to .2° centigrade 

 cooler than the blood in the thoracic aorta, and it will therefore 

 readily be understood that a relaxation in the muscular coats of 

 the cerebral vessels, permitting the more rapid circulation of a 

 larger quantity of blood, would be promptly followed by an increase 

 in the temperature of the superficial parts of the brain. None of 

 the observers I have cited have reported a surface temperature of the 

 head during mental effort that is too high to be accounted for in 

 this way ; and if, as I willingly concede is probable, there is really 

 an increased heat-production in the brain itself, it is wholly masked 

 by the more considerable change due to afflux of blood. 



Now a consideration of the phenomena of blushing, and certain 

 well known sensations in the head, might lead us to expect that 

 emotional and mental conditions would prove to be attended by 

 increased activity in the circulation of the blood in the brain ; yet 

 many difficulties have hitherto been encountered in the attempt to 



