198 On the Propagation of Heat by Conduction, Sec. 



propagation of heat by conduction is not rectilinear, a slight alteration 

 of texture or composition might easily deflect the path of transmission 

 in such a way as to wholly change the relative temperatures of the 

 outer surface which we have given. Differences of thickness, also 

 small, but sufficient to overthrow our calculations — may exist. Lastly, 

 the circulation of the blood, already alluded to, although incapable of 

 checking the outward transmission, might yet, within such narrow 

 limits, bring about a confusion in the external manifestations of the 

 interior change of temperature. It is only when areas of much 

 greater dimensions — for instance, of 50 or 60 millims. square —are 

 taken, that we can look with any degree of confidence to the relative 

 external temperatures as furnishing a key to the relative temperatures 

 of the underlying tracts of cerebral surface.* Moreover, in increased 

 mental activity — whatever may be its kind — the change of tempera- 

 ture on the outer surface of the head is of widespread extent, and not 

 confined to such limited areas as those on which our calculations are 

 based. 



* See the writer's " Eegional Temperature of the Head," pp. 119 and 209. 



