Heat hy Conduction in Bone, Brain-tissue, and Skin. 175 



results from over 700 experiments picked out from a still larger 

 number. It will be seen that M. Franck is quite correct as regards the 

 comparatively good conductivity of brain-tissue, but in error as 

 concerns the conductivity of bone and skin.* 



In approaching this subject, we have at the start, to take into con- 

 sideration what rises of temperature are likely to occur in the brain as 

 the result of increased mental action of different kinds. 



The only direct information in our possession concerning the pro- 

 duction of heat in the brain during increased cerebral activity is 

 furnished by the well-known admirable experiments of M. Moritz 

 Schiff. f As M. Schiff did not reduce his results to a thermometric 

 standard, we are left wholly in the dark as to the degree of the rises of 

 temperature noted by him. It has been rather gratuitously assumed, 

 because M. Schiff did not calculate the thermometric values of the 

 deflections of his galvanometer, that, therefore, these values must have 

 been exceedingly small — too small, in fact, to be easily estimated — and, 

 consequently, that the rises of temperature in the brain were propor- 

 tionally feeble. But a knowledge of the general nature of the galvano- 

 meter and thermo-piles employed by M. Schiff, together with a careful 

 study of the experiments themselves, have failed to prove to the writer 

 that M. Schiff was experimenting with any extraordinary degree of 

 delicacy. To begin with, the electromotive force of the piles employed 

 was not great. Although M. Schiff mentions certain alloys of Rollman, 

 all the results of his experiments on the brain appear to have been 

 obtained with single pairs of either the antimony-bismuth, copper- 

 bismuth, or platinum- German silver combinations. Now the electro- 

 motive forces of these combinations may be expressed by 35, 24, and 

 4*5 respectively, while the electromotive forces of the combinations 

 principally used by the writer are represented by 119*5 and 210. The 

 galvanometer used by M. Schiff was a combination of the principles of 

 the Meyerstein and Wiedemann instruments. These instruments are 

 certainly not superior, even if they are equal, in sensitiveness to the 

 Thomson galvanometer, which the writer has usually employed. The 

 perturbations, arising from external causes, mentioned by M. Schiff, 

 may occur when instruments of the kind are not adjusted to any very 

 great degree of delicacy, and therefore are not necessarily proofs of high 

 sensitiveness. But the principal proof that the galvanometric deflec- 

 tions did not represent very minute values of temperature is to be 

 found in the account of the experiments themselves. It is there stated 

 that with single pairs of German silver and platinum, implanted in 



* The question of the specific heat of the tissues has been purposely omitted, as 

 nothing definite is known on this important point. Yet the writer is strongly in- 

 clined to believe that the differences in the rate of thermal transmission in these 

 tissues are in part owing to differences in their specific heats. 



f "Archives de Physiologie," t. Ill, 1870, p. 6. 



