TRANSACTIONS OF SECTION H. 785 



because the amouut of blood received by each side of the brain depends upon two 

 factors, viz., the physical conditions under which the blood-stream is delivered to 

 the two hemispheres and the calibre of tlie arteries or tubes of supply. Both of 

 these conditions have been stated to be favourable to tlie left hemisphere. It is 

 a matter of common anatomical Iniowledgo that the supply pipes to tbe two sides 

 of the brain are laid down somewhat ditfereutlj', and that tlio angles of junction, 

 (fcc, with the main pipe are not quite the same. Further, it is true that the blood- 

 drains which lead away the blood from the brain are somewhat different on the 

 two sides. Whether tliis would entail any marked difference in the blood-pressure 

 on the two sides I am not prepared to say. This could only be proved experi- 

 mentally ; but, taking all the conditions into consideration, I am not inclined to 

 attach much importance to the argument. It is easy to deal with the loose state- 

 ments which have been made in regard to the size of leading supply pipe (viz., the 

 internal carotid artery). It passes through a bony canal in the floor of the cranium 

 on its way into the interior of the cranial box. Its size can therefore be accurately 

 gauged by measuring the sectional area of this bony tunnel on each side. This I 

 have done in twenty-three skulls chosen at random, and the result shows that 

 considerable differences in this respect are to be found in diflerent skulls. These 

 discrepancies, however, are sometimes in favour of the one side and at other times in 

 favour of the other side ; and when the combined sectional area for all the skuUs 

 examined was calculated it was, curiously enough, found to be 583i- sq. mm, 

 for the left side and 583 sq. mm. lor the right side. 



Leaving out of count the asymmetry in tbe arrangement of the convolutions 

 in the two hemispheres, which cannot by any amouut of ingenuity be twisted into 

 such a form as to give a structural superiority to one side more than the other, the 

 only marked diflerence which appears to possess any degree of constancy is the 

 increase in the territory of the left parietal lobe produced by the more "marked 

 depression of its lower frontier line (Sylvian fissure). That this is in any way 

 associated with right-handedness or with the localisation of the active speech 

 centre in the left hemisphere I am not prepared to urge, because the same con- 

 dition is present in the ape. It is true tliat some authorities '■ hold that the ape is 

 right-handed as well as man, but in the gardens of the Eoyal Zoological Society 

 of Ireland I have had a long and intimate experience of both anthropoid and lower 

 apes, and I have never been able to satisfy myself that they show any decided 

 preference for the use of one arm nrore than the other. 



That difierences do exist in the more intimate structural details of the two 

 hemispheres, which give to the left its functional superiority, there cannot be a 

 doubt ; but these have still to be discovered. Bastian has stated that the grey 

 cortex on the left side lias a higher specific gravity, but this statement has not as 

 yet received corroboration at the hands of other observers. 



I have already mentioned that man's special endowment, the faculty of speech, 

 is associated with striking changes in that part of the cerebral surlace in which the 

 motor centre for articulate speech is located. It is questionable whether the acquisi- 

 tion of any other system of associated muscular movements has been accompanied 

 by a more evident cortical change. The centre in question is placed in the lower 

 and back part of the frontal lobe. We have seen that the insular district is covered 

 over in the hinder three fourths of its extent by the fronto-parletal and temporal 

 opercula, and thus submerged below the surface and bidden from view. The 

 brain of tbe ape and also of the microcephalic idiot with defective speech goes no 

 further in its development. The front part of the insular district remains uncovered 

 and exposed to view on the surface of the cerebrum. In man, however, two addi- 

 tional opercula grow out and ultimately' cover over the fore part of the insula. 

 These opercula belong to the lower and back part of the frontal lobe, and are to be 

 looked upon as being moi-e or less directly called into evidence in connection with 

 the acquisition of articulate speech. 



The active speech centre is placed in the left cerebral hemisphere. We speak 



' Ogle, ' On Dextral Pre-eminence,' Trans. Med. CMrufg. Soc, 1871 ; Aim6 Pere, 

 Zes Courlures lafcrales normales au racMs humuin. ToiUouse, lyoo. 



