TRANSACTIONS OF SECTION H. 891 



have allowed themselves to be affected by extraneous iniiueuces, which have 

 hindered their unanimous agreement upon some one definite liorizontal plane in 

 craniometry. 



The Frankfort plane drawn through the upper margins of the auditory foramina 

 and the lowest points of the orbital borders has the advantage of being easily 

 traced, and difl'ers so little from the plane of A'ision that we may without substantial 

 error adopt it. 



The largest part of the skull is that which is at once the receptacle and the 

 protector of the brain, a part which, when unmodified by external pressure, prema- 

 ture synostosis, or other adventitious conditions, owes its form to that of the cere- 

 bral hemispheres which it contains. Speaking in tbis city of (leorge and Andrew 

 Combe, I need not do more than indicate in this matter that observation and 

 experiment have established on a firm basis certain fundamental points regarding 

 the growth of the brain. The study of its development shows that the convolu- 

 tioning of the cerebral hemisphere is primarily due to the connection, and different 

 rate of growth, of the superficial layer of cells with the underlying layers of white 

 nerve fibres ; and that, so far from the shape being seriously modified by the con- 

 straining influence of the surrounding embryonic skull, the form of the soft mem- 

 branous brain-case is primarily moulded upon the brain within it, whose shape it 

 may however be, to some extent a secondary agent in modifying in later gTowth. 

 We have also learnt that, although in another sense from that of the crude 

 phrenology of Aristotle, Porta, or Gall, the cerebrum is not a single organ acting 

 as a functional unit, but consists of parts, each of which has its specific province ; 

 that the increase in the number of cells in any area is correlated with an increase 

 in the size and complexity of pattern of the convolutions of that area ; and that 

 this in turn influences the shape of the enclosing shell of membrane and subse- 

 quently of bone. 



The anatomist and the physiologist have worked hand in hand in the delimitation 

 of these several functional areas, and pathology and surgery have confirmed what 

 experimental physiology has taught. The topography of each part of the cerebrum, 

 80 important to the operating surgeon, should be pressed into the service of the 

 anthropologist, whose measurements of the brain-case should have definite relation 

 to these several areas. In the discussion which is to take place on this subject, I 

 hope that some such relationships will be taken account of. This is not the place 

 to work out in detail how this may be done ; I only desire to emphasise the funda- 

 mental principle of the method. 



The second factor which determines the shape of the individual skull is the size 

 of the teeth. That these differ among different races is a matter of common 

 observation ; thus the average area of the crowns of the upper-jaw teeth in the 

 male Australian is 1,536 sq. mm., while in the average Englishman it is only 1,286 

 sq. mm., less than 84 per cent, of that size.' 



It is easy to understand how natural selection will tend to increase the size of 

 the teeth among those races whose modes of feeding are not aided by the cook or 

 the cutler; and how, on the other hand, the progress of civilised habits, assisted by 

 the craft of the dentist, interferes with the action of selection in this matter among 

 the more cultured races. 



For larger teeth a more extensive alveolar arch of implantation is necessary ; 

 and as the two jaws are commensurately developed, the lower jaw of the macro- 

 dontal races exceeds that of meso- or micro-dontal races in weight. Thus that of 

 a male Australian exceeds that of the average Englishman in the proportion of 

 100 : 91. 



To work this heavier jaw more powerful muscles are needed. In the average 

 well-developed Englishman with perfect teeth the weight of the fleshy portion of 

 the great jaw-muscles, masseters, and temporals is 00 grammes, while the weight 

 of those as ascertained in two Australians was 74 grammes. 



Correlated with this greater musculature a sharper definition of the areas for 

 the attachments of the jaw-muscles is required. The muscular fascicles are approxi- 



' These and the succeeding averages are from my own measurements, taken from 

 never less than ten individual cases. 



