448 



DR MATTHEW YOUNG. 



form well beyond a third of the length of the frontal bone, and this not in the skull 

 of an aged person. 



We also observe, in such a section, the great variability in the degree of distinct- 

 ness which the impressions corresponding to the various convolutions exhibit, being 

 sometimes quite evident, at other times practically invisible. 



Another feature brought to light and shown well in the sagittal tracings is the 

 variation in the thickness of the skull. As is well known, the skull may become 

 much thicker in old age, but the great variations shown in the series examined is 

 remarkable, even excluding specimens that might have become secondarily thickened 

 owing to advanced years. 



I estimated the thickness with the calipers at six places, viz. : — 



1. At the glabella. 



2. At the inion. 



3. At the middle of the frontal (at the cut edge of the sagittal section). 



4. At the middle of the parietal (at the cut edge of the sagittal section). 



5. In the vicinity of the frontal eminence. 



6. In the vicinity of the parietal eminence. 



The figures shown in the following table are interesting, as they show that the 

 thickness at the glabella is the least variable, using the coefficient of variation as 

 the criterion of variability, in spite of the variation in the degree of development of 

 the frontal sinus in the series. 



Table LIV. 



Position of Measurement. 



Mean Thickness. 



S.D. 



V. 



At glabella ..... 



At inion ...... 



Middle of frontal (mid line) 



Middle of the parietal (mid line) 



At frontal eminence .... 



At parietal eminence .... 



15-88 mm. 



14-25 „ 

 7-01 „ 

 7-24 „ 

 5-65 „ 

 6-41 „ 



2-43 

 336 

 1-88 

 1-42 

 1-61 

 1-65 



15-30 



2358 



26-81 



19-61 



28-5 



2574 



The greater variability of the thickness at the inion is to be attributed largely to 

 the fact that the external and internal occipital protuberances were frequently at 

 different levels, as has been pointed out by various observers, including Sollas (46). 

 Anderson (57), and Klaatsch (43), so that the two eminences did not always come 

 into the measurement of the thickness. 



Anderson (57) measured the thickness of a large number of skulls, making 

 measurements in twelve different places in each. I have taken his mean measure- 

 ments given to the places where I estimated the thickness, converted them from 

 inches into millimetres, and the following table shows the mean thicknesses 

 compared : — 



