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DE. J. CLELAND ON THE VARIATIONS OE THE HUMAN SKULL. 
I. CJKANIUM PROPER. 
Extent of arch ancl base-line (General Table, columns G, 7, 8). — When it is considered 
that one of the most marked peculiarities of the human skull is the great elongation of 
the arch and shortening of the base, it becomes interesting to know what relation the 
length of arch has to the length of base at different ages and in different nations. The 
length of arch has been measured along the middle line of the roof, from the fronto-nasal 
suture to the back of the foramen magnum, while the direct distance between the same 
points has been taken as base-line. The distance from the occipital tuberosity to the 
foramen magnum is best included along with the arch, because, whether its variations 
or its morphological constitution are regarded, it appears to be closely associated with 
the arch, and because it has been found in making the measurements that the tuberosity 
is not a good land-mark, but varies in position according to its prominence. The foramen 
magnum is best considered as part of the base ; for although, when we take into account 
the development of the medullary canal, and the appearance of parts in the lower ani- 
mals, the strict base of the skull must be looked on as commencing at the front of the 
foramen magnum, or opposite the condyloid margins of the basilar ossification of the 
occipital bone, still we shall find that such a close connexion exists between the angles 
at which the foramen magnum and the floor of the anterior cranial fossa respectively lie 
to the intermediate portion of the cranium, that it is convenient to consider all three as 
belonging to the base. 
The general average proportion which the length of the arch bears to the base-line in 
the adult may probably be estimated at about 2*70 : 1 ; but it varies considerably both 
in individuals and nations. 
So far as one may judge from the foetal skull examined, it would appear that before 
the middle period of foetal life the arch is considerably less developed in proportion to 
the base than it is in the adult, but that afterwards the proportion is altered by the great 
growth of the arch, so that in the later months it is about three to one. In new-born 
infants and in children it more frequently exceeds than falls short of this proportion ; 
the average found in five skulls of new-born infants being 3 - Q6 : 1, and the average in 
seven children of ages varying from one to ten years being 3 '07 : 1. The ten-years-old 
skull is the only one of these seven in which the base-line has acquired a length which 
might be permanent, while in four of them the arch is such as might be found in the 
adult. We may judge therefore that the base-line continues to elongate after the arch 
has acquired its permanent dimensions. 
On examining the proportion of arch to base-line in adults we find, as we shall also 
find in all other measurements, that the variations in individuals of one nation are so 
great that the minimum figure in a nation in which the proportion is high is always 
within the limits of variation found in nations in which it is low ; and that therefore it 
is very necessary to compare different specimens of one nation together. Much the 
smallest proportion of arch to base in the series of skulls examined is in the Esquimaux 
skull 77, in which it is 2 - 28 : 1 ; but in the other Esquimaux it is 2‘67, as high as the 
