136 
DB. J. CLELAND ON THE YAEIATIONS OE THE HUMAN SKULL. 
the shortness of the base, are the circumstances which give the characteristic outline to 
its profile. 
Elderly skulls. Gravitation changes . — It is a well recognized fact that the skull con- 
tinues to undergo change of form after adult life is reached. Lavater in the general 
appearance of the head, and Froriep* in the skull, depict the retreat of the forehead 
characteristic of old age, but the precise nature of the change and the causes on which 
it depends have not been recognized. The changes referred to result entirely from the 
operation of mechanical causes, and consist in a yielding of the skull in consequence of 
its own weight. Precisely as the apparently solid glacier flows down its valley at a rate 
too slow to be appreciated by direct observation, so the skull falls gradually down by its 
own weight and that of the contained brain. The condyles of the skull are supported 
on the vertebral column, and by the process of gradual yielding the basal part of the 
skull is driven in, from the occipital tuberosity behind, to the fronto-nasal suture before. 
Thus the postforaminal angle is flattened out, while the angle of the tuberosity and the 
orbito-frontal angle are made smaller. At the same time the skull is increased in breadth, 
being made to bulge out at the squamous suture ; this bulging being partly produced by 
a forcing open of the angle between the squamous and petrous parts of the temporal 
bone, and partly by the depression of the outer end of the petrous part to a lower level 
than its inner end. This lateral bulging is very characteristic, and ought not to be lost 
sight of by the artist in representing old age. In the production of these alterations of 
form it is plain that the cooperating causes are weight of the skull and its contents, 
softness of the bone, and lapse of time ; therefore the larger the skull the more likely 
they are to be developed in a marked manner ; and if the bone be more than usually 
yielding they may be developed at an earlier age than usual, even though the skull be 
not remarkably large. This is probably the explanation of the very considerable gravi- 
tation changes in the skull of Burke the murderer, who, although somewhat past middle 
life, was by no means an old man at the date of his execution. Most probably these 
changes begin in a slight degree in all skulls at an early period of adult life ; and it may 
be the lot of some one who has better opportunities of investigating the subject than 
the present writer, by a comparison of a number of adult skulls of different ages, to 
demonstrate the changes of form which the skull undergoes in the passage from twenty 
to thirty, forty, or fifty years of age. The accompanying Table gives a list of the skulls 
from the study of which the above observations have been made ; and if the angles given 
be compared with the corresponding angles of other skulls, they will illustrate the way 
in which by gravitation changes the base is driven in. In the German 29 the forehead 
appears to have escaped being bent back, and instead of its being so the parietal region 
has become flattened out. 
* Eroriep, Die Charakteristik des Kopfes naek dem Entwieklungsgesetz desselben, 1845. 
