SOCIETY OF THE UNIVERSITY OF ABERDEEN. 177 



the thickness and density of the tissues surrounding it. It would 

 appear, therefore, that there are two important factors concerned in 

 the enlargement of the cranial portion of the head : 



1. An eaepanding force acting from within and due to the growth 

 of the brain and secretion of the cerebro-spinal fluid, and 



2. A restraining force, exerted by the brain capsule. 



If there is an increase in the expanding force, such as occurs in 

 chronic hydrocephalus, the skull will become enormously enlarged, 

 whereas in cases of anencephalus, in which the brain is rudimentary 

 (Plate XL, Figs. 1 and 2), the vault of the skull is -remarkably flattened 

 owing to absence of the distending force. The vault of the skull in 

 these monsters is usually represented by a membrane which covers 

 over the base of the skull and vestiges of the brain, or in rare instances, 

 such "as the specimen represented in Fig. 1, the membrane bones of 

 the vault may be ossified and appear to have fallen in over the base 

 of the skull, the skull as a whole having a curious flat-topped ap- 

 pearance which is very characteristic of this type of monster. The 

 condition appears to be due to an early rupture of the brain capsule, 

 with escape of cerebro-spinal fluid into the anmiotic cavity, resulting 

 in the collapse of the membranes covering the brain and destruction 

 of the brain itself. In those cases of anencephalus which I have 

 examined there has always been -an opening in the membranous 

 covering of the brain, and in most there has been a history of 

 hydramnios. 



Iteturning to the description of the normal development of the 

 skull, we find that about the seventh or eighth week of foetal life 

 ossification of the brain capsule commences by the deposit in it of 

 ossific matter in situations corresponding to the centres of the separ- 

 ate bones which form the vault of the skull, and in certain situations 

 at the base of the skull, where ossification is preceded by the formation 

 of cartilage. Ossification of the membrane bones, forming the vault 

 of the cranium, and of the basal cartilages, proceeds by an extension 

 from the different centres of ossification in a radial direction outwards 



towards the margins of the separate bones. The fibrous layer, ex- 



23 



