176 PROCEEDINGS OF THE ANATOMICAL AND ANTHROPOLOGICAL 



5. Phylogenetic and racial differences. 



6. Variations in form and size associated with variations in the 



form and size of the brain, and with varying degrees of 

 mental ability. 



7. Modifications in the form of the skull produced by the action 



of the temporal muscles. 



8. Deformation : 



(a) Artificial. 



(b) Pathological. 



(c) Posthumous. 



1. DEVELOPMENTAL CHANGES AND CHANGES ATTENDANT ON THE 



ADVANCE OF AGE. 



In order to clearly appreciate certain changes in the form and 

 size of the adult cranium it will be necessary to briefly review some 

 of the principal phases in the normal development of the brain and 

 skull. 



In the early stages of development the brain capsule is formed 

 around the expanded anterior extremity of the primitive cerebro- 

 spinal axis or neural tube. We have at this stage a rapidly-growing 

 tube closed at each end, and at its cephalic extremity there are 

 three expansions called the first, second and third primary vesicles of 

 the brain. The walls of these vesicles are thin, especially along the 

 dorsal aspect, and they are completely filled with cerebro-spinal fluid. 

 The vesicles increase in size partly by the growth of the nerve tissue of 

 their walls and partly by an increase in the amount of fluid which they 

 contain. 



Along with this growth of the cerebral vesicles there is an en- 

 largement of the thin membranous capsule which invests them. This 

 enlargement is partly a true tissue growth ; the growth of the invest- 

 ing capsule, however, is obviously modified by the expanding force of 

 the growing |, ra in within. The capsule yields where growth of the 

 brain is most rapid, and, on the other hand, the enlargement of the 

 brain is modified in certain directions by the resistance afforded by 



