10 ANIMAL MECHANICS 



causing a vibration which runs round the skull 

 and extends to every portion of its contents. 



Were the skull, in its perfect or mature state, 

 softer than it is, it would be like the skull of a 

 child ; were it harder than we find it is, it would 

 be like that of an old man. In other words, as 

 in the former it would be too easily pierced, so, 

 in the latter, it would vibrate too sharply and 

 produce concussion. The skull of an infant is a 

 single layer of elastic bone ; on the approach to 

 manhood it separates into two tables ; and in old 

 age it again becomes consolidated. During the 

 active years of man's life the skull is perfect : it 

 then consists of two layers, united by a softer sub- 

 stance ; the inner layer is brittle as glass, and cal- 

 culated to resist anything penetrating ; the outer 

 table is tough, to give consistence, and to stifle 

 the vibration which would take place if the whole 

 texture were uniform and like the inner table. 



The alteration in the substance of the bones, 

 and more particularly in the skull, is marvel- 

 lously ordered to follow the changes in the mind 

 of the creature, from the heedlessness of child- 

 hood to the caution of age, and even the help- 

 lessness of superannuation. 



The skull is soft and yielding at birth ; during 

 childhood it is elastic, and little liable to injury 

 from concussion ; and during youth, and up to 

 the period of maturity, the parts which come in 



