CHAPTER XIII. 



THE CEANIUM. 



Development of the Skull. — The facial parts of the skull 

 have already been dealt with (Chap. I.). Only those bones which 

 enter into the formation of the cranial cavity and help to form 

 the brain chamber are dealt with here. These bones are the 

 frontal, parietal, occipital, temporal, ethmoid and sphenoid. 



Is the Skull made up of Segments ? — We have just seen 

 that the body is made up of 33 or more segments. Is the skull 

 made up of a series of segments ? The theory supported by 

 Owen and many others that the cranium is really composed of 4 

 modified vertebrae is now no longer tenable. On the other hand 

 the arrangement of the nerves and muscles, the evidence of 

 development and comparative anatomy, indicate that it is com- 

 posed of a number of segments, probably nine in number. 

 The four posterior, which form the occipital region of the 

 skull, are recognisable at an early stage of development, but 

 at no period in the development of the embryo have the anterior 

 five segments been seen to be demarcated. 



The Primitive Membranous Skull. — The brain is developed 

 in the same manner as the spinal cord from the medullary plates 

 of the neural groove (Fig. 69, p. 90). In the same manner 

 the mesoblast grows under and over the cephalic part of the 

 neural canal, and forms for it a membranous covering. The 

 covering of mesoblast thus formed is the primitive basis of the 

 skull in the embryo. 



The Evolution of the Mammalian Cranium. — It is not 

 possible to understand the manner in which the bones of the 

 human cranial cavity are developed without some reference to 



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