164 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



are seen the ossifying fibres of the squamosal. The base of the 

 skull is formed of cartilage which is covered, or ensheathed, by a 

 perichondrium continuous with the membranous capsule. In the 

 cartilage appear the centres of ossification for the sphenoid. 



sup. long. sin. «"£ f ont 



■fibrous capsule 



spicules of_ 

 parietal 



squam. 



pencran. 

 parietal 

 %rdura mater 



temp. fas. 



temp. muse, 

 squam. 



basi-sphen. ^ /(?c md 



Fig. 131. — A coronal section of the Skull of a Foetus, 4£ months old. 



As the bony spicules of the parietal spread out, they divide 

 the primitive cranial capsule into an outer layer — the peri- 

 cranium — and an inner — the periosteal layer of the dura mater. 

 At the periphery of the bone and in the sutures the continuity of 

 these two layers persists. The growth of the spicules of bone 

 keeps time with the growing brain which expands the capsule, but 

 there is, at each corner of the parietal bone, until the end of the 

 first year, a part of the primitive cranial capsule left unossified. 

 These unossified parts of the membranous capsule are the 

 fontanelles. 



The Fontanelles. — There are five fontanelles connected with 

 each parietal bone, one at each of its rounded angles, and one, 

 the sagittal (Fig. 130) which occurs between the radiating fibres 

 of the parietal near the posterior end of the sagittal suture. The 

 parietal foramen marks its position in the adult. In about 15°/ 

 of children this fontanelle is unclosed at birth ; a large parietal 

 foramen may permanently mark its situation. The posterior 

 inferior fontanelle, situated at the asterion (Fig. 130), the anterior 

 inferior at the pterion, and the posterior superior at the lambda, 



