1902.] OLIVER — BLINDNESS FROM MALFORMATION OF SKULL. 163 



cartilage, particularly at the sides and the base of the membranous 

 cranium. During the third stage, true osseous material obtained 

 from both the membrane bones and cartilage bones appears, until 

 finally a more or less completed bony coveri::g containing rem- 

 nants of chondral matter is obtained. 



The occipital bone originates from four centres : the basioccipital, 

 formed from cartilage at about the seventieth day ; the two exoc- 

 cipitals, also derived from cartilage a few days later; and the 

 squamoccipital, composed of two parts, the interparietal and the 

 supraoccipital, which appear from separate nuclei at about the 

 eighty-fourth day, and unite in about twenty-four days' time. At 

 birth all of these parts are connected by cartilaginous strips. They 

 are not fully fused until the seventh year of postnatal life, the two 

 exoccipitals and the squamoccipital becoming ankylosed some two 

 years later. 



The sphenoid bone arises from twelve bone nuclei arranged in 

 pairs, these being divided into two pair for the presphenoidal and 

 four for the postsphenoidal centres. These centres successively 

 appear from the fifty-fourth to the ninety-first day of intrauterine 

 life. 



The parietal bones are of interest, as they constitute a great por- 

 tion of the vault and sides of the skull, and are in direct relation- 

 ship with some of the most important sutures — the sagittal with its 

 fellow, the coronal with the frontal, the lambdoidal with the 

 squamoccipital, and the squamus with the squamal ; the anterior 

 inferior angle articulating with the sphenoid, and the posterior 

 inferior angle articulating with the mastoid portion of the petrosal. 

 As a rule, each parietal bone ossifies from a single earthy spot, 

 situated in the outer layer of the membranous covering of the cra- 

 nium, at about the forty-second day of intrauterine existence. 



The frontal bone, another important suture-bearing roof bone, 

 arises from two earthy spots in the external layer of the membranous 

 covering of the cranium, about a week later than those that are 

 intended for the parietal bones. These two portions, as a rule, 

 inite soon after birth by a median suture-line known as the metoptic. 

 Ankylosis commences at about the second year of postnatal exist- 

 ence. A portion of the bone helps form a part of the orbits and 

 has its main connections with the ethmoidal, the lacrymal, the 

 malar, the superior maxillary, the nasal, the parietal, and the sphe- 

 noidal bones. 



