THE SKULL AS A WHOLE 



143 



anterior ends join to form the cthmoidal plate, which extends forward between the forebrain and 

 the olfactory pits_ Later, the trabeculre meet and fuse below the pituitary body, forming the 

 floor of the pituitary fossa, and so cutting off the ar.erior lobe of the pituitary body from the stoma- 

 todeum. The mesal part of the ethmoidal plate forms the bony and cartilaginous parts of the 

 nasal septum. From the lateral margins of the trabeculae cranii three processes grow out on 

 either side. The anterior forms the lateral mass of the ethmoid and the alar cartilages of the 

 nose; the middle gives-rise to the lesser wing of the sphenoid, while from the posterior the greater 

 wing and external pterygoid plate of the sphenoid are developed (Figs. 109 and 1 10). The bones 

 of the vault are of membranous formation, and are termed dermal or covering bones. They are 

 partly developed from the mesoderm of the primordial cranium, and partly from that which lies 

 outside the entoderm of the foregut. They comprise the upper part of the tabular portion of 

 the occipital (interparietal), the squamous temporals and tympanic plates, the parietals, the 

 frontal, the vomer, the~5nternal pterygoid plates, and the bones of the face. Some of them remain 

 distinct throughout Jife (e. g., parietal and front?!), while others join with the bones of the chondro- 

 cranium (e. g., interparietal, squamous temporals, and internal pterygoid plates). 



Optic foramen. Lesser wing of sphenoid. 



Greater wing of sphenoid. 



Nasal capsule., 

 g 

 Nasal septum.&jk 



For. n. facialis. 

 I Tegmen tymp. 

 I / Incus. 



r^2i 



Meckel's cartilage. 

 Cricoid cartilage.- 

 Thyroid cartilage. 



Styloid process. 

 Fen. cochleae. 



For. hijpogl. 



FIG. 110. The same model as shown in Fig. 109 from the left side. Certain of the membrane bones of the 



right side are represented in yellow. 



Recent observations have shown that, in mammals, the basicranial cartilage, both in the 

 chorda) and prechordal regions of the base of the skull, is developed as a single plate, which 

 extends from behind forward. In man, however, its posterior part shows an indication of its 

 being developed from two chondrifying centres which fuse rapidly in front and below. The 

 relation of this cartilaginous plate to the notochord differs in different animals. In the rat embryo 

 it lies vcntrad of the notochord (Robinson); in the sheep, pig, calf, and ferret the cranial part of 

 the notochord is enclosed within it; in man, the anterior and posterior thirds of the cartilage 

 surround the notochord, but its middle third lies on the dorsal aspect of the notochord, which in 

 this region is placed between the cartilage and the wall of the pharynx. 



Differences in the Skull Due to Age. At birth the skull as a whole is large in pro- 

 portion to the other parts of the skeleton, but its facial portion is small, and equals only about 

 one-eighth of the bulk of the cranium as compared with one-half in the adult. The frontal and 

 parietal eminences are prominent, and the greatest width of the skull is at the level of the latter; 

 on the other hand, the glabella, superciliary ridges, and mastoid processes are not developed. 

 Ossification of the skull bones is not completed, and many of them e. g., the occipital, 

 temporals, sphenoid, frontal, and mandible consist of more than one piece. Unossified 

 membranous intervals, termed fontanelles, are seen at the angles of the parietal bones; these 

 fontanelles are six in number; two, an anterior and a posterior, are situated in the middle 

 line, and four, two on each side, an antero-lateral and a postero-lateral, are placed on either 

 .side. 



The anterior or bregmatic fontanelle (Fig. Ill) is the largest, and is situated at the junction 

 of the sagittal, coronal, and interfrontal sutures; it is lozenge-shaped, and measures about an 

 inch and a half in its antero-posterior and an inch in its transverse diameter. The posterior 

 fontanelle is triangular in form and is situated at the junction of the sagittal and lambdoid sutures. 

 The lateral fon tanelles (Fig. 112) are small, irregular in shape, and correspond respectively with 

 the antero-inferior and postero-inferior angles of the parietal bones. An additional fontanelle 

 is sometimes seen in the sagittal suture at the region of the obelion. The fontanelles are usually 



