THE SKELETAL SYSTEM 



169 



one another during the course of evolution. This also is borne out by 

 the development of the human cranium. 



Ossification of the occipital bone, for example, begins in four centers 

 corresponding with the basioccipital, the paired exoccipitals with their 

 condyles, and a supraoccipital of lower vertebrates. To these are later 

 added a membranous interparietal. Ossification of the occipital begins 

 in the third month but is not completed until the seventh year. 



NASAL 

 LACRIMAL 

 MAXILLA 

 ZYGOMATIC 

 VECKEL'S CART 

 MANDIBLE 



NASAL CONCHA 



(fR; >J^s3jr- OCCIPITAL 



- -A^-i^jf TEMPORAL 



MASTOID PROCESS 



'AUDITORY MEATUS 

 ZYGOMATIC ARCH 



Fig. 159. — The human skull, embryonic (A) and adult (B-D). In the fetal skull 

 (14 weeks) membrane bones are black, and the cartilage cranium (chondrocranium) 

 stippled. Figure B shows the adult skull in basal aspect, figure C in frontal aspect, and 

 figure D in left lateral aspect, approximately one quarter natural size. (Redrawn after 

 Sobotta.) 



The development of the sphenoid bone is even more complex; no fewer 

 than ten centers of ossification are recognized. Six of these arise in the 

 body of the bone and four more in the two paired wings. Membrane bone 

 is added both to the pterygoid processes and to the great wings. Fusion 

 of the separate elements is completed before the second year. 



Ossification of the ethmoid remains throughout life incomplete. 

 Three centers of ossification corresponding to the pro-, epi-, and opisthotic 



