. Chordate anatomy. Chordata; Anatomy, Comparative. 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 C
. Chordate anatomy. Chordata; Anatomy, Comparative. 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. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Neal, Herbert V. (Herbert Vincent), 1869-1940; Rand, Herbert W. (Herbert Wilbur), 1872-1960. Philadelphia : P. Blakiston's Son & Co.
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