THE DEVELOPMENT OF THE HUMAN SKULL. 145 



single centre in its cartilaginous predecessor, but the parietals 



are the result, not of the ossification of cartilage, but of that 

 of the membrane which roofs in the skull. Each has its own 

 centre of ossification in this membrane. 



The presphenoid arises by two separate centres of ossification, 

 one on the inner side of each optic foramen. (Fig. 59, P.S.) 

 These centres coalesce with the orbito-sphenoids of their own 

 side before thev unite with one another.* The osseous orbito- 



* The mode of ossification of the sphenoid bone is one of the most difficult ques- 

 tions in osteogenesis. Meckel has worked out the problem at great length in his 

 "Archiv," Bd. i., and thus sums up his results in the " Handbuch der Menschlichen 

 Anatomie," Bd. ii., pp. 102-4 : — 



" In the third month, the first osseous nuclei appear in the two great wings, and 

 soon afterwards the internal pterygoid processes begin to ossify as separate bones. 

 Next, a third pair of ossifications appears hi the external circumference of the alse 

 minores ; and then, about the fourth month, a seventh and eighth nucleus, which lie 

 side by side in the body of the sphenoid. In the fifth month is formed, alongside 

 this fourth pair, a fifth, between it and the great wings. Upon tins the two median 

 nuclei of the body coalesce. Soon arises a sixth nucleus, on the inner side of the 

 optic foramen, and then a seventh appears between this and the fourth, so that, about 

 the beginning of the seventh month, the sphenoid consists of thirteen separate bony 

 nuclei, since, notwithstanding seven pahs have arisen, the two primary nuclei of 

 the body early coalesced into one. 



"From this time forth the number of the nuclei diminishes still more con- 

 siderably by coalescence. Those nuclei coalesce earliest which give rise to the 

 portions of the sphenoid, which persist in a separate state longest. The fourth, fifth, 

 and seventh pair soon unite into one piece ; the first and second, coalescing on each 

 side, constitute two other pieces ; the third and sixth two others ; whereby, in the 

 eighth month, the sphenoid consists of five pieces — the two greater wings, the lesser 

 wings, and the body. Somewhat later the two lesser wings coalesce into one, and 

 the sphenoid now consists of four pieces ; thereupon the body and the anterior pieces 

 unite, so that in the fully-formed foetus the sphenoid consists of three pieces, the 

 greater wings and internal pterygoid jn'ocesses being still distinct ; but in the first 

 month after birth these three pieces unite into one."' 



The fifth pah of ossifications here mentioned are the lingidse ; the sixth, those 

 which give rise to the presphenoid. Meckel's seventh pair of ossifications, which 

 arise between the fourth (basi-Sphenoidal) and the sixth (presphenoidal), and are 

 said, in the "Archiv, ' to coalesce first with one another, and then with the basi- 

 splienoid, appear not to have been observed by other anatomists. I have not seen 

 them, and they are not mentioned by Virchow, the latest writer on the subject. 

 Virchow writes ( " Entwickelung des Schadel-grundes," 1857;: — "The posterior 

 sphenoid arises (if we leave out of consideration the internal pterygoid processes 

 which are developed as separate and independent bones), according to most authors, 

 from three nuclei, but, according to my observations, from six. Two of these belong 

 to the alse magnae (alse temporales), or lateral arches (Bogenstiicken, of the parietal 

 vertebra. They arise in the third month, and the external pterygoids are produced 

 by direct outgrowths from them. In the third month, I also find two other centres 



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