THE ORGANS OF THE INTERMEDIATE LAYER OR MESENOHYME. 619 s 



It may be stated as a general rule that the ossifications on the base 

 and sides of the cranium are primordial, but that on the roof and in 

 the face covering bones make their appearance. 



The following parts of the human skull belong to the primordial 

 elements : (1) occipitale, except the upper part of the squamous 

 portion; (2) the sphenoidale, except the internal pterygoid plate; 



(3) ethmoidale and turbinatum ; (4) petrosum and mastoid portions 

 of the temporale ; (5) the auditory ossicles malleus, incus, and 

 stapes; (6) the body of the hyoides, with its greater and lesser 

 cornua. 



The following are covering bones : (1) the upper part of the 

 squamous portion of the occipitale ; (2) the parietale; (3) thefrontale; 



(4) the squamous portion of the temporale ; (5) the internal pterygoid 

 plate of the sphenoidale ; (6) the annulus tympanicus; (7) palatinum; 

 (8) vomer; (9) nasale; (10) lachrymale; (11) zygomaticum; (12) 

 maxillae sup. ; (13) maxillae inf. 



I will now, after this survey, give a somewhat more detailed account 

 of the development of the bones of the head enumerated above. 



I. Bones of the Cranial Capsule. 



(1) The occipitale is at first a cartilaginous ring surrounding the 

 foramen magnum ; it begins to ossify early in the third month at 

 four points. One centre of ossification is formed below the foramen, 

 another above, and two more at its sides. In this way there arise 

 four bones, which are joined by broader or narrower bands of carti- 

 lage, according to the degree of their development. In the lower 

 Vertebrates Fishes, Amphibia (fig. 337 Olat) they remain in this 

 condition as separate bones, and are designated as occipitale basilare, 

 oc. superius, and oc. laterale. 



To these are added in Mammals and Man a covering bone, which 

 arises from two centres of ossification in the connective tissue farther 

 above the foramen the interparietale. This begins, even in the third 

 foetal month, to fuse with the superior occipital bone to constitute 

 the squama ; however, up to the time of birth furrows running in 

 from right and from left mark the boundary of the two genetically 

 different parts. In the new-born child squama, occipitalia lateralia 

 and oc. basilare are still separated from each other by thin remnants 

 of cartilage. Then in the first year the squama fuses with the 

 lateral parts (partes condyloideae), and finally there is united with 

 these, in the third or fourth year, the pars basilaris. The occipitale 



