Quain's elements of anatomy . US OF THREE MONTHS, SEEN FROM ABOVE. (From Koiliker.) a, upper half of the squama occipitis ; h, lower half ofthe same ; c, cartilaginous plate extending into it ; d,(in the foramen magnum) the exoccipital ; e, basi-occipital;/, petrous, with the meatus auditorius internus; g, dorsumsella?, with two nuclei belonging to the basi-sphenoidbone ; h, nuclei in the anterior clinoid processes; i, greatwing nearly entirely ossified ; I; small wings ; I, cristagalli; »?, cribrethmoid ; n, cartilaginous nose ; o, strip ofcartilage between the sphenoid and the parietal ; jj,
Quain's elements of anatomy . US OF THREE MONTHS, SEEN FROM ABOVE. (From Koiliker.) a, upper half of the squama occipitis ; h, lower half ofthe same ; c, cartilaginous plate extending into it ; d,(in the foramen magnum) the exoccipital ; e, basi-occipital;/, petrous, with the meatus auditorius internus; g, dorsumsella?, with two nuclei belonging to the basi-sphenoidbone ; h, nuclei in the anterior clinoid processes; i, greatwing nearly entirely ossified ; I; small wings ; I, cristagalli; »?, cribrethmoid ; n, cartilaginous nose ; o, strip ofcartilage between the sphenoid and the parietal ; jj,osseous plate between the lesser wings and the cribriformplate. which surrounds the posterior semicircular canal and extends into themastoid portion. They soon unite into one so as to form the petro-mastoid bone. Here it may be stated that, in addition to the parts of the skull before-mentioned, the ethmo-turbinals and cribriform plate, the styloid processand the tlu-ee auditory ossicles are of cartilaginous origin ; but the. THE CEANIAL FLEXFEES. 811 tympanic ring, the nasal, maxillary and other facial bones are all formedin membrane. To these, however, we shall recur hereafter. In the cranium of the human embryo chondrification begins in thebasilar portion in the fourth, and fifth weeks, and is nearly completedfor the principal parts by the eighth week, soon after which in the courseof the tenth week cartilaginous ossification sets in. But in some of themembrane bones, as for example in the lower jaw, ossification begins ata much earlier date, probably in the sixth week. The form of the head is greatly modified at an early period of deve-lopment by the cranial flexure as well as by the changes which accom-pany the development of new parts. The Cranial Flexures.—The earliest and the most important of thecranial flexures is that, previously mentioned, which takes place at theanterior extremity of the notochord and in the region of the mid-brainor middle encephalic vesicle. A
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