OCR Text |
Show 582 MR. J. B. SUTTON ON THE DEVELOPMENT [June 2, Later a nucleus makes its appearance on the inner side of each optic foramen, on the deep aspect of the perichondrium: these are the presphenoidal centres. They attain some considerable size before involving the cartilage, and can during the first month of their existence be removed without in any way disturbing the subjacent cartilage. The orbitosphenoid quickly sends down two spurs around the optic nerve, which fuse with the piresphenoid. The presphenoids in their turn send a thin shell of bone across the dorsal aspect of the cartilage to fuse with each other ; a small circular spot of cartilage long remains to indicate the point around which they united. It is long before these presphenoidal nuclei fuse below ; a large piece of cartilage, belonging to the ethmo-vomerine plate, separates them, even for some months after birth. Long before this occurs the presphenoids, bearing their allies the orbito-sphenoids, have fused with the basisphenoids; the line of fusion being represented in afterlife by the ridge known as the olivary process. Before dismissing the orbito-sphenoid, it is necessary to draw attention to one circumstance connected with it, of some interest. The lateral extension of the trabeculae to form the side-wall of the chrondro-cranium is in the later stages replaced almost entirely by the alisphenoids. If the region of the side-wall of the skull known as the anterior lateral fontanelle in the foetus (in the adult it is called the pterion) be examined between the fourth and seventh months of intra-uterine life, it will be easily noted that the cartilaginous orbito-sphenoid extends into this fontanelle, so that for a considerable period it helps to form the side-wall of the skull. In m a n the permanent orbito-sphenoid never extends so far outwards as its cartilaginous forerunner, leaving the space to be filled in by the epipteric bone. The details of the ossification of this region I have considered elsewhere **. There remains little to add concerning the later development of the orbito-sphenoid, except to note that eventually the orbito-sphenoids of opposite sides send a thin lamella across that portion of the presphenoid which is anterior to the optic groove, thus excluding it from the cranial cavity. The portions of the sphenoid previously considered are strictly cranial, but it receives an additional element from one of the appendages of the skull, viz. the palato-pterygoid bar. In a paper published in the Proceedings of this Society for 1884, " O n the Parasphenoid " & c , facts were adduced to show that the anterior portion of the palato-pterygoid cartilage in m a n became ossified to form the internal pterygoid plate ; the nucleus for this bone may be detected as early as the commencement of the third month of intra-uterine life. The length of time it m a y remain as a separate ossicle varies within wide limits. I have seen it distinct from the sphenoid as late as the fifth month ; but as a rule it will be found united with the under surface of the alisphenoid at the commencement of the fourth, so that it joins the alisphenoid before that bone 1 " O n the Relation of the Orbito-sphenoid to the Pterion," Journal of Anatomy and Physiology, vol. xviii. p. 219. |