| OCR Text |
Show 120 ON THE VERTEBRATE SKULL. to a s1nall extent, to the formation of the two occipital condvles. In the adult skull the basi-occipital anchyloses com pletoly ~vith the ex-occipital, on the one hano, and with tho next bono of the basi-cranial axis on the other, o that the saw 1nu t be called to our aid in order to demonstrate the bone. Fig. 5 ~{. Fig. 53 .-Crani~-facial axis and ~ ateral elements of the superior ar('hes (as in Fig. 52), with the. pteryg~Id bones, and without the vomer , viewed from below.-e, junction of the basi-sphenoid ant! presphenoid with the internasal cartilage ; C.S., cornua sphenoidaliu, or bones of Bertin. From the synchondrosis a to the point b, in even so young a skull as that here represented, the basi-cranial axis is formed by one continuous ossification, the B asi-sphenoicl bone, excavated superiorly (Figs. 50 and 52) by a saddle-shaped cavity, the sella turcica, which lodges the pituitary body,-an organ of no great physiological moment, so far as we know, but of first-rate morphological significance. On each side of the hinder part of the sella turcica, the basi-sphenoid presents a groove for the internal carotid artery, and this groove is completed in front and externally, by an osseou~ mass, tapering from behind forwards, the lingula sphenott~alis, which lies between the basi-sphenoid and alisphenoid. At the front part of the sella, separating it from the ) I THE STHUOTURE OF THE HUMAN SKULL. 121 depression for the optic commissure, there is a transverse ridge, the tuberculum sellm.* The region between the synchondrosis and the tuberculun1 is the upper surface of the basi-sphenoid. Its under-surface (Fig. 53) exhibits a median, wedge-shaped portion, terminating abruptly at the point e, on each side of which are stuck on, as it were, two delicate bones, shaped somewhat like sugar-bags, with their wide and open ends directed forwards and their apiees backwards. These are the bones of Bertin, or cornua sphenoidalia, which do not. properly belong to the Lasi-sphenoid, but coalesce with it in the course of growth. From the tuberculum sellm (c) to the point (b) in the upper view (Fig. 52), and from the point e, to b of the lower view (Fig. 53), the middle region of the cranio-facial axis belongs to a third bone, the presphenoid (PS) which terminates the basi-cranial axis. I say terminates the basi-cranial axis, because the appearance of a continuation forwards of that axis by the crista galli, or upper margin of the lamina perpendicularis of the ethmoid (see Fig. 50), is altogether fallacious, depending, as it does, upon a special peculiarity of the highest Mammalian skulls, which arises from the vast development of the cerebral hen1ispheres. In the great majority of JJfammalia below the Apes, in fact, the free edge of the lamina perpendicularis is not horizontal, but greatly indined, or even vertical; and in these cases the whole la·mina plainly appears to be, what it really always is, beyond, or anterior to, the floor of the brain-case; while the true basicranial bones are parts of the floor of the brain-case. During footal life, the basi-sphenoid and presphenoid are united only by synchondrosis, traces of which may even be discovered (as Virchow has shown) as late as the thirteenth year, or later. Even before birth the two bones become anchylosed superiorly, their junction being marked by the tuberculum sellm; and the ren1ains of the synchondrosis extend * Where the t erms employed in our ordinary handbooks of Humun Anatomy do not suffice for my purpose, I adopt those used by Henle in Lis classical "Handbuch der Systematischen Anatomic des Menschen," n work of great accuracy and comprehensiveness, now in course of publication. |
