OCR Text |
Show 128 ON THF. VBRTEBRATE SKULfJ. of the pnlatine plates of these bonos, which ev~ntnally unit.e w~th the vomer, give Tise to the apertures, winch are ordinarily called posterior nares. So that in Man, fo~· example, there are three pairs of "11ares :"-the external, situated between the anterior end of the internasal septum, the nasal bones, and the premaxillre, as in the lower Vertebrates;. the me~ian, between the vomer, the palatjnes, and the premax1llm, wlnch correspond with the posterior nares of tho lower V ertcbrates; and the posterior, between the vomer, internally, and tho palatines above, at the sides, aud belovt', which are peculiar to the higher Vertebrates. And, to r0turn to the maxilla, we £nd that it really differs altoo·ether from the other pre-oral bones, and is, as it were, 0 fastened on to the outer sides of the premaxillary and palatine bones, without having any primary dir ct conn ction with the cranio-facial axis. The post-oral bones surround the buccal avity, and form two distinct arches-the mandibular and tho hyoidean. Neither of these arches is directly counected with the cranio-facial axis, nor with the segments of the brain-ca 'e, but both are suspended to different parts of th t m poral Lone, which is so singularly intercalated betw en the n1iddlo and I ost rior of those segments. The lower jaw or Mandible (Mn) con ists of two rami, anchylosed at the symphysis, and each con i ting of a ingle piece, the condyle of which articulates with the squamosal. The Hyoid hone (Hy), con1pos d ofit body and two pairs of cornua, does not articulate directly with the temporal bone, but ligaments connect it with the ty loicl processes, and these last bones unite with the po t rior part of the periotic capsules. Thus, the natural conn ction of th bones by no means allow of the separation of the walls of the lower cha1nbers of the human skull into a ri) of arch s springing from, and corresponding with, th axial part , as we found to be the case with the walls of the upp r chambers. If the temporal bon be detached, the hyoidean and mandibular arches come with it, and exhibit no connection with the occipital or the parietal segm nt . Indeed, the latter is 'l'HE 'l'RUCTURE Oli' rriiE HUMAN SKULL. 12D preoccupied by the pterygoi<l and the palatine, both of whieh are connected with the basi-sphenoid (at least with the lingulm), while the anterior part of tho palatine is also connected, in the adult state, with the presphenoid, by tho intermediation of tho cornua sphenoidalia. Two bones yet remain to be mentioned which come neither into the category of axial bones, nor of superior or inferior arch bones, nor, strictly speaking, of sense-capsule bones. These are the Lachrymal (L ), intercalated between the nasal, Inaxillary, and lateral 1nass of tho ethmoid, and serving to lodge the conduit which places the orbit and the nasal cavity in commu11ication ; and the Jugal or Malar (Ju), which connects the bones of the orbital chamber with the squamosal element of the temporal bone. The skull, thus rom posed, serves as a protection to the organs which are lodged within it, and which are of as great in1portance in their morphological, as in their physiological, aspect. The cerebral hemispheres and cerebellum, with their dependent parts, fill the cranial cavity, the lower lateral margin of the posterior cerebral lobes corresponding with the torcular Herophili and the lateral sinuses, ou the inner surface of the occipital bone; or, in other words, with the line of attachment of the tentorium. Certain axial parts of the brain have definite relations to the axial parts of the cranitun. Thus, the medulla oblongata lies upon the basi-occipital. The pituitary body rests upon the upper surface of the ba i-sphenoid, this bone constituting the chief part of the front as well as of the hinder wall of the sella turcica. The ch~'asma of the optic nerves rests upon the hinder portion of tho upper face of the presphenoid, and the peduncles of the olfactory nerves upon the front portion of that face. The termination of the axial parts of the brain in tho lamina terminalis of the third ventricle corresponds pretty nearly with the termination of the basi-cranial axis in the anterior extremity of the presphenoid. Not less important are the relations of many of the cerebral nerves to the lateral elements of the arches of the brain-case. The filamentH of the olfactory nerves pa~s ont thrmtgh the J( |