The athmoid, Owen (anterior sphenoid,Cuv.: 15,) so highly developed in the carnivorous Alammalia, is in the lower Vertebrata reduced to an extretnely simple condition. In Fishes (fig. 437) it is generally distinct enough, form ing the posterior boundary of the interorbital septum, but sometimes it is quite wanting or represented by membrane. When present, it is generally placed upon the sphenoid, sending off processes to join sometimes the ingrassial bones, sometimes the alar bones, or occasionally to remain suspended in the interorbital mem brane that unites all these parts. The cethmoid appears to be deficient throughout all tribes of Reptiles. In Birds it is recognizable as a bone of considerable size, separating the posterior parts of the orbits, which it assists in forming the two lateral facets that enter into the com position of those cavities corresponding with the ossa plow:, as they are called, of the human subject ; but these are obviously only portions of the cethmoid itself. In the Mammalia, owing to the prodigious developement of the olfactory apparatus, the cethmoid becomes ex tremely increased in size and importance, closing the anterlor extremity of the cranial box, where it is perforated so as to present a central crest and cribriform plate, while inferiorly it has superadded to its body the superior tur binated osseous lamella. that enter so largely into the construction of the olfactory organ.
The vomer (16) is in Fishes a large and im portant bone, joined posteriorly to the sphenoid and above to the mthmoid, forming a vertical portion, on each side of which are situated the organs of smell. Inferiorly it forms part of the roof of the mouth, and is often armed with teeth. Throughout all the Vertebrata this portion of the skeleton holds an analogous position and is re cognized with facility. In Frogs and Lizards the bone is double, but in Tortoises and the higher animals generally there is but a single limner, which enters more or less into the composition of the nasal septum.
The nasal bones, Owen; (cethmoid, Cttv.: 3) in Fishes are represented by a single bone impacted between the mid-frontals and the pre frontals, and inferiorly joined to the vomer, forming a kind of septum between the nasal organs, and thus in position resemble some what the vertical lamella of the cethmoid of Alammalia. Sometimes, as in the Eel and the Conger, the bones in question are inseparably united into one piece. In the higher animals the nasal bones are two in number, covering, the nasal cavity like an arch. They are present in all Reptiles except the Chelonians, and in Birds and AIammals are easily recognizable from their position.
The inferior turbinatcd bones, although in consequence of the construction of their nose quite wanting in Fishes, must not be omitted in enumerating the elements composing the skull in higher animals. In the humbler Rep
tiles, indeed, no traces of it are distinguishable ; but when the olfactory apparatus becomes fully developed, as in the AIammalia, they form an important part of the nasal character, and are found of large size, connected inseparably with the bones that surround the nose.
The bones of the face have been already considered as constituting a very complex framework, destined to lodge the organs of the principal senses or to constitute the instruments appropriated for the prehension or.mastication of food. Seeing, however, that the same bone not unfrequently enters into the composition of several distinct cavities, we are unable to classify them further, and must therefore content our selves with enumemting them seriatim as they occur to our notice.
The maxillary (18) perform only a secondary office in forming the upper jaw of a Fish, being in the finny tribes generally destitute of teeth, which in them are principally implanted upon the intermaxillary (17) that form the greater porticm of the upper jaw. The maxillary in Fishes is moveably articulated with the inter maxillary, the vomer (16), and the palatine (22). Sometimes, as in the Herring and Lepisosteus, this bone is divided into several pieces. In Skates and Rays the whole upper jaw is made up of a single ossified mass, which bears the numerous rows of teeth attached to its under surface.
But in all Reptiles, in T3irds, and in Mam malia the maxillary bones form the principal portion of the upper jaw, more particularly in the Mammalia, where the intermaxillary bones are comparatively of small size. In this portion of the upper jaw are fixed the grinding teeth, where such are present, a circumstance which I in itself demands great strength in this part of, the face; and, consequently, wherever power] of jaw is required to be conferred, it is prin cipally obtained by the increased developement of this element of the skeleton, which thus be comes the largest and, as it were, the central bone of the whole fabric.
The intermaxillary bones (17) form the pri cipal part of the upper jaw in Fishes, and upo their shape depends that of the snout. Som times these bones are flattened horizontally, compressed laterally, or prolonged into a bea their form being modified by circumstanc in almost every genus. In the Chondropterygi nevertheless, they are mere rudiments imbedd in the substance of the upper lip. They a persistent throughout all orders of Reptil Birds, and Alammals, until we arrive at th Quadrumana, where they become anchylose with the maxillary, and in Alan they are quite obliterated at an early period.