. Outlines of zoology. Zoology. 642 MAMMAL/A. ,. the fusion of the parts of each ramus of the mandible into a single bone in the adult, and the three ossicles of the ear. In studying the skull, it is convenient to consider the bones in groups. On the posterior surface of the skull the foramen magnum, through which the spinal cord issues from the cranial cavity, is bounded by the basioccipital beneath, the exoccipital on the sides, the supraoccipital above. The exoccipitals form most of the occipital condyles, but the basioccipital contributes a small part. In many Mammals the ex- occipita
. Outlines of zoology. Zoology. 642 MAMMAL/A. ,. the fusion of the parts of each ramus of the mandible into a single bone in the adult, and the three ossicles of the ear. In studying the skull, it is convenient to consider the bones in groups. On the posterior surface of the skull the foramen magnum, through which the spinal cord issues from the cranial cavity, is bounded by the basioccipital beneath, the exoccipital on the sides, the supraoccipital above. The exoccipitals form most of the occipital condyles, but the basioccipital contributes a small part. In many Mammals the ex- occipitals alone form the condyles. From each exoccipital a parocci- pital process descends, and is applied to the tympanic bullaâa dilatation at the base of the tympanic bone which protects the external auditory tube. Along the roof of the skull from behind forwards lie the supraoccipital, the parietals, the frontals, and the nasals. Between the supraoccipital and the parietals there is a small in- terparietal. On the very front of the skull are the premaxilla, bearing the incisor teeth. Behind each premaxilla is a maxilla, bearing the premolars and molars ; behind this, along the zygo- matic or temporal arch projecting beneath the orbit, is the jugul or malar, which unites posteriorly with the squa- mosal. This zygomatic arch bridges over the deep temporal fossa behind the orbit, and serves for the insertion of muscles, and its '' squamoso-max- illary" structure occurs outside of Mammalia in the Anomodont reptiles only. The fact that in Rodents the malar does not form part of the face is of considerable systematic importance. The squamosals form a great part of the posterior side walls of the skull, and articulate with the parietals, frontals, orbitosphenoids, and ali- sphenoids. At the posterior end of the zygomatic arch is the longi- tudinally elongated glenoid cavity in which the mandible moves backwards and forwards. In connection with the floor of the skull and the roof of
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Keywords: ., bookauthorth, bookcentury1800, bookdecade1890, booksubjectzoology
