. A manual of zoology. Zoology. 522 CHORD ATA. bone, surround the foramen magnum, the opening through which the spinal cord passes to connect with the brain. These are a pair of exoccipitals, right and left, a supraoccipital above and a basioccipital below. The otica depend in their development upon the extent of the otic region. In the fishes, where this part is large, several bones may be present: epiotic, pterotic, sphenotic, prootic, and often opisthotic. In the mammals, on the other hand, these are fused to a single petrosal bone (figs. 466, 467) of small size. Since the otic bones usuall
. A manual of zoology. Zoology. 522 CHORD ATA. bone, surround the foramen magnum, the opening through which the spinal cord passes to connect with the brain. These are a pair of exoccipitals, right and left, a supraoccipital above and a basioccipital below. The otica depend in their development upon the extent of the otic region. In the fishes, where this part is large, several bones may be present: epiotic, pterotic, sphenotic, prootic, and often opisthotic. In the mammals, on the other hand, these are fused to a single petrosal bone (figs. 466, 467) of small size. Since the otic bones usually do not reach the middle line below, the sphenoidalia rest direct upon the basioccipital behind and in front upon a presphenoid bone, both unpaired but arising from. Fig. 561.—Skull of Koat. (Frcim Clans.) Als^ alisphenoid: Bs, basisphenoid: C, occip- ital condyle; EtU, mesethmoid, covering the ectetlinioid: Fo, optic foramen in orbitosphenoid: Fi\ frontal; Inix. premaxillary; 7p, interparietal: Ju, .iugal (malar); -La, lachrymal ; Mx, maxillary; Nn, nasal: (_th, basioccipital; 01. exoc- cipital; Ors, orbitosphenoid; Fa, parietal; Pa/, palatine: ft-, petrosal; P"*, paramastoid process ; , presphenoid ; Pf, pterygoid : sf, frontal sinus in frontal iDone ; ^pb, basisphenoid ; Sq, squamosal; 3'//, tympanic; Vo, vomer. paired centres. With each is connected, right and left, a pair of bones; with the basisphenoid the alisiihenoids, with the presphenoid the orbitosphenoids, just as the exoccipit;ils flank the basioccipital. In the region of the nasal capsule there is an unpaired mesethmoid with a pair of ectethmoids. Hence the cranium of primary bones may bo described as a chain of four median basal bones, basioccipi- tal, basisphenoid, presphenoid, and mesethmoid; right and left of this a row of exoccijiital, alisphenoid, orbitosphenoid, and ecteth- moid. The position of tho otic capsule results in the sum of the otic bones, the petrosal, l)eing wedged in between the exo
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