452 ZOOLOGY sect. 



single large optic foramen (Fig. 1082, opt.fo.). Connected laterally 

 with the basisplienoid and presphenoid are two pairs of thin 

 irregulcir plates, the cdisplunoids (as.) behind and the orbiiosphenoids 

 (o. sph.) in front. The alisphenoids are broad wing-like bones, 

 each produced below into abilaminate process, the pterygoid process. 

 A large foramen, the sphenoidal fissure {sph.), situated between the 

 basispbenoid and the alisphenoid of each side, transmits from the 

 interior of the skull the third and fourth cerebral nerves, the first 

 and second divisions of the fifth, and the sixth nerves. 



The boundary of the anterior part of the brain-case is com- 

 pleted by a narrow plate of bone, the cribriform plate of the 

 ethmoid (Fig. 1083, e^^i.), perforated by numerous small foramina for 

 the passage of the olfactory nerves. This cribriform plate forms a 

 j)art of a median vertical bone, the mescthmoid, the remainder of 

 which, or lamina perpendicularis, forms the bony part of the 

 partition (completed by cartilage in the unmacerated skull) between 

 the nasal cavities. Fused with the mesethmoid are two lateral, 

 thin, twisted bones, the ethmo-turbinals (e. tb), and with its inferior 

 edge articulates a long median bone, with a pair of delicate lateral 

 wings, the vomer (vo.). None of these, with the exception of the 

 cribriform plate, take any share in the bounding of the cavity of the 

 cranium. Roofing over the part of the cranial cavity the walls and 

 floor of which .are formed by the sphenoid elements, is a pair of 

 investing bones, the parietak (Fig. 1082, ^^a.), and further forward 

 is another pair, the frontals (fr.). The parietals are plate-like bones, 

 convex externally, concave internally, which articulate with the 

 supraoccipital behind by a transverse serrated suture, the lamb- 

 doidal. The right and left parietals articulate together by means 

 of a somewhat wavy suture, the sagittal; in front a transverse 

 serrated suture, the coronal, connects them with the frontals. 

 Between the supra-occipital and the parietals is a median osssi- 

 fication or inter-parietal (int. pa.) of varying extent. The frontals 

 are intimately united along the middle line by means of the 

 frontal suture. Laterally their orbital plates form an important 

 part of the upper portion of the inner wall of the orbit ; above 

 this, over each orbit, is a curved, somewhat crescentic process, the 

 supra-orhital process. Between the alisphenoid below, the parietal 

 and frontal above, the frontal and orbitosphenoid in front, and the 

 parietal behind, is a broad bone (sq.), the superior margin of which 

 is bevelled off : this is the squamosal. It is produced in front into a 

 strong zygomatic process, which curves outwards, then downwards, 

 and finally forwards, to unite with the jugal in the formation of 

 the zygomatic arch. Below the root of the process is a hollow, the 

 glenoid fossa. Behind, the squamosal gives off a slender process, 

 the post-tympanic process (p. t. sq.) which becomes applied to the 

 outer surface of the periotic. 



Between the occipital and parietal bones, below and behind the 



