452 ZOOLOGY 



SECT. 



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

 with the basisphenoid and presphenoid are two pairs of thin 

 irregular plates, the alisphenoids (as.) behind and the orbitosphenoids 

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

 each produced below into a bilaminate process, \\\Q pterygoid process. 

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

 basisphenoid 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 .J85^th.\ perforated by numerous small foramina for 

 the passage ^Tthe olfactory nerves. This cribriform plate forms a 

 part of a mediafcn verticil \hpne, the mcscthmoid, the remainder of 

 which, or lamitm perpeh^ctyaris, forms the bony part of the 

 partition (completed by car^ag^-^(tB^teh^ ccrate d skull) between 

 the nasal cavities..^ Fused witla the meiyishmoid are two lateral, 

 thin, twisted WppW>' the ethmo-tiurbinals (e. S^-and with its inferior 

 edge articulates^Mang median bone, with a pair^oMelica^e lateral 

 wings, the vomer\^). None pthese.with the exc&g&vu of the 

 cribriform plate, ta^fce^n^harj^mTfh e bmmtring^rthe cavity of the 

 cranium. Roofing overTrTe^part of the cranial cavity the walls and 

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

 investing bones, the parietals (Fig. 1082, pa.\ and further forward 

 is another pair, the frontals (/?*.). 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-orbital 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 



