ANATOMY OF VERTEBRATES. Ill 



cavity ; its inner, usually cartilaginous, surface lodging part of 

 one of the semicircular canals. It is wedged into the interspace 

 of the ex- and par-occipitals, the petrosal, the alisphenoid, the 

 parietal, the frontal, and postfrontal bones. The projecting pro- 

 cess lodges above the chief mucous canal of the head, and below 

 affords attachment to the epitympanic, or upper piece of the 

 bony pedicle from which the mandibular, hyoid, and opercular 

 bones are suspended. 



The orbitosphenoids, figs. 83, 85, 10, are osseous plates usually 

 of a square shape, sometimes semicircular or semielliptic, as in 

 the Cod; larger in the Malacopteri, fig. 83, 10 ; very small in 

 most Acanthopteri ; and sometimes represented by a descending 

 plate of the frontal, as in the Garpike, or by unossified cartilage, 

 as in mail-cheeked fishes. In the Carp their bases meet, like those 

 of the alisphenoids, above the sphenoid : when osseous matter is 

 developed in the interorbital septum the orbitosphenoids are 

 articulated by their under and anterior part to that bone or 

 bones, fig. 83, io. 1 The olfactory nerves pass forward by the 

 superior interspace of the orbitosphenoids and the optic nerves 

 escape by their inferior interspace, or by a direct perforation ; and 

 the essential functions of the orbitosphenoids relate to the pro- 

 tection of the sides of the cerebrum or prosencephalon, and to 

 the transmission of the optic nerves. The orbitosphenoids 

 frequently bound or complete the foramen ovale. 



Although the frontal always enters into the formation of the 

 cranial cavity, its major part forms the roof of the orbits, which 

 accessory function is the chief condition of the great expanse of 

 this neural spine in fishes. Single, and sending up a median crest 

 in the Cod, the Ephippus, and some other fishes, the frontal is 

 more commonly divided along the median line, the divisions 

 having the form of long and broad subtriangular plates, fig. 76, 

 n, n ; narrower in the lofty compressed skulls, smaller in those 

 with large orbits, and becoming greatly expanded in the fishes 

 with small and deep-set eyes. Each frontal sends up its own crest 

 in the Tunny, 2 the interspace leading to a foramen, penetrating the 

 cranial cavity in front of the single occipital spine : a larger 

 fontanelle exists in the Cobitis and some Siluroids between the 

 frontal and parietal bones. In the Salamandroid fishes (e. g. 

 Pohjpterus) each frontal sends down a vertical longitudinal plate, 



1 The specially developed interorbital septum, or 'cranial rcthmoid' of Cuvier in 

 the Bream and Carp, misled Bojanus into the belief that it was the body of the 

 prosencephalic vertebra (vertebra optica). — Isis, 1818, p. 502. 



2 Reminding one of the double spine of the neural arch of the atlas in Tetrodon. 



