FISHES, 289 



carp, behind the union of the orbitary wings, there is a hole lead- 

 ing into a large canal, which is situated behind under the middle 

 posterior fossa, surrounded superiorly and laterally by lamina of the 

 great wing, inferiorly by the sphenoid, and terminating in a funnel in 

 the basilar. In front it lodges the jjituitary gland, and gives passage 

 into the cranium to the vertebral arteries. The existence of this 

 cavity is not constant ; for instance, it is wanting in the cod, whose 

 pituitary gland is but slightly encated. 



The cavities of the ear lie between the middle and posterior fossa. 

 In the quadrupeds, these cavities are enveloped in the petrous bone, and 

 form a prominence in the interior ; in birds and reptiles, they occupy 

 several of the neighbouring bones ; and in bony fishes they commu- 

 nicate openly with the cranium. These cavities consist, 1st. of two 

 great fossa scooped out beneath the cavity in which the brain is 

 lodged, and are prolonged to the sides of the posterior fossa : they are 

 surroimded by tlie great Aving, the lateral occipital and the basilary, 

 and serve to lodge the sacs containing the large stones of the ear. 

 2d. of different depressions which occupy the lateral posterior angle of 

 the cranium, extending into the external occipital, the mastoidean, the 

 lateral occipital, and even a little into the parietal, the posterior fron- 

 tal, and the great wing, and serve for tlie lodgement of the semi-circu 

 lar canals. 



Foramina of the Cranium. 



According as the closure of the cranium in front is more or less 

 complete, there are varieties, not precisely in the holes which pierce 

 this cavity, but in the mode in which they are surrounded by the 

 bones. Thus in most of the acanthopterygians, and in the perch, 

 which we take as a type, the olfactory and optic nerves, and those of 

 the third and fourth pairs merely pass through the membrane which 

 closes the large opening situated in front, between the frontals, the 

 orbitary wings, and the anterior sphenoid. The same obtains in 

 the cod, in Avhich, moreover, tlae fifth pair passes through a fissure 

 only, of the anterior edge of its great wing ; whilst in the perch, not 

 only is there in the middle of this great wing holes for the branches 

 of the fifth pair, but one near its edge for the sixth. The eight pair 

 passes out by two holes tlirough the side of the lateral occipital ; and 

 the tenth by one in its superior surface, not far from the occipital 

 hole. 



In the bony sculls, we may also observe certain solutions of con- 

 tiouity, which in the fresh state, are only closed by membranes or 

 cartilages ; thus the perch and several other acanthopterygians have a 

 remarkable one on each side, between the parietal, the mastoidean, and 

 the external occipital ; it is likewise seen in the pike, which also 

 possesses another between the posterior frontal, the great wing, and the 

 mastoidean ; it is even in the middle of this cartilage in the pike, that 

 we find suspended a very small vestige of the petrous bone. 



W(' ha\-c already spoken of the very large hole in eacli lateral 

 ci-cipital of the cyprins. These^ fishes liave a small azygos bone, be- 

 tween tlie parietals and the interparietal ; some of the silures present 



VOL. II. u 



