THE SKULL OF OSSEOUS FISUES, 103 



consists of two hollow hemisphcroid pieces, each with two opposite 

 eiiuirgiiiations ; the inner ones circumscribing tlie hole, (analogous 

 to the meatus internus of the petrosal,) for the entry of the nerves 

 and vessels to the essential parts of the organ of vision ; and the 

 outer or anterior emarginations supporting the cornea. As this 

 part of the skeleton of tlie head retains its primitive libro-membranous 

 condition in Man and Mammalia, it is called the sclerotic coat of 

 the eye ; and the osseous plates developed in it in Birds, many 

 lieptiles, and Fishes, are termed ' sclerotic bones,' It bears, 

 however, the same essential relation to the vascular and nervous 

 parts of the organ of sight, which the petrous bone does to the 

 organ of hearing, and which the turbinate bones do to the organ 

 of smell ; the persistent independence of the eye-capsule, which has 

 led to its being commonly overlooked as part of the skeleton, relates 

 to the requisite mobility and free suspension of the organ of vision. 

 In the Cartilaginous Fishes, however, it is articulated by means of 

 a pedicle with the orbi to-sphenoid. The osseous cavity or ' oi'bit ' 

 lodging the eye-ball is formed by the pre-spheuoid, orbito-sphenoid, 

 frontal, post-frontal, pre-frontal, and palatine bones : it opens widely 

 outwards, where it is, often, further circumscribed by the chain of 

 ' sub-orbital ' scale-bones below, and, but less frequently, by a 

 supra-orbital bone above. The bony orbits in most fishes com- 

 municate freely together, or rather with that narrow prolongation of 

 the cranial cavity lodging the olfactory nerves : but, in many 

 Malacopteri, e. (j. the Shads and Erythrinus, the Citharinns and 

 Hydrocyon, tlae Synbranchus, and the genus Cyprinus {fig. 35. 18), 

 an osseous septum divides the orbits. In the Lepidosteus and Poly- 

 pterus tlie orbits are divided by a double septum, forming the proper 

 walls of the olfactory prolongation of the cranium, as we shall find to 

 be the case in the Batrachia. 



The bony capsules of the organ of smell present the same division 

 into cranial and nasal {athmoidal,fi(j. 35. 18, turhhial,fi(/. 30. I'j) por- 

 tions, in Fislics as in INIan, and, as in Man likewise, other bones, the 

 vomer and nasal, for example, contribute an accessary protective func- 

 tion. All the parts of the proper capsule are cartilaginous in cartila- 

 ginous and semi-osseous fishes ; the etlimoidal part continues cartila- 

 ginous in many osseous fislies, closing the foi'e part of the cranium, 

 assisting to form the interorbital septum, and contributing to support 

 the olfactory nerves in their exit from the skull. When ossification is 

 fstablished in the ethmoidal cartilage, it is usually confined in fishes 

 to tlie cranial end, forming there a single symmetrical, slender, bifur- 

 cate or sub-quadrate piece, usually perforated 1)y the olfiictory 

 nerves ; but never in two distinct pieces corresponding to the two 



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