94 LECTURE V. 



the cranial cavity, it usually sends upAvaixls a little process on each 

 side ; or, as in Fistularia, a transverse ridge. Tiie basi-sphenoid is 

 smooth below, where it is usually flattened or convex, but sometimes 

 is produced downwards in the form of a median ridge, and sometimes 

 is perforated for the lodgment of certain muscles of the eye-ball. 

 In the Polypterus both the ali-sphenoids and orbito-sphenoids are 

 anchylosed to the basi-sphenoid, and the result is a bone that answers 

 to the major part of the 'os sphenoides ' in Anthropotomy. As 

 two large and important hfemal arches of the head are suspended 

 from the parapophyses of the second and third cranial vertebrae, this 

 seems to be the condition of the fixation and coalescence of the bodies 

 of those vertebriB in all Fishes. 



The ali-sphenoids (neurapophyses of the parietal vertebra, 

 ih. 6. 6.) are firmly articulated by broad sutural surfaces to the ex- 

 panded sides of the basi-sphenoid, above which their bases usually 

 meet and immediately support the third ventricle or mesencephalon, 

 or leave an interspace for its pituitary prolongation, which then rests 

 in a cavity or ' sella' of the basi-sphenoid. In some fishes, e. g. Perch 

 and Carp, the base of each ali-sphenoid rises some way above the 

 basi-sphenoid, and then sends inwards a horizontal plate, which, 

 meeting that of the opposite ali-sphenoid, forms the immediate support 

 of the mesencephalon, and at the same time the roof of a canal, ex- 

 cavated in the basi-sphenoid, and which traverses the base of the 

 skull, below the cranial cavity from before backwards, opening behind 

 at the under part of the basi-occipital ; this subcranial canal exists in 

 the Salmonoids, Sparoids, Scomberoids, and is very remarkable in 

 most fishes with lofty compressed skulls, as the Ephippus : it exists in 

 some Clupeoids, as the Herring, but not in the Salamandroid Fishes. 

 The subcranial canal resembles, but is not homologous with, being 

 difierently formed from, the posterior prolongation of the nasal pas- 

 sages in the Crocodiles, and it lodges some of the muscles of the 

 eyeball. The form of the ali-sphenoids is influenced by that of the 

 skull : when this is low and flat, their antero-posterior exceeds their 

 vertical extent ; in deep and compressed skulls they are narrow and 

 high plates ; in ordinary shaped skulls they present either a sub- 

 circular form, and are perforated as in the Carp {Jig. 35. e), or are re- 

 niform, the anterior border being deeply notched, as in the Cod {^fig. 

 30. 6) : they form a more definite and fixed proportion of the lateral 

 parietes of the skull than do the petrosals (ib. 16), which are interposed 

 betAveen them and the ex-occipitals ; and they have their essential func- 

 tion in sustaining and protecting the sides of the mesencephalon, and 

 in affording exit to the second and third divisions of the fifth pair of 



