THE SKELETON OF THE VERTEBRATE HEAD. 145 



veloped form, are posterior or inner actinapophyses of the 

 rhinal, vomerine, and ethmoidal haemal arches. These actina- 

 pophyses become included in the nasal fossa by the closure 

 of the metasomatomic clefts ; and, as they subsequently 

 elongate, they abut against one another in the antero-posterior 

 direction. 



I shall, in the sequel, show that the more or less denned 

 space termed orbit, at the side of the mammalian cranium, is 

 fundamentally the metasomatomic fissure between the eth- 

 moidal and pre-sphenoidal sclerotomes. The upper part of 

 this fissure continues permanently open as the lachrymal 

 canal, and drains away the secretion which bathes the front 

 of the eyeball, while that organ, supported by the sclerotic, 

 which is a pre-sphenoidal neuractinapophysis, and surrounded 

 by its accessory structures, is lodged in its dilated portion. 

 From the upper, anterior, and lower orbital margins, which 

 are formed by elements of the anterior of its two bounding 

 primary sclerotomes, a fibrous membrane extends backwards, 

 covered externally by the orbicular muscle, and closing in the 

 contents of the orbit, with the exception of the front of the 

 eye, exposed through the palpebral fissure. This fibrous 

 membrane is a metasomatomic or actinal lamina, extending 

 very obliquely outwards and backwards like an operculum 

 over the orbit. The succeeding metasomatomic membrane 

 assumes the form of the tissue which separates the orbit from 

 the temporal fossa, and which, passing backwards external to 

 that fossa, forms the temporal fascia which constitutes an 

 operculum to that space. The temporal fossa itself is the 

 upper portion of the metasomatomic fissure between the pre- 

 and post-sphenoidal sclerotomes, occupied by the muscles of 

 mastication and the homologous nerve ; the lower part of the 

 fissure on each side remaining permanently open as the mouth, 

 or more correctly as the anterior opening of the isthmus of 

 the fauces. By the extension of ossification from neighbouring 



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