! oS ELEMENTA R Y ANA TOM Y. [LESS. 



distinct bones, the posterior of which is termed in zootomy 

 the basi-sphenoid, while the anterior ossification (in front of 

 the pituitary fossa) bears the name of pre-sphenoid. 



The basi-sphenoid is a point of very great morphological 

 importance, marking as it does the anterior termination of 

 the chorda dorsalis, which never advances further forwards 

 than the hinder margin of the sella turcica. 



Now, there may be no bony representative of the basi- 

 sphenoid at all although more or less of the rest of the skull be 

 ossified as is the case in Lepidosiren and the Batrachia ; or 

 it may be represented by a mere rudiment -a little Y-shaped 

 bone, only forming part of the front of the pituitary fossa, as in 

 the Pike. Thus it need form no part of the inside floor of the 

 skull, nor yet any of the basis cranii, although the last-named 

 part be well ossified, as it is in osseous Fishes and Batrachians. 

 In the Ichthyopsida, in fact, only the median bony investment 



FIG. 104. UNDER SURFACE OF THE SKULL OF A FROG. (After Parker.) 

 c, girdle- bone ; eo, exoccipital ; mx, maxilla; par, parasphenoid ; pm. pre- 

 maxillri : po, pro-otic ; pt, pterygoid ; q, quadrato-jugal ; sns, suspense- ium of 

 lower jaw, the lower end of which represents the quadrate bone ; i>, vomer ; 

 i, optic foramen ; 2, foramen ovale ; 3, condyloid foramen. 



of the basis cranii is a special membranous ossification, termed 

 the para-sphenoid, which extends, both backwards and for- 

 wards, far beyond the limits of the basi-sphenoidal region. 

 This "membrane" bone is also large and conspicuous in 

 Snakes (which have the body of the sphenoid in a cartilagi- 

 nous condition), but it seems to be representated in man, if 

 at all, only by the " lingulae sphenoidales." 



