THE SKULL. 



569 



space is an argument, not without force, which has been appealed to in 

 support of this view. 



In the majority of the lower forms the trabecuhe arise quite 

 independently of the parachordals, though the two sets of 

 elements soon unite ; while in Birds (fig. 325) and Mammals the 

 parachordals and trabeculae are formed as a continuous whole. 

 The junction between the trabeculae and parachordals becomes 

 marked by a cartilaginous ridge known as the posterior clinoid. 



The trabeculae are usually somewhat lyre-shaped, meeting in 

 front and behind, and leaving a large pituitary space between 

 their middle parts (figs. 323 and 325). Into this space there 



tnft 



bbr 



cbr 



FIG. 326. SIDE VIEW OF THE CARTILAGINOUS CRANIUM OF A FOWL ON THE 



SEVENTH DAY OF INCUBATION. (After Parker.) 



pn. prenasal cartilage; aln. alinasal cartilage ; ale. aliethmoid; immediately below 

 this is the aliseptal cartilage, eth. ethmoid ; //. pars plana ; ps. presphenoid or 

 interorbital ; pa. palatine ; pg. pterygoid ; z. optic nerve ; as. alisphenoid ; q. 

 quadrate ; st. stapes ; fr. fenestra rotunda ; hso. horizontal semicircular canal ; 

 psc. posterior vertical semicircular canal : both the anterior and the posterior semi- 

 circular canals are seen shining through the cartilage, so. supraoccipital ; eo. exocci- 

 pital ; oc. occipital condyle ; tic. notochord ; ink. Meckel's cartilage ; ch. cerato- 

 hyal ; bh. basi-hyal ; cbr. and ebr. cerato-branchial ; bbr. basibranchial. 



primitively projects the whole base of the fore-brain, but the 

 space itself gradually becomes narrowed, till it usually contains 

 only the pituitary body. The carotid arteries always pass through 

 it in the embryo ; but in the higher forms it ceases to be 

 perforated in the adult. The trabeculae soon unite together both 

 in front and behind and form a complete plate underneath the 

 fore-brain, and extending into the nasal region 1 . A special 



1 In Man (Kolliker) the trabeculrc form from the first a continuous plate in front 

 of the pituitary space, and the latter very early acquires a cartilaginous floor. 



