THE SKULL. 



469 



an argument, not without force, which has been appealed to in support of 

 this view. 



In the majority of the lower forms the trabeculae arise quite inde- 

 pendently of the parachordals, though the two sets of elements soon 

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

 trabecul* 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 trabeculte 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 primitively projects 



Uln 



bbr 



Fig. 326. Side vikw of the caktilaginous cranium of a Fowl on the 

 SEVENTH DAY OF INCUBATION. (After Parker.) 

 pn. i3renasal cartilage ; aln. aliuasal cartilage ; ale. aliethmoid ; immediately below 

 this is the aliseptal cartilage, eth. ethmoid ; jip. pars plana ; ps. presphenoid or inter- 

 orbital ; pa. imlatine ; pg. pterygoid ; z. optic nerve ; «.s. alisphenoid ; q. quadrate ; 

 St. stapes ; fr. fenestra rotunda ; hso. horizontal semicircular canal ; psc. posterior 

 vertical semicircular canal : both the anterior and the posterior semicircular canals are 

 seen shining through the cartilage, so. supraoccipital ; eo. exoccii^ital ; oc. occipital 

 condyle ; nc. notochord ; vth. Meckel's cartilage ; ch, cerato-hyal ; hh. basi-hyal ; chr. 

 and ehr. cerato-branchial ; hhr. basibranchial. 



the whole base of the fore-brain, but the space itself gradually be- 

 comes 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 trabecular 

 soon unite together both in front and behind and ibrm a com- 

 plete plate underneath the fore-brain, and extending into the nasal 

 region\ A special vertical growth of this plate in the region of the 

 orbit forms the interorbital plate of Teleostei, Lacertilia and Aves 

 (fig. 326, |>6'). on the upper surface of which the front part of the 

 brain rests. The trabecular floor of the brain does not long remain 

 simple. Its sides grow vertically upwards, forming a lateral wall for 

 the brain, in which in the higher types two regions may be distin- 

 guished, viz. an alisphenoidal region (fig. 326, as) behind, growing 

 out from what is known as the basisphenoidal region of the 



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

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



