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 (tig. 325) and Mammals the parachordals and 

 trabeculae are formed as a continuous whole. The junction between 

 the trabeculaj and parachordals becomes marked by a cartilaginous 

 ridge known as the posterior cliimid. 



The trabeculie are usually somewhat lyre-shaped, meeting in front 

 and behind, and leaving a large pituitary space between their middle 

 parts (figs. 323 and 32.5). Into this space there primitively projects 



Fi<;. 32(J. SIDE VIEW OF THK CARTILAGINOUS CRANIUM OF A FOWL ox TIII; 

 SEVENTH DAY OF INCUBATION. (After Parker.) 



l>n. prenasal cartilage ; aln. alinasal cartilage ; ah', aliethmoid; immediately below 

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

 orbital ; pa. palatine; pij. pterygoid; z. optic nerve ; tix. filisphenoid ; q. quadrate; 

 st. stapes; /;. fenestra rotunda; //.so. horizontal semicircular canal; psc. posterior 

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

 seen shining through the cartilage, .so. supraoccipital ; co. exoccipital ; or. occipital 

 condyle; nc. notochord ; mli. Meckel's cartilage ; ch. cerato-hyal ; bli. basi-hyal ; cln: 

 and cln\ cerato-branchial ; bbr. 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 form a com- 

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

 region 1 . A special vertical growth of this plate in the region of the 

 orbit forms the interorbital plate of Teleostei, Lacertilia and Aves 

 (fig. 326, ps). 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 lateralwall 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 trabe-whe form from the first a continuous plate in front 

 of the pituitary space, an:l the latter very early acquires a cartilaginous floor. 



