SKELETON. 61 
brain;. the lower of these, the trabecule cranii, join the anterior 
margin of the basal plate while the dorsal bars, the ale temporales 
or alisphenoid cartilages are eventually connected with the anterior 
wall of the otic capsules. In most vertebrates the trabecule and 
alisphenoids develop as a continuum, but in some elasmobranchs they 
are at first distinct (fig. 59). The two 
trabeculae unite in front to form a 
median ethmoid plate beneath the 
olfactory lobes, beyond which they 
diverge as two horns, the cornua tra- 
beculz, ventral to the nasal organs. 
The floor of the trough is formed by 
the ethmoid plate in front, while behind 
it is usually of membrane, but in the 
elasmobranchs cartilage gradually ex- 
tends from one trabecula to the other, 
closing last below the infundibulum 
and hypophysis, these lying for a time 
in an opening (fenestra, later fossa 
hypophyseos), and after the closure, 
in a pocket in the floor of the chon- 
drocranium, one of the cranial land- - 
marks, the sella turcica. eh TH 
as Fic. 60.—Early (platybasic) chon- 
In the elasmobranchs and amphibians drocranium of ‘an_ elasmobranch, 
the trabecule are widely separated until they ‘t:aightened out. Compare with fig. 
othe cthmota slate: a condition corel 59- als, alisphenoid; cir, cornua tra- 
reach the ethmoid plate, a condition correla. pecule; ep, ethmoid plate; fhyp fenes- 
ted with the anterior extension of the brain. tra pepe: oc, otic papsules ov, 
ee i : occipital vertebre; , notochord; fc, 
This is the platybasic chondrocranium. In pargehonial plate ds uabecdle, ’ 
the other classes the brain does not extend 
so far forward and the two trabecule meet just in front of the hypophysis (fig. 
62) to continue forward as a trabecula communis to the ethmoid region. The 
trabecula communis is usually compressed between the eyes to a vertical interor- 
bital septum. This represents the tropibasic chondrocranium. 
In the more primitive vertebrates the trough is converted into a 
tube around the brain by the extension of cartilages between the ali- 
sphenoid cartilages and the otic capsules of the two sides dorsal to the 
brain. This roof or tegmen cranii is usually incomplete, having one 
or more gaps or fontanelles, closed only by membrane. In the higher 
vertebrates the cartilage roof is at most restricted to a mere arch, the 
synotic tectum, between the otic capsules of the two sides. Later 
ep ot 
