SKELETON. 



6l 



brain; the lower of these, the trabeculae cranii, join the anterior 

 margin of the basal plate while the dorsal bars, the alae temporales 

 or alisphenoid cartilages are eventually connected with the anterior 

 wall of the otic capsules. In most vertebrates the trabeculae 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- 

 beculae, 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. 



t IG. 60. Early (platybasic) chon- 

 drocranium of an elasmobranch, 

 ?tiaightened out Compare with fig. 

 59. als, alisphenoid; ctr, cornua tra- 

 beculse; ep, ethmoid plate; fhyp fenes- 

 tra hypophyseos; oc, otic capsule; ov, 

 occipital vertebrae; n, notochord; PC, 

 parachordal plate; tr, trabeculae. 



In the elasmobranchs and amphibians 

 the trabeculae are widely separated until they 

 reach the ethmoid plate, a condition correla 

 ted with the anterior extension of the brain. 

 This is the platybasic chondrocranium. In 

 the other classes the brain does not extend 

 so far forward and the two trabeculae 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 



