. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. 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 hor
. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. 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 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
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