The trabeculæ 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 the whole base of the fore-brain, but the space itself gradually becomes 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 trabeculæ soon unite together both in front and behind and form a complete plate underneath the fore-brain, and extending into the nasal region[204]. 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 lateral wall for the brain, in which in the higher types two regions may be distinguished, viz. an alisphenoidal region ([fig. 326], as) behind, growing out from what is known as the basisphenoidal region of the primitive trabeculæ, and an orbitosphenoidal region in front growing out from the presphenoidal region of the trabeculæ. These plates form at first a continuous lateral wall of the cranium. At the front end of the brain they are continued inwards, and more or less completely separate the true cranial cavity from the nasal region in front. The region of the cartilage forming the anterior boundary of the cranial cavity is known as the lateral ethmoid region, and it is always perforated for the passage of the olfactory nerves.
The cartilaginous walls which grow up from the trabecular floor of the cranium generally extend upwards so as to form a roof, though almost always an imperfect roof, for the cranial cavity. In the higher types, in Mammals more especially, this roof can hardly be said to be formed at all. The region of the trabeculæ in front of the brain is the ethmoid region. The basal part of this region forms an internasal plate, from which an internasal septum may grow up ([fig. 326]). To its sides the olfactory capsules are attached, and there are usually lateral outgrowths in front forming the trabecular cornua, while from the posterior part of the ethmoidal plate, forming the anterior boundary of the cranial cavity, there often grows out a prefrontal or lateral ethmoidal process.
These and other processes growing out from the trabeculæ have occasionally been regarded as rudimentary præoral branchial arches. I have already stated it as my view that the existence of branchial arches in this region is highly improbable, and I may add that the development of these structures as outgrowths of the skull is in itself to my mind a nearly conclusive argument against their being branchial arches, in that true branchial arches hardly ever or perhaps never arise in this way.
The sense capsules. The most important of these is the auditory capsule, which, as we have seen, fuses intimately with the lateral walls of the skull. In front there is usually a cleft separating it from the alisphenoid region of the skull, through which the third division of the fifth nerve passes out. This cleft becomes narrowed to a small foramen ([fig. 327], V). The sclerotic cartilage is always free, but profoundly modifies the region of the cranium near which it is placed. The nasal investment forms in Elasmobranchs ([fig. 327], Na) a capsule open below, and continuous with the ethmoid region of the trabeculæ. In most types however it becomes more closely united with the ethmoid region and the accessory parts belonging to it.
Fig. 327. Skull of adult Dogfish, side view. (From Parker.)
O.C. occipital condyle; Au. periotic capsule; Pt.O. pterotic ridge; Sp.O. sphenotic process; S.Or. supraorbital ridge; Na. nasal capsule; P.N. prenasal cartilage; II. optic foramen; V. trigeminal foramen; Pl.Pt., Qu. pterygo-quadrate arcade; M.Pt. metapterygoid ligament (including a small cartilage); Pl.Tr. ethmo-palatine or palato-trabecular ligament; Mck. lower jaw; Sp. spiracle; H.M. hyomandibular; C.Hy. ceratohyal; m.h.l. mandibulohyoid ligament; Ph.Br. pharyngobranchial; E.Br. epibranchial; C.br. ceratobranchial; H.Br. hypobranchial; B.Br. basibranchial; Ex.Br. extra-branchial; l1, 2, 3, 4, 5. labial cartilages; the dotted lines within Mck. indicate the basihyal.
The cartilaginous cranium, the development of which has been thus briefly traced, persists in the adult without even the addition of membrane bones in the Cyclostomata, Elasmobranchii ([fig. 327]) and Holocephali. In the Selachioid Ganoids it is also found in the adult, but is covered over by membrane bones. In all other types it is invariably present in the embryo, but becomes in the adult more or less replaced by osseous tissue.
The most primitive type of branchial skeleton in any existing form would appear to be that of the Petromyzonidæ, which is developed in a superficial subdermal tissue, and consists of a series of bars united by transverse pieces, so as to form a basketwork. It is known as an extra-branchial system, and an early stage of its development in the Lamprey is shewn in [fig. 47]. In the higher forms this system is replaced by a series of bars, known as the branchial bars, so situated as to afford support to the successive branchial pouches. Outside these bars there may be present in some primitive forms (Elasmobranchii) cartilaginous elements, which are supposed to be remnants of the extra-branchial system ([fig. 327], Ex.Br); while a series of membrane bones is also usually added to them, which will be dealt with in a separate section. The branchial bars are developed as simple cartilaginous rods in the deeper parts of the mesoblast which constitutes the primitive branchial arches.