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 behind, growing out from what is known as the basisphenoidal region of the primitive trabeculæ, and an orbito-sphenoidal region in front, growing out from the presphenoidal region of the trabeculæ. These plates form at first a continuous lateral wall of the cranium. 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. The basi-cranial cartilaginous skeleton reduces itself always into trabeculæ and parachordals with olfactory and auditory cartilaginous capsules. {123}In addition, a branchial skeleton exists, which consists of a series of bars known as the branchial bars, so situated as to afford support to the successive branchial pouches. An anterior arch known as the mandibular arch (Fig. 50, _Mn._), placed in front of the hyo-mandibular cleft, and a second arch, known as the hyoid arch (_Hy._), placed in front of the hyo-branchial cleft, are developed in all types; the succeeding arches are known as the true branchial arches (_Br._), and are only fully developed in the Ichthyopsida. In all cases of jaw-bearing (gnathostomatous) vertebrates the first arch has become a supporting skeleton for the mouth (Fig. 51), and in the higher vertebrates in combination with the second or hyoid arch takes part in the formation of the ear-bones. [Illustration: FIG. 50.--HEAD OF EMBRYO DOG-FISH, ELEVEN LINES LONG. (From PARKER.) _Tr._, trabecula; _Mn._, mandibular cartilage; _Hy._, hyoid arch; _Br_1._, first branchial arch; _Na._, olfactory sac; _E._, eye; _Au._, auditory capsule; _Hm._, hemisphere; _C_1_, _C_2_, _C_3_, cerebral vesicles.] [Illustration: FIG. 51.--SKULL OF ADULT DOG-FISH, SIDE VIEW. (From PARKER.) _cr._, cranium; _Br._, branchial arches; _Mn._ + _Hy._, mandibular and hyoid arches.] The true branchial arches persist, to a certain extent, in the Amphibia, and become still more degenerated in the Amniota (reptiles, birds, and mammals) in correlation with the total disappearance of a branchial respiration at all periods of their life. {124}Their remnants become more or less important parts of the hyoid bone, and are employed solely in support of the tongue. In no single animal is there any evidence that the foremost arch, the mandibular, is a true branchial arch. As low down as the Elasmobranchs it becomes divided into two elements which form respectively the upper and lower jaws; the hyoid arch, on the other hand, although it has altered its form and acquired the secondary function of supporting the mandibular arch, still retains its respiratory function.