470 



THE TRABECUL.E. 



primitive trabeculae, and an orbitosphenoidal region in front 

 growing out from the presplienoidal region of the trabeculy3. 

 These plates form at first a continuous lateral wall of the cranium. 

 At the front end of tlie 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 trabeculce 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 

 (Hg. 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 pre- 

 frontal or lateral ethmoidal process. 



These and other processes growing out from the trabeculce have 

 occasionally been regarded as rudimentary prseoral branchial arches. I 

 have already stated it as my view that the existence of bi-anchial arches 

 in this region is highly impi-ol)able, and 1 may add that the develo[>nient 

 of these structures as outgrowths of the irkull is in itself to my mind a nearly 



Fig. 327. Skull of adult Dogfish, side view. (From Parker.) 

 0.6'. occipital condyle ; A%i. periotic capsule ; Pt.O. pterotic ridge ; Hp.O. sphenotic 

 process; S.Or. supraorbital ridge; Na. na^al capsule ; P.N. prenasal cartilage ; II. optic 

 foramen; V. trigeminal foramen ; Fl.Pt., Qu. pterygo-quadrate arcade ; Jl/.Pf. meta- 

 pterygoid ligament (including a small cartilage); Pl.Tr. ethmo-palatine or palato- trabe- 

 cular ligament; i)/cfc. lower jaw; Sp. spiracle; ff.il/. byomandibular; Ci/y. ceratohyal ; 

 711. h.L mandibulo-hyoid ligament; Ph. Br. pharyngobrancbial ; E.Br, epibranchial; C.br. 

 ceratobranchial ; H.Br, hypobrancbial ; B.Br, basibranchial ; Ex.Br. extrabranchial ; 

 ''. '\ ■*, ■*! '• labial cartilages ; the dotted lines within Mch. indicate the basihyal. 



