THE SKELETON 431 



and fuses with the trabeculae, forming the septum interorbitale 

 and septum nasi (Fig. 243). The free posterior border of this 

 plate lies in front of the optic nerves; an interorbital aperture 

 arises in the plate secondarily (Fig. 243). 



In the ethmoidal region the septum nasi arises as an anterior 

 continuation of the interorbital plate; and the trabecular plate 

 is continued forward as a prenasal cartilage in front of the olfac- 

 tory sacs. Curved, or more or less rolled, plates of cartilage 

 develop in the axis of the superior, middle, and inferior turbinals 

 (see olfactory organ), and these are continuous with the lateral 

 wall of the olfactory capsules, which in its turn arises from the 

 dorsal border of the septum nasi (Figs. 243 and 244). 



(2) The Origin of the Visceral Chondrocranium (Cartilaginous 

 Splanchnocranium) . The visceral portion of the cartilaginous 

 skull arises primarily in connection with the arches that bound 

 the cephalic portion of the alimentary tract, viz., oral cavity 

 and pharynx. In the chick, cartilaginous bars are formed in 

 the mandibular arch, hyoid arch, and third visceral arch. In 

 fishes, the posterior visceral arches also have an axial skeleton, 

 but in the chick the mesenchyme of these arches does not develop 

 to the stage of cartilage formation. The elements of these arches 

 are primarily quite distinct. The upper ends of the mandibular 

 and hyoid skeletal arches are attached to the skull; and the lower 

 ends of the three arches concerned meet in the middle line. Two 

 medial elements or copulae are formed in the floor of the throat, 

 one behind the angle of the hyoid arch, and one behind the 

 third visceral arch (Fig. 245). 



Mandibular Arch. Two skeletal elements arise in the man- 

 dibular arch on each side, a proximal one (the palato-quad- 

 rate) and a distal one (Meckel's cartilage). The former is 

 relatively compressed, and the latter an elongated element (Fig. 

 243, 10). The palato-quadrate lies external to the antero-ver- 

 tral part of the auditory capsule, and soon develops a triradiate 

 form. The processes are: the processus oticus, which applies 

 itself to the auditory capsule, the processus articularis, which 

 furnishes the articulation for the lower jaw, and the processus 

 orbitalis, which is directed anteromedially towards the orbit. 

 A small nodule of cartilage of unknown significance lies above 

 the junction of the processus oticus and otic labyrinth. Meckel's 

 cartilage is the primary skeleton of the lower jaw, corresponding 



