SKULL BIRDS 



171 



The history of the chondrocranium is partly known and the following account 

 is tentative. The basal plate (fig. 184, b) arises as a continuum of notochord, 

 parachordals and three occipital vertebras, the posterior the most prominent, 

 while in some birds the arch of the first vertebra arises independently and forms 

 the metotic piUar, with nerves IX and X between it and the otic capsule. The 

 foramen for the nerves is completed in the usual way, and is later divided by a 

 process from the basal plate into glossopharyngeal and jugular foramina. The 

 synotic tectum chondrifies separately, soon uniting with the capsules and the 

 dorsal ends of the fused occipital vertebras, which become shortened and tele- 

 scoped, accompanying the change in the plane of the foramen magnum, the 

 tectum becoming nearly vertical. 



Fig. 184. — Earlier (right) and later (left) stages of chondrocranium of Tinnunculus 

 (Suschkin, '99). al, 'alisphenoid' cartilage; ai, foramen for internal ophthalmic 

 artery; h, basal plate; hpl, basipterygoid process; iorh, interorbital septum; itr, inter- 

 trabecula; mc, middle concha of nose; ot, otic capsule; ov, occipital vertebrae; pc, posterior 

 semicircular canal; sorh, supraorbital bar; sir, supratrabecula (with al, forms spheno- 

 lateral) ; tr, trabecula. 



The prechordal parts appear later than the chordal. As a result of the 

 cephalic flexure, the trabeculte (formed independently) are at first nearly at right 

 angles with the basal plate, the angle soon flattening to about 160°. Some 

 birds have a separate intertrabecular plate betw^een the anterior ends of the 

 trabeculae, this forming the ethmoid region; in others the trabeculae unite 

 directly to a trabecula communis which reaches to the hypophysis where 

 the two trabeculae separate, forming the sides of the hypophysial fenestra, and 

 joining the anterior margin of the basal plate. 



Several chondrifications occur in the interorbital region (fig. 184), the 

 relations of which are uncertain. An interorbital septum grows up from the 

 common trabecula, dividing dorsally into supraseptal plates. The septum 

 extends back to the optic nerves which pass on either side of it. In Tinnunculus 

 a large cartilage {sorb) extends back from the nasal region as two diverging 



