74 ZOOLOGY 



SECT. 



formed by the basi-occipital below, the ex-occipitals at the sides > 

 and the supra-occipital above ; the parietal segment (C), formed by 

 the basi-sphenoid below, the alisphenoids laterally, and the parietals 

 above ; and the frontal segment (D) constituted by the pre-sphenoid 

 below, the orbito-sphenoids on either side, and the frontals above. 

 It must be observed that this segmentation of the cranium is quite 

 independent of the primary segmentation of the head, which is 

 determined by the presence of myomeres and by the relations of 

 the cerebral nerves. 



The cranial bones have constant relations to the cerebral nerves. 

 The olfactory nerves (A, Nv. 1) pass out one on either side of the 

 mesethmoid, the optic nerves (Nv. 2) through or immediately 

 behind the orbito- sphenoids, the fifth nerves (Nv. 5) through or 

 immediately behind the alisphenoids, and the tenth nerves (Nv. 10) 

 through or immediately in front of the ex-occipitals. 



It will be seen that a clear distinction can be drawn between 

 the primary cranium or chondrocranium, formed by the fusion of the 

 parachordals, auditory capsules, and trabeculae, and consisting of an 

 undivided mass of cartilage more or less replaced by cartilage 

 bones, and the secondary cranium modified by the super-addition 

 of membrane bones. 



A similar distinction may be drawn between the primary and 

 secondary jaws. The primary upper jaw or palato-quadrate be- 

 comes ossified by three chief cartilage bones on each side, the 

 palatine (A. PL) in front, then the pterygoid (PTG), and the quad- 

 rate (QU) behind, the latter furnishing the articulation for the 

 .lower jaw or mandible. In the higher classes the primary upper 

 jaw does not appear as a distinct cartilaginous structure, and the 

 palatine and pterygoid are developed as membrane bones. The 

 secondary upper jaw is constituted by two pairs of membrane bones, 

 the premaxilla (PMX) and the maxilla (MX), which in bony skulls 

 furnish the actual anterior boundary of the mouth, the primary jaw 

 becoming altogether shut out of the gape. The proximal end o 

 the primary lower jaw ossifies to form a cartilage bone, the articular 

 (ART), by which the mandible is hinged ; the rest of it remains as 

 a slender, unossified Meckel's cartilage (Mck. C), which may dis- 

 appear entirely in the adult. The secondary lower jaw is formed by 

 a variable number of membrane bones, the most important of 

 which is the dentary (DNT). In Mammalia the dentary forms the 

 entire mandible, and articulates, not with the quadrate, but with 

 a large membrane bone formed external to the latter, and known 

 as the squamosal (SQ). 



In the hyoid arch a cartilage bone, the hyomandibular (HY. M), 

 appears in the cartilage of the same name, and ossifications are 

 also formed in the various segments of the hyoid cornua (EP. HY, 

 c. HY, H. HY, B. HY) and of the branchial arches (PH. BR, EP. BR, 

 C. BR, H. BR, B. BR). In the air-breathing forms both hyoid and 



