494 COLLEGE ZOOLOGY 



The cartilage and bones of the skull may be grouped into two 

 main divisions: (i) the brain case and auditory and olfactory 

 capsules, which constitute the cranium; and (2) the jaws and 

 hyoid arch, which constitute the "visceral skeleton. 



CRANIUM. A large part of the cranium consists of cartilage 

 (dotted in Fig. 418). The bones are either ossifications of the 

 cartilage (the exoccipitals (ex), prootics (pro), and ethmoid), 

 or are developed from membranes and invest the cartilage and 

 cartilage bones. The spinal cord passes through a large open- 

 ing, the foramen magnum (Fig. 418, fm), in the posterior end of 

 the cranium. On either side of this opening is a convexity of 

 the exoccipital bones (ex), called the occipital condyle, which 

 articulates in life in a concavity of the first vertebra (av), and 

 enables the frog to move its head. 



The cranial bones of the dorsal side are the prootics (Fig. 418, 

 pro) which inclose the inner ears, the frontoparietals (pf) which 

 form most of the roof of the cranium, the sphenethmoid (sp. et) 

 which forms the posterior wall of the nasal cavity, and the. nasals 

 (na) which lie above the nasal capsules. The ventral surface of 

 the cranium discloses the central, dagger-shaped paras phenoid 

 (par) and the vomers (vo) which bear the vomerine teeth. 



THE VISCERAL SKELETON. The jaws and hyoid, which con- 

 stitute the visceral skeleton, are also preformed in cartilage and 

 then strengthened by ossifications. The upper jaw or maxillary 

 arch consists of a pair of premaxilla (Fig. 418, pmx), a pair of 

 maxilla (mx), and a pair of quadratojugals (qj). The maxillae 

 and premaxillae bear teeth. The lower jaw or mandibular arch 

 consists of a pair of cartilaginous rods (Meckel's cartilages) 

 invested by a pair of dentary bones, and a pair of angulo-splenials. 

 The jaws are attached to the cranium by a suspensory apparatus 

 consisting of the squamosals (Fig. 418, sq), the pterygoids (ptg), 

 and the palatines (pal). 



The visceral arches are represented in the adult by the hyoid 

 and its processes (Fig. 418, D). The cartilaginous basilingual 

 plate lies in the floor of the mouth cavity. The hyoid arches 



