344 



COLLEGE ZOOLOGY 



ingful to the student of gross anatomy. 



There are two main subdivisions of the 

 skeleton: (1) the axial and (2) the appen- 

 dicular skeleton. 



The axial skeleton comprises the skull, 

 vertebral column, and sternum.* The ap- 

 pendicular skeleton consists of the pectoral 



and pelvic girdles, and the bones of the 

 limbs which they support. 



The cartilage and bones of the skull may 

 be grouped into two main divisions: (1) the 

 brain case, and auditory and olfactory cap- 

 sules, which constitute the cranium; and 

 (2) the visceral skeleton (Fig. 228). 



Premaxllla 

 Maxilla — 



Nasal 



Vomer 



Vomerine teeth 

 Palatine - 



Dorsol 



Sphenethmoid 

 Frontoparietal 



Parasphenold 



Squamosal 



Pterygoid 



Prootic 



J Columella i,v^ 



^Quadratojugal -^ ■ 



Exoccipital 



Quadrate cartilage 

 Occipital condyle 

 Foramen magnum 



Figure 227. Skull of the bullfrog in dorsal and ventral views. 



Ventral 



Cranium 



A large part of the cranium consists of 

 cartilage. The bones are either ossifications 

 of the cartilage, the cartilage bones, or have 

 developed into connective tissue without 

 passing through a cartilage stage, the mem- 

 brane bones. The spinal cord passes through 

 a large opening, the foramen magnum, in 

 the posterior end of the cranium. On either 

 side of this opening is a convexity of the 

 exoccipital bones called the occipital con- 

 dyle, Vk'hich articulates in life in a concavity 

 of the first vertebra and enables the frog 

 to move its head. 



The cranial bones of the dorsal side are 

 the prootics, which enclose the inner ears; 

 the frontoparietals, which form most of the 



* The sternum may be regarded as a part of the 

 appendicular skeleton because it supposedly orig- 

 inated in the early history of the land vertebrates 

 from the pectoral girdle. However, it may be con- 

 sidered a part of the axial skeleton because of its 

 medial (central) location. 



roof of the cranium; the sphenethmoid, 

 which forms the posterior wall of the nasal 

 cavity; and the nasals, which lie above the 

 nasal capsules. The ventral surface of the 

 cranium discloses the central, dagger-shaped 

 parasphenoid and the vomers, which bear 

 the vomerine teeth (Fig. 227). 



Visceral skeleton 



The jaws, hyoid, and cartilages of the 

 larynx, which constitute the visceral skele- 

 ton, are preformed in cartilage and then 

 strengthened by ossifications. The upper 

 jaw (maxilla) consists of a pair of premaxil- 

 lae, a pair of maxillae, and a pair of quad- 

 ratojugals. The maxillae and premaxillac 

 bear teeth. The lower jaw (mandible) con- 

 sists of a pair of angulosplenials and a pair 

 of dentary bones which overlap the angulo- 

 splenials and extend forward to meet a pair 

 of mentomecklian bones. The visceral arches 

 are represented in the adult by the hyoid 

 and its processes (Fig. 228 ) . 



