56 GENERAL ZOOLOGY 



Cranium of the Frog. — The skull is composed of two parts: 

 (1) the cranium, which encloses the brain, the inner ears, and 

 olfactory organs, and (2) the visceral skeleton, made up of the 

 upper and lower jaws and the hyoid apparatus. The cranium 

 of the frog occupies the central region of the skull. Beginning 

 at the posterior end are two bones, a right and left exoccipital 

 bone, between which is a large opening, the foramen magnum, 

 through which the spinal cord passes from the cranium to the 

 neural canal of the vertebral column (Fig. 31). Each exoccipital 

 bone, on its posterior face has a rounded projection, the occipital 

 condyle, one on each side of the foramen magnum. The two 



frarf/o-parfefoU^^ exoccipital condyles articulate with the 



columella $£~\ 1 ^*\/^$r pro6t ' c atlas or first vertebra. At the 

 quadratoH^^X^^'^^^ t i uamosal side and just in front of each 



9q \{//r „,„,. occipital condyle is a foramen for 



the passage of the ninth and 

 tenth cranial nerves. 



Fig. 31. — Posterior view of skull of The prootic bones, which form 



Rana catesbeiana. ^ auditory capsu l eS) are short 



cylindrical bones at the sides and in front of the exoccipital 

 bones. On the lower lateral surface of each prootic bone near 

 the outer edge is an opening, the foramen ovale, which faces the 

 middle ear (Fig. 31). The foramen ovale is closed by a cartilage 

 which in turn articulates with the broad, inner end of the colu- 

 mella, the principal ossicle of the middle ear. From the broad 

 base, the columella extends outward as a slender, slightly curved 

 rod to the tympanic membrane to which it is attached. Vibra- 

 tions of the tympanic membrane are transmitted through the 

 columella and the cartilage at its inner expanded end to the inner 

 ear. At the front and toward the ventral side of each prootic 

 bone is an opening serving as a passageway for the fifth, sixth, and 

 seventh cranial nerves. 



The frontoparietal bones, one on either side, extend forward 

 from the exoccipital bones to form the greater part of the roof of 

 the portion of the cranium enclosing the brain (Fig. 30). These 

 two bones are firmly united in the mid-line by a median suture. 

 From the side of each a wing extends downward to form a portion 

 of the side wall of the cranium. The greater part of the floor 

 of the cranium is formed by the unpaired parasphenoid bone. 

 This bone consists of a transverse process spanning the region 



