xni THE SKELETON 229 



CHAPTER XIII 

 THE SKELETON 



IN the skeleton, or bony framework, of the frog we com- 

 monly distinguish two main divisions, the axial, consisting 

 of the skull and vertebrae, and the appendicular, composed 

 of the limbs and their girdles or supports. We shall begin 

 our description with the skull. 



Bones of the Cranium. - - In the skull we may distinguish 

 the cranium, or part inclosing the brain and principal sense 

 organs, and the viscera! skeleton, which forms the jaws and 

 hyoid arch. The cranial portion of the skull is relatively 

 small, and is narrowest in the central part, between the very 

 large spaces, or orbits, which lodge the eyes. At the posterior 

 end is a large aperture, the foramen magnum, through which 

 the spinal cord passes. On either side of this opening are 

 the exoccipital bones, which are separated from each other 

 above and below by a small piece of cartilage. At the sides 

 of the foramen magnum these bones bear a pair of rounded 

 prominences, the occipital condyles, which articulate with the 

 atlas, or first vertebra. Just external to each condyle is a 

 small aperture for the exit of the vagus nerve. 



At the sides and in front of the exoccipitals lie the prootic 

 bones, each of which forms a ring-like lateral projection on 

 each side of the skull, which incloses the inner ear. Anteri- 

 orly each prootic is perforated by a large aperture, through 

 which pass the 5th, 6th, and yth cranial nerves. On the 

 outer side there is ah opening, the foramen ovale, which 



