18 THE STRUCTURE OF THE FOWL 



first cervical vertebra, or atlas. The fact that 

 birds have only one occipital condyle permits 

 of the well-known extensive rotation of the 

 skull on the vertebral column. Each lateral 

 part of the occipital bone is pierced by a 

 common opening leading into the jugular 

 foramen and the carotid canal. 



The roughly triangular sphenoid bone forms 

 the greater part of the base of the cranium, 

 and consists of a posterior portion made up 

 of a body and a pair of temporal wings, and an 

 anterior part represented by a body only. 

 On each side of the body there is a slightly 

 projecting, smooth surface for articulation with 

 the pterygoid bone. The two optic nerves 

 leave the cranium by a single opening which 

 represents the two optic foramina of mammals. 

 The common aperture occurs opposite the 

 edge of the interorbital septum by which the 

 two optic nerves are separated immediately 

 on leaving the cranium. It should be remarked 

 that a wide basi= temporal bone, developed in 

 membrane, covers a great part of the sphenoid, 

 and forms the broad part of the cranium 

 visible when this is viewed from the ventral 

 aspect. 



Filling the interval between the occipital 



