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. 



Tlie 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 



