THE CLASS BIEDS. 309 



The mode of articulation of the head with the vertebral 

 column admits of much more extended movements in birds 

 than in mammals ; the articulation is formed of a single con- 

 dyle, a sort of semi- spherical pivot placed in the mesial line 

 of the body, and received into a corresponding articular cavity 

 in the atlas. 



Orbit. Inter-Orbitar Septum, 

 i / 



Lachrymal Bone. 



^^^^ J^ Cranium. 



Sup. Maxil. 



" '"Exoccipital. 



Os Quadratum, 



_ or Tympanic. 



IS asal Fossae. JugalBone. Infer. Maxil. 



Fig. 284. Skeleton of the head of the Eagle. 



429. The neck in birds is generally much longer than 

 in mammals. The higher they are elevated on their limbs, 

 the longer must the neck become, the jaws being the principal 

 organs of prehension (Fig. 283) ; in the swan, the neck ex 

 ceeds the height of the body, thus enabling it to seek its 

 prey at considerable depths while swimming. Thus the 

 number of cervical vertebrae varies greatly, according to the 

 species ; from twelve to fifteen is the usual number, but there 

 may be fewer, and occasionally there are as many, or more, 

 than twenty; they are extremely moveable on each other, 

 and this they owe to the forms of their articular surfaces.* 

 This arrangement is remarkable in wading birds, as in storks ; 

 numerous processes for the insertion of muscles assist in these 

 motions. 



On the other hand, in almost all birds the vertebrse of the 

 back are nearly fixed or immovable, and this, no doubt, is to 

 enable the wings to find in this part of the trunk a point of 

 support. In general, they are consolidated or united into 

 one, but in birds which do not fly, as the ostrich and cassowary 



* These articular surfaces are concave on one aspect and convex on the 

 other (ball-and-socket). In the upper part of the neck they permit of free 

 flexion forwards, but about the middle of the neck they admit only of 

 flexion backwards ; whilst again, towards the base of the neck, they admit 

 only of flexion forwards. 



