THE CLASS BIRDS. 2d 



in mammals; and the inferior, instead of being articulated 

 directly with the cranium by means of condyles, is connected 

 therewith through the intermedium of a distinct bone, called 

 tympanic or os quadratum, generally considered to be a 

 portion of the temporal bone (the osseous meatus), and remain- 

 ing distinct throughout life. Moreover, each branch of the 

 lower jaw is composed of two segments, and it is by a fossette 

 that it articulates with the tympanic bone. 



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. 



Lachrymal Bone. 



Nostril 

 Sup. Maxil. 



"*-.. Tympanum. 



Os Qua- 

 dratum. 

 Nasal Fossa?. JugalBone. Infer. Maxil. 



Fig. 223. 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. 222) ; 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 ; 



* 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. 



