|
|
The Head
The orientation of the brain in birdsIn humans, the bones of the face have stretched and migrated onto what would be the ventral surface in a primitive vertebrate. Because we stand upright, it is convenient to have our eyes, nose and mouth facing forward when we walk. Owls and some birds have undergone a similar process but where the human brain is greatly expanded and curled forward, the bird’s brain retains its original shape and posture in line with the spinal chord. As a result, in birds that appear to face forward, the brain stands on end and is pressed towards the “back” of the skull.
There are many birds, such as cormorants whose heads retain the format of a primitive vertebrate. The nostrils, mouth, brain, and spinal cord all lie in a line and the eyes look to the side.
The variations in facial arrangements leads to variation in the geography of the base of the skull, also called the nuchal surface. The location of the opening through which the spinal cord leaves the skull (foramen magnum) and the size and shape of the special structure (occipital condyle) with which the skull articulates with the atlas are particularly variable.
Some birds that use their heads in unusual ways have developed special structures on the nuchal surface. Hornbills have developed a second condyle so that the neck can support the weight of the crests or casques that they carry on their heads. This second condyle may also help to reduce the risk of injury in species that butt heads during aerial jousts. Cormorants place great stress on their necks when they catch fish. They have developed a large boss above the foramen magnum that looks like an accessory condyle but is a point of attachment for unusually heavy neck ligaments that strengthen the neck. Gallery of Skulls With Facial Rotation
The Dens ProcessBirds are the only animal in which two vertebrae articulate with the base of the skull. As in mammals the first vertebra of the neck (atlas) is securely bound to the skull by connective tissue, creating a strong joint that is slightly flexible. In the bird, a finger of bone (dens process) from the second vertebra of the neck (axis) passes through the base of the atlas. The dens process stiffens the neck and protects the fragile spinal cord by preventing the atlas from rotating on the axis.
Avian Jaw ActionThe action of the lower jaw in birds is not very different from that of a mammal except that there is a hinge partway along the mandible that allows it to bow outwards and accommodate larger pieces of food. On the other hand, there is also independent movement of the upper jaw where the action is somewhat similar to some reptiles and fishes. The upper jaw is able to move because of a structure between the braincase and the upper bone of the beak. In parrots and some other birds, this structure is an actual hinge with a pad of connective tissue. In other birds there is no hinge and the bones are fused together but remain very thin so that they form a bending plate. Movement of the upper jaw results from contraction of muscles attached to the quadrate bone beneath the eye socket. That bone is connected to two pairs of pushrods that have no muscles of their own. The outer pair consists of the jugal bars. When the quadrates move, force is transferred through those bars to the lower part of the beak and it rotates upward at the hinge. The pterygoid bones are also connected to the quadrate but meet near the midline of the skull. They also transfer force to the lower part of the beak but may be more important in keeping the whole apparatus aligned. A similar structure can be found in the upper jaw of dinosaurs but the hinge of the skull is apparently above the orbit of the eye.
|
