A brief consideration of the bones, joints, and muscles involved in the flexions will help in avoiding certain mistakes.
The bars, on which rest the bits, are the distal part of the lower jaw, between the molar teeth and the incisors. In conformation they are of three types. In one sort the bone is small, and covered by a thin mucous membrane. Such bars are said to be "sharp," and are especially sensitive to the pressure of the bits. Another sort has a large bone, somewhat flattened where it meets the bits, and covered with thick mucous membrane. This sort is commonly little sensitive, and is described as "fleshy." The best type of bar is intermediate between the two.
The temporo-maxillary articulation which connects the lower jaw with the skull lies between the ears and the eyes, just behind the frontal bone. It allows the jaw, moved by the digastricus, masseter, and temporalis muscles, to open and shut, to move laterally for mastication, and to glide back and forth. This joint plays an important part in equitation.
Another important set of bones are the vertebrae of the neck. The first cervical vertebra, the atlas, articulates with the occipital bone of the skull. Next to it comes the axis. These two vertebrae form the atlo-axoid articulation which permits the head to rotate upon the axis, this remaining fixed. The occipito-atloid articulation, on the other
