BITS AND BITTING (Presented by Dr. Hilary Clayton at the 1998 USDF National Dressage Symposium and re-printed in USDF Connection, May 1999) Many parts of the bridle play a role in our communication with the horse, with the horse's acceptance of the bit being a key factor in dressage training. This series of articles will explore the structure and function of the horse's head as it relates to the position and action of the bit, and the development problems associated with acceptance of the bit. Several years ago I studied the action of the bit using a technique called fluoroscopy. Fluoroscopy uses x-ray techniques to produce a video image that allows us to visualize the position and movements of the bit within the horse's mouth. In this article we'll look at the anatomy of the horse's oral cavity and the implications for fitting a bit. Anatomy of the Oral Cavity The bit lies within the horse's oral cavity, which contains the gums, the teeth and the tongue. Unlike the teeth and gums, the tongue is soft, which allows it to change shape to accommodate the presence of the bit. The tongue is composed of muscles that give it tremendous mobility as I'm sure you've noticed. This mobility allows the horse to use his tongue to change the position of the bit or to stabilize the bit if it causes discomfort. The hard palate, which forms the roof of the mouth, consists of bone covered by softer connective tissue. It is vulnerable to pressure from the bit, a fact that is applied in the construction of certain types of bits that are designed to exert pressure against the palate. Anatomy of the Jointed Snaffle The jointed snaffle consists of the rings (or cheeks) that lie outside the oral cavity. The two arms of the mouthpiece meet at the joint. Mouthpieces vary in length (the distance between the rings or shanks) and in width (the fatness of the arms). Figure 1: Fluoroscopic view of the position of the jointed snaffle bit in the horse's mouth. P hard palate, M mandible, R rings of bit, J joint of bit, UC upper canine tooth, LC lower canine tooth Position of the Jointed Snaffle in the Oral Cavity Figure 1 shows the position of the jointed snaffle bit in the horse's mouth as viewed fluoroscopically. The horse is facing to the right. The image resembles a conventional x-ray except that the light and dark colorations are reversed. Structures that do not allow X rays to pass through readily, such as the metal of the bit, the teeth and the bones are dark in color in the fluoroscopic picture. Air-filled spaces are light in color and muscular structures, like the tongue, are an intermediate grey color. The interpretive diagram beside the fluoroscopic image indicates the positions of the various anatomical structures. Figure 2: Indentation of the tongue by applying tension to both reins. The light area indicated by the arrow to the right of the joint of the bit is an air pocket between the bit and the hard palate.   Within the horse's mouth the jointed snaffle lies between the tongue and the hard palate, with a slight downward angulation toward the incisor teeth. Tension in the reins causes the mouthpiece to indent the tongue more deeply, pulling the bit away from its contact with the hard palate and leaving an air-filled pocket above the joint of the bit (figure 2). As someone pointed out at a seminar recently, this could explain why some horses pull against the reins; by doing so they are able to reduce uncomfortable pressure on their hard palate. In the next article we'll explore the intricacies of fitting a jointed snaffle bit. Figures are reprinted with permission from the Journal of Equine Veterinary Science