Thursday, December 31, 2009

Hidden Structures of the Equine Skull

All about the guttural pouch and the hyoid apparatus

You are probably wondering what in the world these two parts are. These interesting structures play a very important role in protecting the sensitive structures of the skull, and in supporting the structures of the skull and neck.

The guttural pouch

There are 2 guttural pouches in the horse, each placed on either side at the back of the skull. These pouches are air-filled extensions of the auditory tubes. They act as a puffy cushion, protecting the sensitive structures of the head from harsh contact with the bony structures of the skull and hyoid apparatus.

The hyoid apparatus

The hyoid apparatus consists of the two hyoid bones that are fused together to form one solid bony structure. It is suspended from the base of the skull (from the styloid processes of the temporal bone) by ligaments and is suspended in the soft tissue of the neck between the mandible and the larynx. The pharynx passes between the two arms of the hyoid bone. The hyoid apparatus functions to provide attachment points for the muscles and ligaments of the tongue, pharynx, neck and sternum.

Interestingly the hyoid bone/s derived from a gill arch in jawless primitive fish. Through evolution of jaws the hyo-mandibular gill arches evolved into the hyoid bones. It is thanks to the hyoid apparatus that a frog can fire its tongue to capture it's prey. Humans too have a hyoid. Unlike the horse's ours is horse- shoe shaped. You can feel the hyoid connections to your sternum by placing your finger at the notch in the base of your neck, opening your jaw and wiggling your tongue.

The hyoid is a very important structure to consider when riding the horse. As mentioned the tongue attaches to the hyoid. The hyoid is capable of very little movement (equivalent to that of teeth) and is fixed in relation with the base of the skull. Thus, if the horse's head is strongly repositioned for exaggerated flexions, such as the rollkur, it will result in the tongue being drawn up. This has a negative effect, because it hinders the mouth and inhibits the horse from having total relaxation in the mouth. It also decreases the available space in the “throat” area (as a secondary effect to extreme flexion), making breathing very difficult. Another reason why the hyoid has to be considered is, because the six muscles of the sternum attaches to it (as mentioned). These muscles form the underline of the horse and greatly affect the mobility of the forehand of the horse.

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