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On the Contact 2: Specific Injuries to Dressage Horses
From Being Ridden Behind the Vertical for Prolonged Periods (with veterinary citations)

On the Contact 1: General Anatomy

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A.
Horse and rider showing the anatomical basis for some connections from the hand and leg to the horse's mouth. The rider is shown with the inside leg left) slightly ahead of the outside leg and is seated with pelvis curled under. Saddling should take account of the position of the cartilage cap of the shoulder blade and assure that the front of the saddle does not interfere with the range of motion of the foreleg by constriction from the front edge of the saddle.

The height at which the reins are held supports muscles such as the ventral serratus (9). The thoracic serratus (10) is influenced by the rider's calf. The connection from the leg to jaw is known from practice but its detailed anatomy is not discussed in riding books.

The connection is from the deep caudal pectorals (11) attached to the sternum, to the hyoid group (5) of muscles attached to the hyoid apparatus (4) between the jaws, which is embedded in the mylo- and genio-hyoid muscles (3) under the tongue (2). The muscles forming the base of the tongue attach to the jaw behind the lower incisors (1). There is also a connection from the leg to hindquarters via the abdominus rectus, which runs along the sternum to the pelvis.
B.
Enlarged head and neck, showing the connections between the sternum, hyoid apparatus and tongue. An active connection is seen when the horse makes a soft munching motion of the jaw with the lips closed or when the horse licks its lips with one or two light strokes of the tongue. Both these actions signal acceptance of the rider's aids from the lower leg.
1- attachment of genio- and mylo-hyoid muscles to jaw
2-tongue
3-genio- and mylo-hyoid muscles
4-bones of hyoid apparatus (hangs like a swing from the underside of the skull between the jaws)
5-hyoid group of muscles (3 on each side)
6-brachiocephalicus (attached to arm, neck and back of head) and sternocephalicus muscles
7-poll
8-nuchal ligament
9- cervical serrate muscle
10- thoracic serrate muscle
11-deep caudal pectoral muscles
12-sternum (cartilage)
13-humerus (arm bone)
14-shoulder blade
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