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.
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
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
9- cervical serrate muscle
10- thoracic serrate muscle
11-deep caudal pectoral muscles
13-humerus (arm bone)