Construction of the body axis can be more accurately compared with the “Bow and String” theory.
The thoracic and lumbar vertebrae, their articulations, epaxial muscles and ligaments associated with them constitute the “bow”. The “string” is made up of abdominal muscles particularly the straight muscle (rectus abdominis) that extends from the thorax to pelvis and linea alba. The bow is connected with the string, laterally by ribs, cranially by sternum and caudally by the pelvic bones.
Contraction of the abdominal muscles causes flexion of the bow, whereas contraction of the epaxial muscles straightens the bow. Further, the weight of the viscera attached to the vertebral column makes to straighten the bow, while at the same time the weight of the viscera on the abdominal muscles bends the bow.
Flexion of the bow is assisted by the contraction of protractor muscles of the forelimb (biceps brachii) and the retractor muscles of the hind limb (biceps femoris). Whereas, the bow is straightened by the contraction of retractor muscles of forelimb (latissimus dorsi) and protractor muscles of hind limb (psoas major, psoas minor and ilio-pasoas).
Co-ordinated contraction of all these muscles resulted in elasticity to the trunk and thus maintains the equilibrium of the body. This is proved in the horse, when the rider sits over the back of the horse, it does not sag of bend under the rider; instead it is curved in a dorsal direction as a result of increased tension of the string muscles. The caudal part of the bow and string construction is attached to the sacrum in horse (sacro-transverse articulation).
Excessive flexion of the bow is prevented by dorsal spinous process and their ligaments. Similarly, over-extension of the bow is prevented by the abdominal musculature and to some extend by the ventral longitudinal ligament of the vertebral column. Between these two extreme positions, the tension on the bow and string is constantly adjusted to the external circumstances.
The same “bow and string” theory can be applied to the cervical region also. But the curve is in the opposite direction to that of thoraco-lumbar region. The cervical vertebrae and their articulations constitute the bow, while the nuchal ligaments act as the string. The weight and active lowering of the head straightens the bow, while the nuchal ligament flexes it.
