The Missing Link in the Head Suspension of the Cat: A De Facto Nuchal Ligament

Abstract

The nuchal ligament in quadrupedal mammals suspends the head from the trunk while the neck serves as a lever arm. Although well known as a deep, elastic “yellow” ligament”, especially in larger herbivores, it is either not mentioned or considered absent in cats. Based on biomechanical considerations, we hypothesized that the cat must have, if not an actual nuchal ligament, at least an analogous structure. We tested our hypothesis by micro‐dissecting specimens under a stereomicroscope fitted with a fiber‐optic ring‐light with a polarizing filter to visualize and trace individual muscle‐tendon fascicles and collagenous fiber bundles. We found that the cat actually has a de facto nuchal ligament that shares similarities with the nuchal ligament of humans instead of being of the yellow ligament variety. The de facto nuchal ligament of the cat originates on the spinous process of the first thoracic vertebra (or the preceding or following one) via the aponeurosis of the paired cervical trapezius muscles where it merges with the muscle fiber fascicles of the paired thoracic trapezius muscles. It continues cranially as two portions: (1) A superficial plait of interweaving tendon fiber fascicles, which inserts on the medial two‐thirds of the nuchal crest of the skull via the aponeurosis of the clavotrapezius muscle and is comparable to the dorsal raphe of the human nuchal ligament with its interweaving fibers; and (2) a deep, expansible but tenuous connective tissue lamina, which is loosely attached to the cervical vertebrae and is comparable to the midline fascial septum of the human nuchal ligament. The interweaving fibers of the superficial plait of the de facto nuchal ligament of the cat include myotendinous fibers of the right and left clavotrapezius and cervical trapezius muscles, with contributions from the cutaneous platysma muscle. While the superficial muscles creating the dorsal raphe of the human nuchal ligament are often described as crossing the dorsal midline and continuing on the opposite side as deeper muscles, we observed in the superficial plait of the de facto nuchal ligament of the cat that the tendon fiber fascicles of the superficial muscles actually join the endo‐, epi‐, and perimysium of the deeper muscles on the opposite side. These tendon fiber fascicles change orientation as the position of the clavotrapezius muscles change with movements of the limbs or the neck. The tendon fiber fascicles are transversely oriented when the head and neck are in the resting upright posture and the de facto nuchal ligament is relaxed, but are diagonally angled and pointing toward the head when the head and neck are lowered and the de facto nuchal ligament is stretched. Our findings suggest that the mechanisms of suspending the head and neck from the thorax while allowing the head to be lowered to feed or drink and then be returned to the upright position through resilience differ among mammals and encourage further investigations, particularly of mammals in which the nuchal ligament is thought to be absent, such as in the pig.Support or Funding InformationLSU Foundation to DGH