The triceps brachii muscle complex represents a significant anatomical feature in the forelimb of the domestic cat, Felis catus. This muscle is classified as a powerful extensor, a common characteristic across many four-limbed vertebrates. In the cat, the triceps brachii is a group of muscle bellies that converge on a single attachment point near the elbow. Understanding this muscle is fundamental to comparative anatomy, as its structure influences the cat’s unique movement capabilities and predatory mechanics.
General Location and Role in Feline Locomotion
The triceps brachii is prominently situated along the caudal aspect of the cat’s humerus, the long bone of the upper forelimb. It occupies a large triangular space formed by the shoulder blade, the humerus, and the elbow joint. This positioning allows it to act as the primary muscle mass of the upper arm, linking the shoulder region to the forearm bones at the elbow.
The size and location of the muscle complex make it fundamentally important for feline locomotion. As the main extensor of the elbow joint, the triceps provides the force necessary to straighten the limb. This extension is essential for weight-bearing during stance, ensuring the forelimb remains rigid as the cat supports its body weight.
During active movement, the triceps brachii fires powerfully to push the cat’s body forward, generating propulsion for walking, running, and jumping. The muscle’s strength and coordination allow the cat to achieve its characteristic bursts of speed and impressive vertical leaps.
Specific Anatomy: Identification of the Four Heads
A distinguishing feature of the triceps brachii in the cat, and other carnivores, is its composition of four distinct muscle bellies, or heads. The presence of a fourth head is a notable difference from the human triceps, which typically has only three.
The four heads are:
- Long head
- Lateral head
- Medial head
- Accessory head
The long head is generally the largest and is the only head that originates from the scapula. Due to its origin crossing the shoulder joint, this head also plays a secondary role in shoulder movement. The lateral head and the medial head both originate directly from the shaft of the humerus.
The accessory head is positioned deep within the muscle complex, often nestled between the medial and lateral heads. It arises from the proximal caudal part of the neck of the humerus. The close proximity of the medial and accessory heads is noteworthy, as the main nerve supply for the entire complex must pass between them.
Muscle Attachments, Action, and Nerve Supply
Attachments
The muscle bellies of the triceps brachii complex have different origins but share a common insertion point. The long head originates from the infraglenoid tubercle on the caudal edge of the scapula. The lateral and medial heads arise from different surfaces along the shaft of the humerus, with the accessory head originating from the proximal caudal neck of the humerus.
These four distinct muscle bellies converge distally into a powerful, single tendon. This collective tendon attaches exclusively to the olecranon process, a large bony projection found on the proximal end of the ulna. This unified point of insertion allows the combined force of all four heads to be channeled into a single mechanical action at the elbow joint.
Action
The primary action of the triceps brachii is the extension of the elbow joint, which means straightening the forelimb. This forceful straightening action allows the cat to push off the ground during locomotion and maintain a rigid limb for supportive stance.
Because the long head originates on the scapula, it also crosses the shoulder joint and contributes to the flexion of the shoulder. This dual action allows the cat to pull the humerus backward, contributing to forward momentum. The combined action of the muscle complex is crucial for generating the rapid, explosive power needed for hunting and escaping predators.
Nerve Supply
The triceps brachii complex, including all four heads, receives its motor innervation from the radial nerve. The radial nerve is the largest nerve of the brachial plexus, a network of nerves originating from the spinal cord that supplies the forelimb. After branching, the radial nerve travels down the arm, passing between the medial and accessory heads of the triceps before continuing its course.
The path of the radial nerve is significant because it allows for the coordinated and simultaneous firing of all four heads, which is necessary for the powerful, unified extension of the elbow. Damage to the radial nerve can cause paralysis of the triceps brachii, resulting in the inability to bear weight on the forelimb. This highlights the importance of this single nerve in controlling the primary extensor function of the cat’s limb.