A Bilateral Cervical Accessory Biceps Muscle: anatomical description and clinical implications
This study reports the rare discovery of bilateral accessory biceps muscles located between the levator scapulae and serratus posterior superior in a cadaver, characterizing their distinct morphology, innervation, and clinical significance as unique anatomical variants rather than simple slips of adjacent muscles.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
The human body is a masterpiece of biological engineering, yet it is rarely built to a single, rigid blueprint. While textbooks describe a standard set of muscles that move our shoulders and neck, nature often adds its own unique flourishes. In the back of the neck and upper chest, a group of muscles works to lift the shoulder blade and stabilize the spine. Among these, the levator scapulae acts like a strap that pulls the shoulder blade upward, while the serratus posterior superior sits slightly deeper, helping to expand the rib cage during breathing. Sometimes, extra bands of muscle tissue appear between these established structures. These variations are not errors, but rather evidence of the complex ways our bodies develop. For doctors and surgeons, knowing exactly where these muscles are and how they connect is vital, as mistaking a normal variation for a tumor or a standard muscle can lead to unnecessary complications during surgery or confusion in medical imaging.
A team of researchers at the Federal University of Espírito Santo in Brazil recently uncovered a rare example of such a variation during a routine examination of a human body. Working in a laboratory with a formalin-preserved adult male cadaver, the scientists carefully peeled back the layers of tissue in the upper back and neck. Their goal was to map the exact layout of the muscles in this region. Instead of finding the standard two muscles they expected, they discovered a pair of distinct, extra muscles running down the back, one on the left and one on the right. These were not simply extra strands attached to the known muscles; they were fully formed, independent structures with their own starting points and paths.
The researchers traced these new muscles from their origin to their end. Each one began at a specific bony bump on the second vertebra of the neck, sitting just in front of the main shoulder-lifting muscle. From there, they traveled downward and inward, running parallel to a large neck muscle called the splenius capitis. Unlike the main shoulder-lifting muscle, which attaches directly to the shoulder blade, these new muscles did not reach the bone. Instead, they stopped short, ending just above the deep breathing muscle. A small, fluid-filled sac, known as a bursa, separated the end of the new muscle from the deep muscle below it, acting as a cushion to prevent friction. This separation was crucial; it proved that the new muscle was a separate entity and not just a fused extension of the one beneath it.
To understand the nature of these muscles, the team measured them with precision. The muscle on the right side was nearly 110 millimeters long and about 15 millimeters wide at its thickest point. The one on the left was slightly smaller, measuring roughly 100 millimeters in length and 11 millimeters in width. Both had a spindle shape, thick in the middle and tapering at the ends, and both were innervated by the same nerve branches that supplied the nearby splenius capitis muscle. This shared nerve supply suggested that these muscles developed alongside the standard neck muscles rather than as random additions. The researchers noted that while some previous studies had described extra muscle slips that seemed to belong to the shoulder-lifting muscle, these new findings were different. They were bilateral, meaning they appeared on both sides, and they lacked the broad, flat connections to other structures that usually characterize those known variations.
The team concluded that these structures should be viewed as unique, paired accessory muscles rather than mere variations of the muscles they sit next to. They proposed a new name for this discovery: the "biceps cervicodorsalis." This classification matters because it changes how medical professionals might interpret scans or perform surgery in this area. If a surgeon sees a muscle in this spot and assumes it is just a normal part of the shoulder-lifting muscle, they might cut it without realizing it is a distinct structure with its own nerve supply. Similarly, radiologists looking at an MRI might mistake these symmetrical muscles for a disease or a tumor if they are not aware that such bilateral variations exist. The presence of a fluid-filled sac between the muscle and the tissue below it is a key sign that this is a normal, albeit rare, anatomical feature rather than a pathological growth.
This discovery adds a new piece to the puzzle of human anatomy, reminding us that the body's design allows for significant individuality. The researchers suggest that these muscles likely formed during development when the muscle fibers followed a slightly different path than usual, guided by signals that directed them to attach to the second neck vertebra and stop short of the shoulder blade. While the exact reason for this variation remains a mystery without further embryological study, the physical evidence is clear. The existence of these muscles, with their specific size, shape, and nerve connections, expands the known map of the human body. For anyone working in the field of medicine, from surgeons to radiologists, recognizing these silent, symmetrical variations ensures that the human body is understood in its full, complex reality, not just in its textbook ideal.
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