The Complexities of Canine Neck Anatomy

The canine neck is a remarkable structure, comprising seven cervical vertebrae that allow for a wide range of motion. These vertebrae are designed to absorb shock and provide flexibility.

Each vertebra in the canine neck is uniquely shaped, with a specialized structure that allows for the transmission of forces from the head to the body. The neck's flexibility is also influenced by the presence of intervertebral discs, which act as shock absorbers.

The cervical spine is divided into three main regions: the cervical, thoracic, and lumbar regions. The cervical region is the most mobile, with the ability to rotate, flex, and extend in multiple directions.

The canine neck's unique anatomy enables dogs to perform a variety of activities, including running, jumping, and playing. This flexibility is essential for many dog breeds, which rely on their necks to navigate agility courses or chase prey.

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Muscle properties in the canine cervical spine show a significant range of inter-individual variability, with some muscles having as few as three divisions and others having five. This variability is observed in the complexus muscle, where some dogs have five divisions, while others have four or three.

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In a bilateral dissection of one dog, subtle differences were found between the right and left sides of the same dog, with the dorsal division of the scalenus superficialis originating from different ribs on each side. However, the similarity of results after normalization of fascicle lengths confirmed the precision of the dissection method.

Muscle mass, PCSA, and fascicle lengths were scaled by body mass to allow comparison between dogs of different sizes. The scaled values of PCSA, fascicle length, and muscle mass were found to be quite similar for all dogs studied, but PCSA magnitude did not always correlate with body mass.

Muscle properties can vary significantly between individuals. Some muscles in the canine cervical spine have a large range of inter-individual variability in their anatomical divisions, attachment sites, and the number of tendinous inscriptions crossing them.

For example, the complexus muscle was found to have five divisions in some dogs, but only four or even three divisions in others. This variability didn't seem to have major functional significance.

Bilateral dissection of one dog revealed only subtle differences between the right and left side of the same dog in features such as muscle attachment sites. The similarity of results for all muscle pairs in the bilateral dissection confirmed dissection precision and supported the assumption that the musculature of the canine neck is approximately symmetrically constructed.

Muscle mass, PCSA, and fascicle lengths were scaled by body mass to allow comparison between dogs of different sizes. The scaled values of PCSA, fascicle length, and muscle mass were found to be quite similar for all dogs of this study.

However, in some muscles, PCSA magnitude did not correlate with body mass. For example, the romboideus cervicis muscle had a PCSA value of 4.56 cm in dog D, but only 2.65 cm in dog C, despite dog C being much heavier.

In some cases, our findings differ significantly from what's described in veterinary anatomy textbooks.

The serratus ventralis in dogs has an additional division at C2 that hasn't been reported before.

We also found that the cervical part of the longus colli contains only three divisions, each spanning two vertebrae, not four as previously described.

This variation doesn't seem to have a substantial impact on the animal's function.

Interestingly, the longissimus capitis originates from the articular processes of T1-T3 vertebrae, not the transverse processes as textbooks suggest.

Comparative morphometry reveals significant differences in the skeletal configuration of dogs and humans. The centre of mass of a dog's head and neck is located in front of the vertebral column, requiring active muscle contraction to support its weight.

In contrast, the human head is carried more directly over the trunk, with much of its weight borne passively. This difference is crucial when trying to extrapolate the function of muscles from similarities in location between the two species.

The rectus capitus dorsalis and splenius muscles in dogs have higher PCSA values compared to their human counterparts, due to their role in maintaining head position. This is evident in Fig. 2, which shows that all muscles except the trapezius have higher PCSA values in dogs.

The trapezius muscle in dogs is less well-developed than in humans, possibly because it serves only to stabilize the forelimb in dogs, whereas in humans it is involved in both stabilization and grasping motions.

The dissection process was a crucial step in understanding canine neck anatomy. Immediately following euthanasia, the skin and subcutaneous tissues of the right forequarter were carefully removed.

Each of the muscles of the right-hand side of the neck was identified, carefully isolated and removed. This process took between 5 and 7 hours to complete.

Tissues were kept moist with physiological saline throughout the dissection and measurement processes. This helped prevent any damage to the muscles and ensured accurate measurements.

The list of muscles studied is given in Table 2.

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Muscles from the rib cage to the vertebral column are fascinating to study.

The scalenus supracostalis muscle originates on the first costal cartilage and inserts on the thyroid cartilage, with a smaller PCSA than the scalenus hypaxialis muscle.

This muscle can be divided into three distinct divisions, each inserting on the caudal aspect of the transverse processes of C4 and C5 by means of two tendons.

The scalenus primae costae muscle has a variable insertion site, found to be C6 in some dogs and C7 in others.

In dogs A and C, the scalenus supracostalis muscle originated on the upper third of the fourth rib, while in dogs B, D, and F, it originated on the third and fourth ribs.

The scalenus supracostalis muscle has a muscular attachment to the fourth rib in dog A, and to the fifth rib in some cases.

The scalenus supracostalis muscle can be partitioned into three divisions, all of which originate on the first rib and insert on C6 or C7.

The scalenus supracostalis muscle is smaller and thinner than the other two divisions of the muscle, resulting in a lower PCSA value.

The scalenus supracostalis muscle forms a triangle between C7 and the tubercle of the first rib in all dogs.

In some dogs, the scalenus supracostalis muscle inserts on the caudal part of the transverse process of C6, while in others, it inserts on the transverse process of C7.

Dissection is a crucial step in understanding the anatomy of a species. Immediately following euthanasia, the skin and subcutaneous tissues of the right forequarter were carefully removed.

The researchers took great care to identify, isolate, and remove each muscle of the right-hand side of the neck. Measurements and evaluations were performed within hours of euthanasia, avoiding the effects of rigor mortis.

Duration of each dissection ranged between 5 and 7 hours. Tissues were kept moist with physiological saline (0.9%) throughout the dissection and measurement processes.

The list of muscles studied is extensive, with 11 muscles analyzed in total. Here's a breakdown of the muscles studied:

The researchers used a specific equation to calculate the PCSA (Physiological Cross-Sectional Area) of each muscle.