The Shoulder of the Dog

The Shoulder of the Dog: Anatomy and Functional Adaptations

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The shoulder of the dog is a complex anatomical structure that plays a crucial role in the animal's mobility and agility. Understanding these components is essential for appreciating the dog's evolutionary adaptations for running and hunting.

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Muscles of the Dog's Shoulder

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The shoulder muscles in dogs are responsible for a wide range of movements, including flexion, extension, abduction, and rotation. Key muscles include:

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1. Deltoid Muscle: This muscle covers the shoulder joint and is involved in flexing the limb and lifts the humerus. It originates:

• On the scapular spine and inserts on the deltoid tuberosity of the humerus.
• At the acromion inserting to the deltoid tuberosity

It inserts at the humeral crest.

2. Supraspinatus Muscle: Originating on the supraspinous fossa of the scapula spine and inserting at the greater tubercle of the humerus, this muscle aids in the extension and flexion. It is one of the muscles that works to stabilize the shoulder joint. Active dogs are prone to injuring this muscle

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3. Infraspinatus Muscle: Found in the infraspinous fossa of the scapula, it functions as flexion and abduction of the forelimb. It originates at the infraspinous fossa and the scapula spine. It runs down the shoulder blade before crossing over to the humerus, where it inserts. The muscles works closely with the supraspinatus and is important for shoulder stabilization.

4. Subscapularis Muscle: Situated on the subscapular fossa, this muscle helps in the internal rotation of the humerus and also provides joint stability. It inserts at coracobrachial muscle on the lesser tubercle of the humerus after crossing the shoulder joint. It adducts and extends the shoulder and helps keep flexion.

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5. Coracobrachialis Muscle: It originates from the distal part of the scapula and inserts at the proximal part of the humerus. It helps stablize the joint and adduct the shoulder.

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6.Teres Major and Minor Muscles:  The major originates at the caudal margin of the scapula and inserts at the body of the humerus. The minor originates at the distal infraspinous fossa and infraglenoid tubercle, inserting at the teres minor tuberosity. Both muscles work to keep the shoulder stable and flex.

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Absence of the Clavicle in Dogs

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Unlike humans and many other mammals, dogs do not possess a bony clavicle. Instead, they have a vestigial structure that is not connected to other bones but is embedded within the muscles of the shoulder. The absence of a functional clavicle in dogs allows for greater flexibility and range of motion in the forelimbs. This anatomical adaptation is particularly advantageous for running and leaping, as it enables the scapula to move freely in a dorsoventral direction, increasing stride length and efficiency.

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Functional Implications

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The combination of powerful muscles, robust ligaments, and flexible tendons allows dogs to perform a variety of complex and dynamic movements. The musculoskeletal design of the shoulder is optimized for speed and endurance, reflecting the evolutionary pressures of predation and survival.

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The lack of a clavicle contributes significantly to the efficiency of the canine gait. It reduces the weight of the shoulder girdle and minimizes resistance during limb movement. This adaptation is crucial for endurance running, which is a key survival strategy for many canine species.

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Final Thoughts

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The shoulder of the dog is a remarkable example of evolutionary specialization. Its unique structure, characterized by the absence of a clavicle and the presence of well-developed muscles, ligaments, and tendons, underpins the dog's exceptional locomotive capabilities. Understanding these anatomical features provides insight into the functional adaptations that have enabled dogs to thrive in diverse environments.

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Resources

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1. Dyce, K. M., Sack, W. O., & Wensing, C. J. G. (2017). Textbook of Veterinary Anatomy. Saunders.

2. Evans, H. E., & de Lahunta, A. (2013). Miller’s Anatomy of the Dog. Saunders.

3. Hermanson, J. W., de Lahunta, A., & Evans, H. E. (2019). Miller and Evans Anatomy of the Dog: E-Book. Elsevier Health Sciences.

4. IMAIOS, vet-Anatomy

5. Anatomy, biomechanics, physiology, diagnosis and treatment of teres major strains in the canine, Laurie Edge-Hughes,

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