Muscular System (Part 3) - Muscles of the Thoracic Limb

Summary

This presentation segment delves into the anatomy of the thoracic limb muscles, distinguishing between intrinsic and extrinsic muscles and their roles around the shoulder, elbow, carpus, and digits. It highlights key muscles like the trapezius, rhomboid, serratus ventralis, and latissimus dorsi, detailing their functions and interactions. The discussion also covers muscles affecting the elbow, such as the triceps brachii and biceps brachii, emphasizing their extensor and flexor actions. The narrative concludes with a reminder of the multi-joint influence of these muscles, encouraging viewers to continue to the next module on pelvic limb muscles.

Takeaways

• 💪 The muscles of the thoracic limb are divided into intrinsic and extrinsic muscles, with intrinsic muscles located entirely within the limb and extrinsic muscles connecting different regions of the body.
• 🦾 Muscles can be further grouped by the joints they act upon, including muscles of the shoulder girdle, shoulder joint, elbow, carpus, and digits.
• 🏋️‍♀️ The trapezius muscle, a flat triangular muscle, swings the scapula forward and backward and is divided into two parts: the cervical (larger) and thoracic (smaller) parts.
• 🔍 Rhomboideus is a deeper, heavier muscle located beneath the trapezius and has three parts based on their location: capitis, cervicis, and thoracis.
• 🏗️ Serratus ventralis, a large fan-shaped muscle, forms a sling supporting the trunk between the thoracic limb, crucial for weight-bearing.
• 🏃‍♂️ Brachiocephalicus is the principal extensor of the shoulder, also acting as a flexor of the neck when the limb is weight-bearing, subdivided into clydobrachialis and clydoscephalicus.
• 🚶‍♀️ Latissimus dorsi is a strong flexor of the shoulder and assists in pulling the limb caudally or advancing the trunk when the limb is fixed.
• 🤸‍♂️ Triceps brachii is the strongest extensor of the elbow with four heads in dogs, including a long, lateral, medial, and accessory head, all inserting on the olecranon of the ulna.
• 🦵 Extensors of the carpus are found on the craniolateral aspect of the limb, including muscles like extensor carpi radialis and common digital extensor.
• 🤲 Flexors of the carpus and digits are found on the caudomedial side of the limb, including flexor carpi radialis and two digital flexors (superficial and deep digital flexor).

Q & A

What are the two types of muscles that make up the muscles of the thoracic limb?

The muscles of the thoracic limb are composed of intrinsic and extrinsic muscles. Intrinsic muscles lie completely within one region of the body, having their origin and insertion in the bones of the thoracic limb. Extrinsic muscles run from one region of the body to another.

How are the muscles of the limb further grouped based on their function?

The muscles can be grouped based on the joint of the limb they act with, which includes the muscles of the shoulder girdle, shoulder joint, elbow, carpus, and the digits.

What is the function of the trapezius muscle and how is it divided?

The trapezius muscle helps swing the scapula forward and backward. It is divided into two parts: the cervical part (trapezius cervicalis), which is larger, and the thoracic part (trapezius thoracis), which is smaller.

What is the serratus ventralis muscle and its role?

The serratus ventralis is the largest and most important muscle attaching the thoracic limb to the trunk. It is a large fan-shaped muscle that forms a sling supporting the trunk between the thoracic limb.

What is the main action of the brachiocephalicus muscle?

The brachiocephalicus muscle is the principal extensor of the shoulder area and acts as a flexor of the neck. It raises and advances the shoulder.

Describe the latissimus dorsi muscle and its action.

The latissimus dorsi is a wide triangular muscle and is a strong flexor of the shoulder. It also pulls the thoracic limb caudally or advances the trunk if the limb is fixed.

What are the two parts of the deltoid muscle and their respective locations?

The deltoid muscle has two parts: the spinal part, which is thicker and fusiform in shape, and the acromial part, which is thinner and strap-like. They fuse before inserting and act together as a single unit.

Which muscle is considered the strongest extensor of the elbow and what are its heads?

The triceps brachii is the strongest extensor of the elbow. It has three heads: the long head, which originates from the caudal border of the scapula, the lateral head from the lateral humeral diaphysis, and the medial head from the medial humeral diaphysis.

What is the role of the tensor fasciae antebrachii muscle?

The tensor fasciae antebrachii muscle tenses the antebrachial fascia. It assists the triceps brachii in the extension of the elbow through its fascial connections.

How does the location of the pectoral muscles relate to their function?

The pectoral muscles are strong adapters of the forelimb. The deep pectoral muscle (ascending pectoral) advances the trunk when the limb is fixed on the ground, while the superficial pectoral muscle, which is subdivided into a descending part and a transverse part, also plays a role in forelimb adaptation.

What is the main action of the superficial digital flexor muscle?

The superficial digital flexor is the only muscle that flexes the distal interphalangeal joint. It also secondarily flexes the more proximal joints of the digit and the carpus, and supports the fat lock.