Part 3: Proximal Thoracic Limb

Return the forelimb to its normal position. Identify the deltoideus m. on the lateral aspect of the shoulder region. (Note that deltoideus is made up of two fused portions that together give it a ‘checkmark’ shape. Also, note that deltoideus is a muscle that crosses the shoulder joint.) (See DG Fig. 2-15)

• Transect the combined portions of deltoideus just distal to the acromion of the scapula (the palpable ‘bump’ on the distal end of the spine of the scapula) and reflect the stumps.

• Free the scapular portion of deltoideus and reflect it to the spine of the scapula, revealing the infraspinatus m. underneath.

Deep to the deltoideus, identify the infraspinatus m. which lies in the infraspinous fossa of the scapula (below the spine of the scapula). Note that infraspinatus is another muscle that crosses the shoulder joint and there is a subtendinous (synovial) bursa (shiny pocket w/fluid intended to reduce friction) between its tendon of insertion and the greater tubercle of the humerus. (See DG Fig. 2-16)

• Transect the infraspinatus m. through its approximate middle.

• Free and reflect the more distal half of the muscle by scraping it away from the scapula with a scalpel handle (not the blade, but instead use the flat end of the handle).

• Continue reflecting the distal half of infraspinatus until you reveal the subtendinous (synovial) bursa where its tendon crosses over the shoulder joint.

Note the small, wedge-shaped teres minor m. caudal to the shoulder joint as you dissect the infraspinatus m. (Teres minor is another muscle that crosses the shoulder joint.) (See DG Fig. 2-16) The teres minor will also be dissected on a demonstration animal for you to view.

Identify the supraspinatus m. which lies in the supraspinous fossa of the scapula (above the spine of the scapula); note that this muscle also crosses the shoulder joint. (See DG Fig. 2-16)

On the deep face/medial side of the scapula, identify the subscapularis m. occupying the subscapular fossa. Note that the subscapularis m. crosses the shoulder joint on the medial side. (See DG Fig. 2-19)

Caudal to the subscapularis m., identify the teres major m. which also crosses the shoulder joint. (See DG Fig. 2-19)

In demonstration animals, note the small, spindle-shaped coracobrachialis m. crossing the medial surface of the shoulder joint.

On the medial aspect of the forelimb, in the region of the brachium, identify the thin, flat tensor fasciae antebrachii m. (See DG Fig. 2-19)

• Transect the tensor fasciae antebrachii m. proximally and reflect it distally (toward the elbow) to view the underlying heads of the triceps brachii m.

On the medial side of the forelimb, reflect the pectoral muscles cranially.

• As you reflect the pectorals cranially, look for the axillary ln. on the dorsal edge of the pectorals, along the caudal muscle border shared with the latissimus dorsi m. (Note that this lymph node may be difficult to find, so be sure to look at a demonstration animal as well.)

Identify the triceps brachii m. group which is made up of four heads (instead of the usual three heads seen in humans); these heads are the long, lateral, accessory, and medial heads of the triceps brachii m. The triceps brachii are visible on both medial and lateral sides of the brachium. (See DG Figs. 2-15, 2-16 and 2-19)

• Note that on the medial side of the brachium, you may need to reflect the tensor fasciae antebrachii m. (if you have not already done so), to see the full extent of the triceps brachii m. group. (See DG Fig. 2-19)

• Identify the long head of the triceps brachii m. This is the largest head and is seen from both medial and lateral sides of the brachium. This head of the triceps spans two joints, extending from the scapula down to the ulna, hence acts on both the shoulder and elbow joints.

• Identify the lateral head of the triceps brachii m. As the name implies, the lateral head is found on the lateral side of the brachium.

  • Transect the lateral head of the triceps at its origin and reflect it distally to reveal/identify the accessory head of the triceps brachii m. just deep to it.

  • Dissection Note: Try to preserve the nerves emerging on the lateral side of the brachium, just deep to the reflected lateral head of the triceps brachii m. These are branches of the radial nerve which will be discussed later in this lab.

• Identify the medial head of the triceps brachii m. The medial head is found on the medial side of the brachium and is smaller than the lateral head.

• Note that all four heads of the triceps brachii m. insert on the olecranon tuber via a common tendon, hence the triceps brachii m. group is the main extensor of the elbow joint.

  • Comment: Recall that the long head of the triceps brachii m. crosses both the shoulder and elbow joints, so in addition to extending the elbow joint, the long head of the triceps brachii m. also flexes the shoulder joint.

On the medial aspect of the limb, cranial to the triceps brachii m. group, identify the biceps brachii m. The biceps brachii has only one head (instead of the usual two heads seen in humans) and is found on the medial (and cranial) aspect of the humerus. (See DG Fig. 2-19)

• Note that the biceps brachii m. spans two joints, extending from the scapula down to the radius and ulna, hence acts on both the elbow and shoulder joints. It is well known for being a major flexor of the elbow joint, but is also an extensor of the shoulder joint.

• Transect the biceps brachii m. through its middle and reflect the proximal half toward its origin from the scapula.

• As you reflect the biceps brachii proximally, identify the transverse humeral retinaculum (strong band of fibrous tissue) that holds the tendon of the biceps in the intertubercular groove. If needed, cut through this retinaculum to trace the tendon of the biceps brachii to its origin from the supraglenoid tubercle of the scapula. (See DG Fig. 2-28)

  • Comment: As you trace the tendon of origin of the biceps brachii m. proximally you may encounter synovial fluid; this area is an extension of the shoulder joint capsule surrounding the tendon of origin.

Be sure to observe the aforementioned muscle tendons that cross the shoulder joint (hence result in action on the shoulder joint). Specifically, observe the tendon of origin of the biceps brachii m. Also, observe the tendons of insertion of the infraspinatus m. and the subscapularis m. which help provide collateral support to the shoulder joint. 

On the medial side of the forelimb, return to the axilla (armpit) region and use blunt dissection (using a probe or the tip of your scissors), to carefully dissect and identify the axillary a. and axillary v. (See DG Figs. 3-27, A & C, 3-28, 3-29, A,B, & C). Note that you will need to reflect the pectoral muscle mass cranially to fully visualize the path of these vessels travelling distally into the brachial region.

• Trace the path of the axillary a. and v. distally in the forelimb.

  • If necessary, transect the axillary vein to view the path of the axillary artery.

• As you trace the axillary a. distally, identify the large subscapular a.

  • Dissection Note: Note that only a short trunk of the subscapular a. is seen before the thoracodorsal a. and the caudal circumflex humeral a. are given off. The subscapular a. will then continue caudally between the subscapularis and teres major mm. (See DG Fig 3-29, A, B, & C)

  • If needed, transect the teres major and reflect both halves of the muscle to expose the continuation of the subscapular a.

• Going back to the axillary a., identify the cranial circumflex humeral a. (Note that the cranial circumflex humeral a. can branch from the axillary either before or after the subscapular a.) It courses cranially to supply the biceps brachii m. and the joint capsule of the shoulder. (See DG Figs. 3-27, A & C, 3-28, and 3-29, A, B, & C)

As you continue to trace the vessels on the medial side of the forelimb, between the biceps brachii and triceps brachii mm., note that the name of these vessels changes. In the brachium, these vessels are now called the brachial a. and brachial v. Continue to trace these vessels to the level of the elbow. (See DG Figs. 3-27, A & C, 3-28, and 3-29, A, B, & C)

• Note that for the arterial supply, the name change from axillary a. to brachial a. occurs after the cranial circumflex humeral artery is given off (i.e., the brachial a. is the continuation of the axillary a.).

Return to the axillary region and note the various nerves entering the limb.

• Comment: The origination of the group of nerves in this region is called the brachial plexus. The brachial plexus is typically formed by the ventral branches of the 6th, 7th and 8th cervical spinal nerves along with the 1st and 2nd thoracic spinal nerves. The brachial plexus gives rise to the nerves in the thoracic limb that will be dissected. (See DG Fig. 3-27, A, B & C)

• Using blunt dissection techniques, on the medial aspect of the forelimb, continue by dissecting the nerves described.

• Transect the supraspinatus m. near its insertion and reflect its distal end to see the suprascapular n. entering the muscle.

• Comment: The musculocutaneous n. also gives off the medial cutaneous antebrachial n. which is sensory to the skin on the medial aspect of the antebrachium. (This nerve is often removed with the skin and need not be dissected or identified.)

On the medial aspect of the limb, identify and trace the median n. (which arises from a common trunk with the ulnar n., which you will dissect next.) The median n. is located more cranially than the ulnar n. and courses distally alongside the brachial artery. The median n. will continue distally into the antebrachium to supply several of the flexor muscles of the carpus and digits in that region. (See DG Figs. 3-27, A, B, & C, 3-30A, and 3-32)

Identify and trace the ulnar n.; this is the more caudal branch of the common trunk shared with the median n. The ulnar n. courses caudally toward the elbow and it will continue distally into the antebrachium to supply several flexor muscles of the carpus and digits. (See DG Figs. 3-27, A, B, & C, 3-30A, and 3-34)

• Comment: The ulnar n. also gives off the caudal cutaneous antebrachial n. near the middle of the brachium. This small nerve supplies the skin of the distal medial brachium and the caudal antebrachium. (This nerve need not be dissected or identified.)

Identify the large radial n. entering the triceps brachii muscle mass, just distal to the teres major m. The radial n. will supply the triceps brachii, tensor fasciae antebrachii, and the anconeus mm. as well as the extensors of the carpus and digits. (See DG Figs. 3-27, A, B, & C, and 3-33)

• Comment: It is important to note that the radial n. will dive into the triceps m., pass around the caudal surface of the humerus, and emerge distally on the lateral side of the brachium where it terminates as deep and superficial branches.

  • • Reflect the lateral head of the triceps brachii m. distally to observe where the radial n. appears laterally. (See DG Fig. 3-31) Observe in this location where the radial nerve divides into two terminal branches, the deep and superficial branches, which innervate extensors of the carpus and digits. (See DG Figs. 3-31, and 3-33)

    • This terminal branching will also be dissected on one of the demonstration animals for you to observe.

Identify the axillary n. entering the space between the subscapularis and teres major mm. (and traveling to the lateral aspect of the limb). Of the muscles we have dissected, the axillary n. supplies the teres major, part of the subscapularis and (laterally) the deltoideus mm. (See DG Fig. 3-27, A, B, & C)

Before moving on to Part 4, to the dissection of deeper antebrachial structures, identify and trace the cephalic vein on the craniolateral aspect of the antebrachium as it carries venous blood from the distal limb toward the heart. (See DG Figs. 3-3, 3-26, 3-30A, 3-35, 3-36B, 3-37, and 3-38)

• The cephalic vein begins on the palmar aspect of the paw, curves around to the cranial aspect of the antebrachium and courses proximally. The cephalic vein continues on the craniolateral aspect of the limb and then disappears as it courses deep to the brachiocephalicus m. to enter the external jugular v. near the thoracic inlet. There is also an accessory cephalic v. arising from the dorsal aspect of the paw that joins the cephalic v. on the cranial surface of the distal third of the antebrachium. (See DG Figs. 3-36B, and 3-37)

• Note that the cephalic vein is a common venipuncture site in the cranial antebrachial region of the dog and cat.

On the flexor surface of the elbow joint, identify the median cubital v. The median cubital v. forms a connection between the cephalic and brachial veins. Trace the median cubital v. laterally to locate where it joins the cephalic vein, then trace it medially to locate where it connects with the brachial v. (See DG Figs. 3-30A, 3-35, and 3-36B)