Part 2: Extrinsic Muscles and Associated Structures

Identify the pectoral muscles. Note that within this muscle group there are superficial pectoral as well as deep pectoral muscles. (See DG Fig. 2-12) As you complete the dissection and reflection of the pectoral muscles, take note of the blood vessels and nerves entering the deep face of the forelimb in the axillary region (armpit) and be careful not to cut through them!

• Identify the superficial pectoral m.

  • Note that this muscle has two parts, descending and transverse, but you do not need to differentiate them, as they can be tricky to separate – especially in cat specimens. (See DG Fig. 2-12)

  • Transect the superficial pectoral m. (both parts) ~1cm from the sternum and reflect the main muscle mass distally on the forelimb.

• Identify the deep pectoral m. lying deep to the superficial pectoral m.

  • Transect the deep pectoral m. ~2cm from the sternum and reflect the main muscle mass toward the forelimb.

Crossing the cranial surface of the shoulder, identify the brachiocephalicus m. (See DG Figs. 2-12 and 2-15) extending from the brachium to the head. Define its borders. Be careful to avoid cutting the cephalic vein as you dissect in this area. Note that this muscle has several subdivisions/parts that will be identified.

• Comment: This muscle has different parts named according to their location in reference to what would be the clavicle (collar bone). In the dog, within this muscle there is a faint line called the clavicular intersection, whereas in the cat there is a small clavicle bone, located at the point of the shoulder.

  • Cleido- is a word part that refers to clavicle.

  • The portion of muscle extending from the clavicular intersection/clavicle to the brachium is the cleidobrachialis m.

  • The portion of muscle extending from the clavicular intersection toward the head and neck is the cleidocephalicus m. The cleidocephalicus also has two parts, pars cervicalis and pars mastoidea, named with reference to their respective insertion points.

• Identify the clavicular intersection (dog) or the small clavicle (cat).

• Identify the cleidobrachialis m. extending from the clavicular intersection to the brachium.

• Identify the cleidocephalicus m. extending from the clavicular intersection toward the head and neck.

  • Comment: As mentioned previously, cleidocephalicus has two parts, pars cervicalis and pars mastoidea, which you do not need to differentiate in your dissection. These two parts will be dissected on demonstration animals for you to observe.

• Transect the brachiocephalicus m. 2-3cm cranial to the clavicle/clavicular intersection so it can be reflected to see deeper structures.

• In cat specimens, after transecting the brachiocephalicus muscle, you should palpate the small clavicle found on the medial side.

Identify the omotransversarius m. extending from the shoulder region to the head. (See DG Fig. 2-15) Transect the omotransversarius m. through its middle and reflect both halves.

Identify the superficial cervical lymph nodes (aka prescapular lymph nodes) deep to the omotransversarius m., just cranial to the scapula/shoulder region. (See DG Fig. 3-26)

As you proceed with the following muscle dissection, note the median raphe of the neck which is a fibrous septum between the left and right sides of epaxial muscles in the neck (along dorsal midline). It may be a helpful landmark to orient yourself to the muscles when looking at them from the dorsal midline perspective.

• Transect both parts of trapezius with an arching cut, from the cranial aspect of the cervical part to the caudal aspect of the thoracic part, and reflect both parts (reflect the majority of the muscle distally, toward the limb). This will allow you to view the underlying rhomboideus m.

Identify the rhomboideus m. deep to the trapezius, attaching to the dorsal border of the scapula. The rhomboideus m. has three parts, capitis, cervicis, and thoracis, each named for the place it originates from; all three parts insert on the dorsal border of the scapula.

• Identify the rhomboideus capitis which is a small ‘strap’ of muscle that extends toward the caudal aspect of the head.

• Identify the rhomboideus cervicis extending from the neck to the dorsal border of the scapula.

• Identify rhomboideus thoracis extending from the first seven thoracic vertebrae to the dorsal border of the scapula.

Just caudal to the forelimb, identify the large, roughly triangular, latissimus dorsi m. covering part of the thoracic wall. (See DG Fig. 2-16)

• Transect the latissimus dorsi m. ~1cm caudal to the forelimb and reflect it dorsally (toward the midline of the back).

Abduct the forelimb slightly (pull it away from the body) and, again, note the axillary vessels and nerves entering the deep face of the limb (try not to cut through them!).

As you abduct the forelimb, identify the large, fan-shaped serratus ventralis m. on the lateral aspect of the trunk. The serratus ventralis m. has two parts, cervicis and thoracis; these names refer to the regional origins from the neck and thorax respectively. Identify both parts of the serratus ventralis m. (See DG Figs. 2-16 and 2-76, A)