Superficial pectoral muscles

The superficial pectoral muscles (Musculi pectorales superficiales) occupy the space between the ventral part of the thoracic wall and the proximal part of the thoracic limb, forming the ventral aspect of axilla.

The superficial pectoral muscles comprise two muscles:

• The descending pectoral muscle (Musculus pectoralis descendens) that originates form the manubrium of the sternum and terminates on the crest of the greater tuberclce of the humerus
• The transverse pectoral muscle (Musculus pectoralis transversus) arises caudal to the descending pectoral muscle from the ventral aspect of the sternum and blends with the fascia of forearm.

In carnivores, the descending pectoral muscle is smaller, as a a narrow band-shaped muscle, and is hardly discernable form the thicker transverse pectoral muscle.

Origin: Paramedially on the cranial end of the sternum.

Insertion: Entire crest of the greater tubercle of the humerus.

Action: Supports the limb, draw back the limb inward, draw the limb forward or backward according to its position, and draw the trunk sideward.

Nerve: cranial pectoral nerves and branches form cervical nerves 7 and 8

Note: The disposition of the pectoral muscles is very different according to humans and animals, causing difficulties of nomenclature (Barone et al.):

• The pectoralis major in human results from the union of the descending and ascending pectoral muscles in animals.
• Humans have a small pectoral muscle that does not exist in quadrupeds.
• Quadrupeds have a transverse pectoral muscle that is not present in humans.