The deep pectoral muscle (or ascending pectoral) is a strong muscle which originates from the sternum, the xiphoid cartilage adn the costal cartilages and inserts on the medial or lateral aspect of the proximal humerus in different species. It is deep to the superficial pectoral muscles (descending and transverse pectoral muscles).
In dogs :
Origin: First to last sternebra and with a superficial marginal portion as the pars abdominalis, from the deep fascia of the trunk to the region of the xiphoid cartilage.
Insertion: Lesser tubercle of the humerus.
Action: During locomotion, to move the trunk cranially over the advanced limb, extend the shoulder joint; draw the limb backward.
Nerve: Pectorales caudales.
Note: The disposition of the pectoral muscles is very different according to humans and animals, causing difficulties of nomenclature (Barone et al.):