Pectoralis major

The pectoralis major muscle is the largest muscle in the anterior aspect of the thorax. It is thick and fan-shaped.  It attaches superiorly to the medial half of the clavicle, and medially to the anterior aspect of the sternum and cartilage of the first to sixth or seventh rib, extending inferiorly to attach to the aponeurosis of the external oblique muscle. Laterally, this muscle attaches to the lateral lip of the intertubercular groove (bicipital groove) of the humerus by a two-layered tendon which inserts each of the two heads of the muscle.

The superficial tendon attaches the clavicular head (red in the accompanying image), which extends between the intertubercular groove of the humerus and the clavicle. The deep tendon attaches the sternocostal head (purple in the accompanying image), which extends between the humeral intertubercular groove and the attachments in the sternum, costal cartilages, and the aponeurosis of the external oblique muscle. There is usually a well-defined interval between the two heads of the pectoralis major.

The pectoralis major is innervated by the medial (C8-T1) and lateral pectoral nerves (C5-C7).

This muscle is covered by the pectoral fascia which extends in different directions. An extension of this fascia is the clavipectoral fascia. In both male and female, the mammary gland is situated anterior to and anchors in the pectoral fascia.

Pectoralis major muscle - Red: clavicular head. Purple: Sternocostal head - Image modified from the original by Henry VanDyke Carter, MD. Public domain
Pectoralis major muscle - Red: clavicular head. Purple: Sternocostal head
Click on the image for a larger depiction 
The word pectoral arises from the Latin term "pectum" meaning "chest, breast". In its true meaning, pectoral or pectoralis refers to a "chest plate" or an "adornment of the chest".

Sources:
1. “Gray’s Anatomy” Henry Gray, 1918
2. "Tratado de Anatomia Humana" Testut et Latarjet 8th Ed. 1931 Salvat Editores, Spain
3. "Gray's Anatomy" 38th British Ed. Churchill Livingstone 1995
4. “An Illustrated Atlas of the Skeletal Muscles” Bowden, B. 4th Ed. Morton Publishing. 2015

Image modified from the original by Henry VanDyke Carter, MD. Public domain