Pectoralis Major and Pectoralis Minor
From the surface of pectoralis major, some superficial fibers arise from the fascia of the clavicular head and are inserted into the fascia of the deltoid muscle. M. Sternalis (in 3-5% of individuals) is a fleshy band of longitudinal fibers, often bilateral, of varying length and width, located close to and, generally, parallel to the sternum. When typical, it arises from the sheath of the rectus abdominis, fascia of the external oblique, pectoralis major, or costal cartilages (3-7) and ends above in fascia, upper costal cartilages, or manubrium, or may be joined to the the sternal or medial head of sternocleidomastoideus. The muscle may exhibit tendinous inscriptions. It is considered by some authors to be a vestige of the panniculus carnosus. In a study of 1000 cadavers (438 females and 562 males), sternalis was found more frequently in females (8.7%) than males (6.4). Its reported incidence in the white population is about 4.-7%, the black population about 8.4%, and in the Asian population about 11.5%. (see also m. supracostalis).
Turner reported finding 21 sternalis muscles in 650 subjects or about 3%. Of these, 7 were male and 11 female. In 12 subjects the muscle was single and in 9 double. Turner reported that Cabrolius (Anatomes elenchus accuratissimu, Obs. 8, p. 96, 1604) was the first observer of sternalis muscle. The muscle was not precisely described until 1726, when DuPuy (Historie de l'Acad. Royale des Sciences, Amsterdam) gave an account of a well defined example, which he found in its bilateral form. Shortly afterwards other specimens were observed by Weitbrecht (Comment. Acad Petropolitanae, 1729, Vol. 4, p. 258), Albinus (Historia Musculorum, 1734, p. 291) De la Faye (Historie de l'Acad. Roy. des Sciences, 1736, p. 82), and Wilde (Com. Acad. Petropol. 1740, Vol. 12, p. 320). In 1751, Turner points to an important memoire on the subject by Kaau Boerhaave (Idem., 1751, p. 257). Based on his account and descriptions and figures by Boerhaave, and Sandifort (Exercitationes Academicae, 1783, p. 82 called the muscle Thoracicus, a name which is frequently been applied to it by subsequent authors (e.g., Vesalius).
In a study be Saeed, et al. The prevalence of sternalis in the Kingdom of Saudi Arabia is about 4% based on a study of 75 adult cadavers and a retrospective study of 1580 adult females (none of the 1580 were found to have sternalis muscles). Saeed found the Sternalis muscle to be innervated by intercostal nerves.
The pectoral sheet is divided into a superficial and a deep layer. The outer sheet (pectoralis major) is divided into clavicular, manubrial, sternal, and abdominal portions, all of which may be more or less separable (see illustration of Leonardo da Vinci and compare with that of Huntington).
The clavicular head may extend laterally on the clavicle as far as the deltoideus and be fused with it, or its origin may be confined to its sternal end. It is occasionally divided into two distinct parts; it may also decussate across the midline.
Its sternal and costal heads may be absent and its complete absence unilaterally and bilaterally has been recorded. The sterno-costal head may extend laterally to the latissimus dorsi. The clavicular head is least likely to be absent.
Rarely , the whole muscle may be absent. In one study, pectoralis major was absent in three of 15,000 cases and in another study, the muscle was absent in five of 54,000 cases. On average, the muscle was absent in about 0.01% or one in 10,000 individuals.
Tiedemann reported in Meckels archives that pectoralis major may also be doubled.
The abdominal portion of the muscle is considered to be a derivative of the panniculus carnosus. The abdominal portion may be fused with rectus abdominis or with external oblique. Sometimes a slip from pectoralis major extends to biceps, pectoralis minor, the coracoid process, capsule of the shoulder joint, or the brachial fascia.
Bing reported that the sternocostal part of pectoralis major defective or absent more than any other muscle.
Pectoralis quartus usually arises near the costochondral junction of the fifth and sixth ribs. It extends laterally along the border of pectoralis major, but is entirely separate from it. It crosses the axilla to be inserted on or near the deep surface of the tendon of pectoralis major. The quartus forms a long flat band with an average width of 1 cm. The origin and insertion of pectoralis quartus are frequently joined by a common band of connective tissue to an axillary arch and/or to a sternalis muscle when these muscles are present.
M. chondrofascialis (Macalister) extends from the insertion of pectoralis major to the medial intermuscular septum of the arm.
A muscular slip, costoepitrochlearis, chondroepitrochlearis (Duvernoy) or chondrohumeralis, has been described arising from one or more ribs, crossing the axilla, and inserting into the median intermuscular septum or onto the medial epicondyle of the humerus. It has been reported that in these cases the usual twist of the tendon of pectoralis major may fail to occur. It may occur in 12-20% of bodies (LeDouble).
The deep layer off the pectoral sheet consists of the pectoralis minor and certain muscles that appear only occasionally (See M. Subclavius).
Pectoralis minor reportedly originates from ribs 2,3, and 4, or 2, 3, 4, and 5 in 67% of subjects examined; in 24%, it arises from ribs 3,4, and 5. It rarely arises from the first or sixth rib. Pectoralis minor sometimes has its insertion extended beyond the coracoid process. The tendon, in such cases, passes over the process, from which it is separated by a bursa, through the coracoacromial ligament to the capsule of the shoulder joint and /or greater tubercle of the humerus. Pectoralis minor may send slips to the subclavius and pectoralis major and It may also insert onto the inferior surface of the middle of the clavicle.
The unilateral and bilateral absence of pectoralis minor has been reported. Chondrocoracoideus (Wood) or costocoracoideusarises by one or more slips from ribs 6 to8 and the rectus sheath to insert with the short head of the biceps. The manubrial part of the deep layer is normally absent, but maybe represented in rare cases by anomalous slips. The sternoclavicularis anterior (Gruber), which Gruber reported to be in about 3% of bodies, arises from the border of the manubrium and is inserted onto the anteroinferior border of the clavicle as far as, or even beyond, the middle of the bone.
Pectoralis intermedius, a fleshy slip that arises from the third and fourth ribs, between pectoralis major and minor, and is inserted onto the coracoid process.
Pectoralis minimus (Gruber) arises from the first rib cartilage and is inserted into the coracoid process.
The tensor semivaginae articulationis humeroscapularis arises from the first , first and second, second and third, or third and fourth rib cartilages and from the border of the sternum, to be inserted by an aponeurosis into the capsule of the shoulder joint or into the wall of the subacromial bursa.
Another slip, sternohumeralis, has been observed passing from the first cartilage and border of the sternum to the fascia of the arm.
Finally, the sternochondrocoracoideus arises from the third rib cartilage and/or border of the sternum and is inserted onto the coracoid process. Pectoralis major has been found to have more defects in form than any other muscle in the body (Bing). In fact, the pectoral muscles were anomalous six times more frequently than any other muscle or muscle group. See also Panniculus Carnosus and Pectoralis Major.
Syn.: P.Ma., m. pectoralis (Albin), Grosser Brustmuskel.
P. Mi., m. serratus anterior (Albin), serratus anterior minor, coracopectoralis, Kleiner Brustmuskel
