THE ANTERIOR THORACIC REGION 461 



extending from its attachment to the first rib to the coracoid process is often whiter 

 and denser than the rest; this is sometimes called the costocoracoid ligament. 

 Below, it is thin, and at the upper border of the Pectoralis minor it splits into 

 two layers to invest this muscle; from the lower border of the Pectoralis minor 

 it is continued downward to join the axillary fascia, and outward to join the fascia 

 over the short head of the Biceps. The costocoracoid membrane is pierced by 

 the cephalic vein, the acromiothoracic artery and vein, superior thoracic artery, 

 and anterior thoracic nerve. 



The Pectoralis minor (m. pectoralis minor) (Fig. 349) is a thin, flat, triangular 

 muscle, situated at the upper part of the thorax, beneath the Pectoralis major. 

 It or/.sr.v by three tendinous dictations from the upper margin and outer surface 

 of fTT^ fhirr^ frmrth^gnrl fiftli rib,fl ( n^ar their cartilages, and from the aponeurosis 

 covering the Intercostal muscles; the fibres pass upward and outward, and con- 

 verge to form a flat tendon, which is itiscrtcd into the inner border and upper 

 surface of the coracoid process of the scapula. 



Relations. By its superficial surface, with the Pectoralis major and the thoracic branches of 

 the acromiothoracic artery. By its deep surface, with the ribs, Intercostal muscles, Serratus 

 magnus, the axillary space, and the axillary vessels and brachial plexus of nerves. Its upper 

 border is separated from the clavicle by a narrow triangular interval, occupied by the costocoracoid 

 membrane, behind which are the axillary vessels and nerves. The longjthoracic ~artery~run"s 

 parallel to the lower border of this muscle and the anterior thoracic nerve pierces it." 



The" costocoracoid membrane should now be removed, and the Subclavius muscle will be 

 exposed. 



The Subclavius (m. subclavius) is a small triangular muscle, placed in the 

 interval between the clavicle and the first rib. It qrisos. in front of the rhomboid 

 ligament by a short, thick tendon from the first rib and its cartilage at their 

 junction; the fleshy fibres proceed obliquely upward and outward, to be insi-rfcl 

 into the groove on the under surface of the clavicle. An extension from the apon- 

 eurosis of this muscle lies upon the subclavian vein. 



Relations. Its deep surface is separated from the first rib by the subclavian vessels and 

 brachial plexus of nerves. Its superficial surface is separated from the Pectoralis major by the 

 costocoracoid membrane, which, with the clavicle, forms an osseofibrous sheath in which the 

 muscle is enclosed. 



If the costal attachment of the Pectoralis minor be divided across, and the muscle reflected 

 outward, the axillary vessels and nerves are brought fully into view, and should be examined. 



Nerves. The Pectoral muscles are supplied by the external and internal anterior thoracic 

 nerves; the Pectoralis major through these nerves receives filaments from all the spinal nerves 

 entering into the formation of the brachial plexus: the Pectoralis minor receives its fibres from 

 the eighth cervical and first thoracic nerves through the internal anterior thoracic nerve. The 

 Subclavius is supplied by a filament derived from the fifth and sixth cervical nerves. 



Actions. If the arm has been raised by the Deltoid, the Pectoralis major will, conjointly 

 with the Latissimus dorsi and Teres major, depress it to the side of the thorax. If acting alone, 

 it adducts and draws forward the arm, bringing it across the front of the thorax, and at the same 

 time rotating it inward. The Pectoralis minor depresses the point of the shoulder, drawing 

 the scapula downward and inward to the thorax, and throwing the inferior angle backward. 

 The Subclavius depresses the shoulder, drawing the clavicle downward and forward. When 

 the arms are fixed, all three muscles act upon the ribs, drawing them upward and expanding the 

 thorax, and thus becoming very important agents in forced inspiration. During an attack of 

 asthma patients always assume an attitude which fixes the shoulders, so that all these muscles 

 may be brought into action to assist in increasing the capacity of the thorax. 



