294 THE MUSCLES 



1. The pectoral fascia, a thin membrane which forms the sheath of the pec- 

 toralis major, and is attached to the clavicle al)Ove; while below it passes over the 

 free edge of tlie great pectoral muscle, and there unites with the axillary fascia. 



2. The clavi-pectoral fascia arises in two sheets from the anterior and pos- 

 terior borders of the under surface of the clavicle. These sheets unite after enclos- 

 ing the pubclavius muscle, and form a single membrane (the costo-coracoid) , which, 

 after bridging the space between the subclavius and pectoralis minor, divides again 

 to form the sheath of the pectoralis minor. At the lower border of this muscle, the 

 clavi-pectoral joins the axillary fascia. The costo-coracoid membrane, which is 

 the middle portion of the clavi-pectoral fascia, is of a quadrilateral shape, and is 

 attached above and below to the sheaths of the subclavius and the pectoralis minor 

 muscles; externally, where it blends with the sheath of the axillary vessels, it is 

 very strong; internally, where it joins the fascia covering the front part of the first 

 and second intercostal spaces, it is thin and weak. 



3. The axillary fascia is a strong membrane which stretches across the triangu- 

 lar floor of the axilla. In front, it joins the pectoral and. clavi-pectoral fasciae; 

 behind, it joins the sheath of the latissimus dorsi; above, it is continuous with the 

 deep fascia of the upper arm, and below with that of the thorax. 



The concavity of its surface, which is directed downwards and outwards, is 

 maintained to a great extent by the attachment of the clavi-pectoral fascia above 

 mentioned. 



First Layer 



PECTORALIS MAJOR 



The pectoralis major — named from its being the larger of the two muscles 

 which arise from the front of the chest (pgrtw.s = breast) — is a thick, triangular, fan- 

 shaped sheet; or, more accurately, it may be likened to the segment of a circle on 

 account of the curved origin, from which all the fibres converge to the upper part 

 of the humerus as a centre. 



Origin. — (1) The anterior surface of the inner half of the clavicle and the 

 adjacent part of the sterno-clavicular joint; (2) the side of the front of the 

 sternum, from the sterno-clavicular joint to the lower extremity of the gladiolus; 

 (3) the front of the cartilages of the second to the sixth ribs; (4) a small part of 

 the outer surface of the sixth rib close to its anterior extremity; (5) the upper part 

 of the aponeurosis of the external oblique muscle which forms the front of the 

 sheath of the rectus abdominis. 



• Insertion. — (1) The external bicipital ridge of the humerus from the greater 

 tuljerosity down to the impression for the deltoid; (2) adjacent fibrous structures, 

 esiiecially the tendon of insertion of the deltoid muscle. 



Structure. — The clavicular portion is distinct from the rest, and might be 

 described as a separate muscle. It forms a thick band of parallel fibres, which, 

 arising tendinous from the clavicle, become almost immediately fleshy, and are 

 inserted into the humerus by short tendinous fibres in front of the rest of the 

 tendon of insertion. 



The rest of the muscle (the sterno-costal portion as it is usually named) con- 

 sists of fleshy fil)res which, arising directly from the four other ])arts of the origin 

 enumerated, converge and cross one another to be inserted into the humerus l)ehind 

 the clavicular portion V)y means of a peculiar tendon. 



In a vertical section (fig. 271) near the humerus this tendon is seen to be folded 

 upon itself into a com] iressed horseshoe-shape. The convexity of the folded tendon 

 is directed downwards, and the anterior segment of the horseshoe is shorter than 

 the posterior. 



The anterior segment receives the muscular fibres which proceed from that 

 part of the origin of the sterno-costal portion which lies above the third costal 

 cartilage, and Avith the front of this segment, the insertion of the clavicular portion 

 is closely lilended. The posterior segment receives the fibres from the lower part 

 of the sterno-costal portion, the lowest fibres (viz. those from the external oblique 

 aponeurosis) being inserted highest, and the highest (viz. those irom the middle of 



