THE PECTORALIS MAJOR. 209 



sterno-costal, which are separated at their origin by a slight interval opposite the 

 sterno-clavicular articulation, but are united externally at the insertion of the muscle. 

 The clavicular portion arises from an impression occupying the inner half of the 

 anterior surface of the clavicle ; the sterno-costal portion from the anterior surface of the 

 sternum, from the cartilages of the upper six ribs, and from the aponeurosis of the 

 external oblique muscle of the abdomen. The fibres of the two portions converge, 

 and form a thick mass which is inserted by a tendon of considerable breadth into the 

 pectoral ridge of the humerus. The tendon of insertion is composed of two layers, 

 which are only united for a limited extent along their lower margins. -The anterior 

 layer is formed by the upper sterno-costal fibres, and is also joined on its front sur- 

 face by the descending clavicular portion of the muscle, the latter being prolonged 

 downwards beyond the level of the sterno-costal fibres and becoming closely united 

 with the tendon of the deltoid. The posterior layer of the tendon is formed by the 

 lower sterno-costal fibres, which turn backwards successively behind the upper part ; 

 this layer of the tendon reaches higher on the humerus than the anterior, and from 

 its upper border an expansion is given off, covering the long head of the biceps 

 muscle, to the great tuberosity of the humerus and the capsule of the shoulder- joint, 

 while its deepest fibres are reflected inwards at their attachment to the bone, lining 

 the bicipital groove, and meeting the insertion of the latissimus dorsi. From the lower 

 border of the tendon also a slip is prolonged to the fascia of the arm. 



Relations. The folded inferior border of the pectoralis major forms the anterior margin 

 of the axilla ; the superior is nearly parallel to that of the deltoid muscle, the two being 

 separated only by a slight interval which becomes wider towards the clavicle, and in which 

 run the cephalic vein and the humeral branch of the acromio-thoracic artery. The anterior 

 surface is subcutaneous in the greater part of its extent, being covered only by some of the 

 fibres of the platsyma myoides and by the mamma. The posterior surface rests chiefly on the 

 pectoralis minor, and with that muscle forms the anterior wall of the axilla. 



Varieties. The more frequent varieties of this muscle consist in the greater or less 

 extent of its attachments to the ribs or sternum, and the greater or less separation of its 

 clavicular and sterno-costal parts. The clavicular part is occasionally incorporated com- 

 pletely with the deltoid. The tendinous and fleshy fibres of origin of opposite sides sometimes 

 meet and even decussate in front of the sternum. In some instances additional muscular slips 

 take origin from the aponeurosis of the external oblique muscle. Numerous cases are 

 recorded of considerable deficiency or even absence of the sterno-costal portion. Absence of 

 the clavicular part is less frequent. In rare cases the whole muscle has been wanting. The 

 slips of connection with the latissimus dorsi have already been noticed. 



Costo-corac'oideus. This is a muscular band springing from one or more of the ribs 

 between the origins of the pectoralis major and latissimus dorsi, or from the aponeurosis of 

 the external oblique, and inserted into the coracoid process, either separately, or in conjunc- 

 tion with the pectoralis minor or coraco-brachialis and biceps muscles. It may be repre- 

 sented by a fasciculus given off from the pectoralis major, or from the latissimus dorsi. 



Chondro-epitrochleari*. This name has been given to a muscular slip which is 

 occasionally seen, springing from one or two rib-cartilages, or from the aponeurosis of the 

 external oblique, below the pectoralis major, or sometimes given off from the lower border 

 or from the tendon of the muscle itself, and passing to a variable insertion on the inner side 

 of the arm, to the fascia, to the intermuscular septum, or even to the internal condyle of the 

 humerus. 



The sternalis muscle is a fasciculus not unfrequently present on one or both sides, fleshy 

 in the middle and tendinous at both ends, lying on the surface of the pectoralis major, 

 parallel to the margin of the sternum. It springs below, variably, from the sheath of the 

 rectus and some of the costal cartilages between the third and seventh, and terminates above 

 in the sterno-mastoid, or sternum and upper rib-cartilages, or more rarely in the pectoralis 

 major. When two are present, they frequently unite in front of the manubrium. Its nervous 

 supply is derived from the anterior thoracics, and in some cases also from the intercostals. 

 The sternalis appears to be a displaced portion of the pectoralis major, which has undergone 

 a rotation downwards and inwards. (D. J. Cunningham, Journ. Anat., xxii, 391.) 



The pectoralis minor muscle arises from the upper margins and slightly from 

 the external surfaces of three ribs near their cartilages usually the third, fourth, and 



