THE PECTORAL MUSCLES. 



569 



a band, the costo-'coracoid ligmnent (Fig. 560), which passes obHquely downward 

 and laterally from the clavicle to the coracoid process. The coraco-clavicular fascia 

 occasionally contains muscle-tibres (the m. coraco-claviciilaris) , and is usually perfo- 

 rated by the cephalic vein on its way to join the axillary, by the thoraco-acromial 

 artery, and by the external anterior thoracic nerve. 



I. Pectoralis Major (Fig. 557). 



Attachments. — The pectoralis major is a strong fan-shaped muscle situated 

 on the anterior thoracic wall. It is composed of three portions : (i) \}l\^ pars cla- 

 vicularis, which arises from the inner half of the anterior border of the clavicle ; 

 (2) the pars sterno-costalis, which arises from the anterior surface of the sternum 

 and tUe upper six costal cartilages ; and (3) x\\e. portio abdotniitalis, which arises from 



Fig. 557. 



Deltoid 



Clavicle 



Pectoralis major 

 clavicular portiui 



Brachial 

 fascia 



Sternum 



Pectoralis 

 major, sterno- 

 costal portion 



Serratus magnus 



Latissimus dorsi 



Pectoralis major, 

 abdominal portion 



Dissection of thoracic wall, showing pectoralis major. 



the upper part of the anterior layer of the sheath of the rectus abdominis. From 

 these origins the fibres are directed laterally to be inserted into the external bicipital 

 ridge which extends downward from the greater tuberosity of the humerus, the lower 

 fibres of the sterno-costal and the abdominal portions of the muscle passing behind 

 those of the clavicular and upper portions, so that the tendon of insertion is U-shaped 

 in section, consisting of two layers separated above but continuous below. A bursa 

 is usually interposed between the posterior surface of the tendon and the anterior 

 surface of the long head of the biceps humeri. 



Nerve-Supply. — From the external and internal anterior thoracic nerves by 

 fibres from the lower four cervical and the first thoracic nerves. 



