296 MUSCLES AND FASCIJE. 



Relations. By its anterior surface, with the Platysma, the mammary gland, the 

 superficial fascia, and integument. By its posterior surface: its thoracic portion, 

 with the sternum, the ribs and costal cartilages, the Subclavius, Pectoralis minor, 

 Serratus magnus, and the Intercostals ; its axillary portion forms the anterior 

 wall of the axillary space, and covers the axillary vessels and nerves. Its upper 

 border lies parallel with the Deltoid, from which it is separated by the cephalic 

 vein and descending branch of the thoracico-acromialis artery. Its lower border 

 forms the anterior margin of the axilla, being at first separated from the Latissimus 

 dorsi by a considerable interval ; but both muscles gradually converge towards the 

 outer part of this space. 



Peculiarities. In muscular subjects, the sternal origins of the two Pectoral muscles are 

 separated only by a narrow interval ; but this interval is enlarged where these muscles are ill- 

 developed. Very rarely, the whole of the sternal portion is deficient. Occasionally, one or two 

 additional muscular slips arise from the aponeurosis of the External oblique, and become united 

 to the lower margin of the Pectoralis major. A slender muscular slip is occasionally found 

 lying parallel with the outer margin of the sternum, overlapping the origin of the Pectoral 

 muscle. It is attached, by one end, to the upper part of the sternum, near the origin of the 

 Sterno-mastoid ; and, by the other, to the anterior wall of the sheath of the Rectus abdominis. 

 It has received the name "Rectus sternalis." 



Dissection. The Pectoralis major should now be detached by dividing the muscle along its 

 attachment to the clavicle, and by making a vertical incision through its substance a little external 

 to its line of attachment to the sternum and costal cartilages. The muscle should then be reflected 

 outwards, and its tendon carefully examined. The Pectoralis minor is now exposed, and imme- 

 diately above it, in the interval between its upper border and the clavicle, is a strong fascia, the 

 costo-coracoid membrane. 



The costo-coracoid membrane protects the axillary vessels and nerves, and is 

 very thick and dense externally, where it is attached to the coracoid process, and 

 is continuous with the fascia of the arm ; more internally, it is connected with the 

 lower border of the clavicle, as far as the inner extremity of the first rib : traced 

 downwards, it passes behind the Pectoralis minor, surrounding in a more or less 

 complete sheath, the axillary vessels and nerves ; and above, it sends a prolongation 

 behind the Subclavius, which is attached to the lower border of the clavicle, and 

 so incloses the muscle in a kind of sheath. The costo-coracoid membrane is 

 pierced by the cephalic vein, the acromial-thoracic artery and vein, superior 

 thoracic artery, and anterior thoracic nerve. 



The Pectoralis minor (fig. 170) is a thin, flat, triangular muscle, situated at 

 the upper part of the thorax, beneath the Pectoralis major. It arises, by three 

 tendinous digitations, from the upper margin and outer surface of the third, fourth, 

 and fifth ribs, near their cartilages, and from the aponeurosis covering the Inter- 

 costal muscles : the fibres pass upwards and outwards, and converge to form a flat 

 tendon, which is inserted into the anterior border of the coracoid process of the 

 scapula. 



Relations. By its anterior surface, with the Pectoralis major, and the superior 

 thoracic vessels and nerves. By its posterior surface, with the ribs, Intercostal 

 muscles, Serratus magnus, the axillary space, and the axillary vessels and nerves. 

 Its upper border is separated from the clavicle by a triangular interval, broad 

 internally, narrow externally, bounded in front by the costo-coracoid membrane, 

 and internally by the ribs. In this space are seen the axillary vessels and nerves. 



The costo-coracoid membrane should now be removed, when the Subclavius muscle will be seen. 



The Subclavius (fig. 170) is a long, thin, spindle-shaped muscle, placed in the 

 interval between the clavicle and the first rib. It arises by a short, thick tendon, 

 from the cartilage of the first rib, in front of the rhomboid ligament ; the fleshy 

 fibres proceed obliquely outwards to be inserted into a deep groove on the under 

 surface of the middle third of the clavicle. 



Relations. By its upper surface, with the clavicle. By its under surface, it is 

 separated from the first rib by the axillary vessels and nerves. Its anterior 



