THE CHEST. 19 



SUPERFICIAL ANATOMY OF THE TRUNK. 



THE CHEST. 



On the front of the chest, the greater part of the thoracic wall is concealed on 

 each side by the pectoralis major, the uppermost portion of the muscle extending 

 over the inner half of the clavicle from which it arises, while inferiorly, it forms a 

 prominent curved margin, which follows the direction of the fifth costal cartilage. 

 The interval between the clavicular and sterno-costal portions can often be seen 

 when the muscle is at rest, and always when it is put into action Externally, the 

 upper and lower borders of the muscle converge as it narrows to its insertion ; the 

 former is at first separated from the adjacent anterior margin of the deltoid by the 

 wfradavicular fossa, but lower down the two muscles become closely united ; the 

 lower margin of the pectoralis major leaves the chest opposite the fifth rib (at which 

 spot the lowest slip of the pectoralis minor often appears on the surface) and forms, 

 as it passes upwards and outwards to the arm, the rounded anterior axillary fold, 

 ending in the sharp tendon, which becomes apparent when the muscle is in action. 

 The nipple is placed over the outer and lower part of the pectoral muscle, generally 

 between the fourth and fifth ribs, about three-quarters of an inch (2 cm.) external 

 to the junction of the bone and cartilage, and rather more than four inches (10 cm.) 

 from the middle line ; but its'position varies considerably in different individuals, and 

 it is not unfrequently, especially in fat persons and in females, at a much lower level. 



Along the middle line, the sternum is subcutaneous at the bottom of the sternal 

 groove or furrow between the great pectoral muscles. The furrow is interrupted 

 towards the upper part by a slight, but distinct, transverse ridge, which marks the 

 sternal angle formed by the union of the manubrium and the body of the sternum, 

 and on each side of this the second costal cartilage, which projects forwards more 

 than the others, continues the prominence outwards. Inferiorly, the sternal furrow 

 opens out, as the pectoral muscles diverge from one another, exposing the lower end 

 of the body of the sternum, a spot which marks the articulation of the seventh 

 costal cartilage, and which is always to be readily felt, and usually distinctly seen, 

 owing to the formation of the infrasternal depression immediately below it. The 

 infrasternal depression (epigastric fossa, scrobiculus cordis) is a generally well- 

 marked, although variable, hollow between the seventh costal cartilages and the 

 upper ends of the recti muscles, and is placed over the ensiform process, which is 

 itself seldom visible on the surface. It will be remembered that the upper margin 

 of the sternum is on a level (during expiration) with the disc between the second 

 and third dorsal vertebrae ; the junction of the manubrium and body is opposite the 

 fifth dorsal vertebra ; and the xiphi-sternal articulation generally corresponds to the 

 lower part of the ninth dorsal vertebra. 



To the outer side of the pectoralis major, the ribs are covered by the serratus 

 magnus. Of the digitations of this muscle, the first to appear, at the lower margin 

 of the pectoralis major, is the one attached to the fifth rib ; the following one, the 

 sixth, is the largest and most prominent, and they become less marked below this. 

 Below the pectoral muscle, the wall of the thorax is covered by the rectus abdominis 

 internally, and the external oblique laterally, the pointed slips of the latter muscle 

 being received between the digitations of the serratus magnus. More posteriorly, 

 the latissimus dorsi ascends over the hinder part of the serratus, and, winding round 

 the teres major muscle, forms the thick posterior fold of the axilla. 



The ribs may generally be followed without difficulty over the front and sides 

 of the chest ; but only a very small portion of the first can be distinguished, as it is 

 almost completely covered by the clavicle and scapula. The width of the inter- 

 costal spaces, and the form of the subcostal angle vary greatly in accordance with 



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