SURFACE FORM OF THE UPPER EXTREMITY. 



409 



conjunction with the Lumbricales, flex the first phalanges at the metacarpo-phalan- 

 geal joints, and extend the second and third phalanges in consequence of their 

 insertion into the expansion of the extensor tendons. The Extensor communis 

 digitorura is believed to act almost entirely on the first phalanges. 



SURFACE FORM OF THE UPPER EXTREMITY. 



The Pectoral is major largely influences surface form and conceals a considerable part of the 

 thoracic wall in front. Its sternal origin presents a festooned border which bounds and deter- 

 mines the width of the sternal furrow. Its clavicular origin is somewhat depressed and flattened, 

 and between the two portions of the muscle is often an oblique depression which differentiates 

 the one from the other. The outer margin of the muscle is generally well marked above, and 

 bounds the infraclavicular fossa, a triangular interval which separates the Pectoralis major from 

 the Deltoid. It gradually becomes less marked as it approaches the tendon of insertion, and 

 becomes more closely blended with the Deltoid muscle. The lower border of the Pectoralis 

 major forms the rounded anterior axillary fold, and corresponds with the direction of the fifth 

 rib. The Pectoralis minor influences surface form. When the arm is raised its lowest slip of 

 origin produces a local fulness just below the border of the anterior fold of the axilla, and so 

 serves to break the sharp line of the lower border of the Pectoralis major muscle, which is 

 produced when the arm is in this position. The origin of the Serratm magnits produces a very 

 characteristic surface marking. When the arm is raised from the side in a well-developed 

 subject, the five or six lower serrations are plainly discernible, forming a zigzag line, caused by 

 the series of digitations, which diminish in size from above downward, and have their apices 

 arranged in the form of a curve. When the arm is lying by the side, the first serration to 

 appear, at the lower margin of the Pectoralis major, is the one attached to the fifth rib. The 

 Deltoid, with the prominence of the upper extremity of the humerus, produces the rounded 

 outline of the shoulder. It is rounder and fuller in front than behind, where it presents a 

 somewhat flattened form. Its anterior border, above, presents a rounded, slightly curved 

 eminence, which bounds externally the infraclavicular fossa ; below, it is closely united with the 



FIG. 244. The Dorsal interossei of left hand. 



FIG. 245. The Palmar interossei of left hand. 



Pectoralis major. Its posterior border is thin, flattened, and scarcely marked above ; below, it 

 is thicker and more prominent. When the muscle is in action, the middle portion becomes 

 irregular, presenting alternate longitudinal elevations and depressions, the elevations correspond- 

 ing to the fleshy portions, the depressions to the tendinous intersections of the muscle. The 

 insertion of the Deltoid is marked by a depression on the outer side of the middle of the arm. 

 Of the scapular muscles, the only one which materially influences surface form is the Teres 

 major, which assists the Latissimus dorsi in forming the thick, rounded fold of the posterior 

 boundary of the axilla. When the arm is raised, the Coraco-brachialis reveals itself as a long, 

 narrow elevation which emerges from under cover of the anterior fold of the axilla and runs 

 downward, internal to the shaft of the humerus. When the arm is hanging by the side, its 



