496 THE MUSCLES AND FASCIA 



phalanx and aponeurotic expansion of the common Extensor tendon of the same 

 finger. The First arises from the ulnarside of the second metacarpal bone, and 

 is inserted into the same side of the first phalanx of the index finger. The Second 

 arises from the radial side of the fourth metacarpal bone, and is inserted into the 

 same side of the ring finger. The Third arises from the radial side of the fifth 

 metacarpal bone, and is inserted into the same side of the little finger. From 

 this account it may be seen that each finger is provided with two Interosseous 

 muscles, with the exception of the little finger, in which the Abductor muscle 

 takes the place of one of the pair. 



Nerves. The two outer Lumbricales are supplied by the sixth cervical nerve, through the 

 third and fourth digital branches of the median nerve; the two inner Lumbricales and all the 

 Interossei are supplied by the eighth cervical nerve, through the deep palmar branch of the 

 ulnar nerve. The third Lumbrical frequently receives a twig from the median. 



Actions. The Palmar interossei muscles adduct the finger to an imaginary line drawn 

 longitudinally through the centre of the middle finger; and the Dorsal interossei abduct the 

 fingers from that line. In addition to this, the Interossei, in conjunction with the Lumbricales, 

 flex the first phalanges at the metacarpophalangeal joints, and extend the second and third 

 phalanges in consequence of their insertion into the expansion of the Extensor tendons. The 

 Extensor communis digitorum is believed to act almost entirely on the first phalanges. 



SURFACE FORM OF THE UPPER EXTREMITY. 



The Pectoralis major muscle 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 determines 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 infradavicular fossa (Fossa of Mohrenheim), a triangular interval which 

 separates the Pectoralis major from the Deltoid. It gradually becomes less marked as it 

 approaches the tendons 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 muscle 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 Pec- 

 toralis major muscle, which is produced when the arm is in this position. The origin of the 

 Serratus magnus muscle produces a very characteristic surf ace. 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 muscle, 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 infraclaviciilar 

 fossa; below, it is closely united with the 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 corresponding 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 front and inner part presents the prominence of the Biceps, 

 bounded on either side by an intermuscular depression. This muscle determines the contour of 

 the front of the arm, and extends from the anterior margin of the axilla to the bend of the elbow. 

 Its upper tendons are concealed by the Pectoralis major and the Deltoid, and its lower tendon 

 sinks into the space at the bend of the elbow. When the muscle is in a state of complete con- 

 traction that is to say, when the forearm has been flexed and supinated it presents a rounded 

 convex form, bulged out laterally, and its length is diminished. On each side of the Biceps, 



