THE MIDDLE PALMAR REGION. 



497 



that each finger is provided with two Interossei 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 median nerve ; the 

 rest of the muscles of this group, by the ulnar. All the Interossei are supplied by 

 the ulnar. 



Actions. The Palmar interossei muscles adduct the fingers 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 



FIG. 317. The Dorsal interossei of left hand. 



FIG. 318. The Palmar interossei of left hand. 



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 

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



SURFACE FORM OF THE UPPER EXTREMITY. 



The Pi-cfornJifi 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 Pi-ftoi-fili.t utinn,- 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 Serratus niagnus 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 extremit\ T of the humerus, produces the rounded 

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

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

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



