Limbs : Upper Limb 65 



It has been pointed out more recently, that there is a mirror-relation between the limbs one 

 limb corresponding with the other " the other way round " and this is a suggestive conception 

 of the limbs that does not depend on homology : but the'question is rather outside the province of 

 this work, and it is not possible to devote further space to its consideration. 



The primary elements of the girdles and of the free limb skeletons are laid down 

 in cartilage, and the process of ossification follows in a general way the evolution of 

 the limb as sketched above that is, the centres for the proximal segments of the 

 free limbs appear first, followed shortly by those for the next segment, and at about 

 the same time ossification begins in the girdles : in the extreme segments the process 

 is variable in its onset and delayed in parts, as it is in some portions of the girdles. 



2. UPPER LIMB. 



The buried pectoral girdle consists of two bones on each side the Scapula or 

 shoulder blade, on the back and outer side of the upper part of the thorax, supported 

 here by muscles and by the clavicle or collar bone, which articulates at one end with the 

 scapula and by the other with the top of the sternum : so the girdle is completed in 

 front by the manubrium. 



The humerus is the bone of the upper arm, articulating with the scapula and 

 carrying at its lower end the radius and ulna, that constitute the bony skeleton of the 

 forearm. The Radius is the preaxial bone, the Ulna postaxial. The radius carries 

 the skeleton of the hand, and is capable of pronation and supination by rotating round 

 the ulna. 



The bones of the upper limb in men are frequently a very little longer on the right 

 than on the left side. In the adult the limb, hanging by the side, extends to about 

 half-way down the thigh. 



CLAVICLE, 



A long bone connecting the acromion process of the scapula with the upper end 

 of the sternum and directed outwTIl'lB ind backwards : there is frequently a slight 

 downward direction also to be found in female skeletons. 



The bone has a double curve whence its name * in the horizontal plane. The 

 inner curve takes about the inner two-thirds of the bone and has a posterior concavity 

 that arches over the axillary sheath (see Fig. 51). The outer curve is shorter and 

 sharper, with an anterior concavity, and takes the outer third of the bone. The inner 

 two-thirds of the bone forms a somewhat rounded bar, but the outer third is flattened, 

 so that it can be described as possessing upper and lower surfaces, with anterior and 

 posterior margins : the " margins " are really thick, rounded surfaces. This part has 

 also an outer margin which articulates with the acromion. 



The lower aspect of the clavicle can be recognised at once (Fig. 50), by the promi- 

 nent ligamentous markings on it and by the presence of a groove in its middle part. 



The bone presents a shaft and two extremities for examination. The inner end 

 has an articular surface at its extremity for the sterno-clavicular fibro-cartilage, 

 concavo-convex, and prolonged on to the under aspect of the bone for articulation 

 with the first costal cartilage. Round this articular surface are markings for the 

 sterno-clavicular ligaments : the Sterno-hyoid origin extends from these ligaments 

 behind on to the back of the bone for a little distance. 



* Clavis = a key : the Roman key was S-shaped. 

 F.A. ' - * 5 



