CHAPTER IV 



LIMBS: UPPER LIMB 

 i. THE LIMB-GIRDLES AND SKELETON OF THE FREE LIMBS. 



THE term " limb " is popularly applied to one of the appendages that are freely 

 movable on the " body " and distinct from it, but it must not be forgotten that these 

 are only what one may call " free " limbs, and are carried by a more fixed part related 

 to them and embedded in the walls of the trunk. So in dealing with the skeleton of 

 the limbs we can talk of that of the free limb, and that of the embedded portion with 

 which the skeleton of the free limb articulates. 



These embedded parts, really applied to the surface of the proper body-wall, 

 constitute the pectoral and pelvic girdles : in the former the skeleton is capable of 

 movement and not firmly articulated with the trunk skeleton, but in the latter, where 

 stability and strength are necessary for carrying the weight of the body, the girdle is 

 firmly fixed to the sacrum and forms with it the bony pelvis. 



The pectoral girdle consists of Scapula and Clavicle on each side : the clavicle is 

 the only bony connection between the scapula and the trunk skeleton, articulating 

 with the sternum. The upper arm, the proximal segment of the free limb, has the 

 humerus as its bony skeleton, articulating with the Scapula and carrying the bones 

 of the forearm (radius and ulna) at its distal end. 



The pelvic girdle is formed by two innominate bones which articulate with each 

 other ventrally and with the sacrum dorsally. Each os innominatum is composed of 

 three bones joined together, the Ilium, Ischium, and Pubis, and has a deep articular 

 surface (acetubulum) which carries the femur that supports the thigh. The Ilium is on 

 the dorsal and upper side of the acetabulum, the Ischium and Pubis on its ventral side, 

 and the junction of one innominate bone with the other is effected by the meeting of 

 their pubic portions in the symphysis. All the three parts of the bone are concerned in 

 forming the acetabulum. 



The resemblance in details between the upper and lower limbs are matters of common obser- 

 vation, and the general resemblance between their bony parts is very apparent. The meaning of 

 such likeness and the nature of the relationship and morphological identities of the limbs, if any, 

 has been the subject of much controversy and many theories, and appears likely to remain in that 

 position for an indefinite time : those interested should consult the special works that deal with the 

 matter, but some of these views will be shortly spoken of when considering the individual bones of 

 the limbs. 



The mode of development of the vertebrate limbs is hidden in the aeons that have passed since 

 the earliest chordate came into existence, but we may, perhaps, assume that they were evolved from 

 lateral appendages used by the hypothetical early vertebrate as a means of moving through the 

 water in which he is supposed to have dwelt. 



As the bulk of the animal increased, or the size of the appendages, it became necessary to 

 render them firmer by the growth of a skeletal frame-work in them, and, as a result of the continued 

 operation of such causes, to provide a more resistant basis in the wall of the body from which the 

 enlarged and strengthened appendage could act with greater force : thus we have a skeletal basis 

 of a free limb movable on a skeletal support embedded in the body-wall. 



Since such limbs were serving the same purpose (with very slight differences as the result of 

 position), the limbs and their skeleton would develop on more or less parallel lines, and their 

 increasing strength would be correlated with parallel growth of their embedded support. We are 



