HOMOLOGY OF THE BONES OF THE LIMBS 207 



It is necessary to trace next the further changes which take place in the segments of the hmbs 

 up to the time when they assume their permanent positions. They may be arranged in stages 

 as follows: — 



(1) Each segment of the limb is bent upon the one above it. The humerus and femur 

 remain unchanged. The forearm segment, however, is bent so that the ventral surface looks 

 medially and the dorsal surface laterally. Moreover, the joints between these segments — i. e., 

 elbow and knee — -form marked projections. The terminal segments (hand and foot) are bent 

 in the opposite direction to the middle one, so that the primitive position is retained, and the 

 ends of the digits directed laterally. It will be noticed that in this series of changes the relations 

 of the pre-axial and post-axial borders of the limbs remain as before. 



(2) This stage consists in a rotation of the whole limb from the proximal end, though in an 

 exactly opposite direction in each case. The anterior extremity is rotated backward so that the 

 humerus lies parallel with the trunk; the elbow is directed toward the caudal end, the pre-axial 

 (radial) border becomes lateral, and the post-axial border medial. The ends of the digits point 

 backward. The posterior extremity undergoes a rotation forward to the same extent, so that 

 the femur is also nearly parallel with the trunk; the knee is directed toward the head end, the 

 pre-axial (tibial) border becomes medial, and the post-axial border lateral. The tibia and 

 fibula are parallel, the ends of the digits are directed forward, the great toe is on the pre-axial 

 and the little toe on the post-axial border of the limb, and in this position the posterior extremity 

 remains, the changes being finally completed by the extension of the Umb at the hip-joint as the 

 body attains its full development. 



(3) This stage affects the anterior extremity alone and consists in a rotation of the radius, 

 carrying the hand round the ulna so that the digits are brought round from the back to the front 

 of the limb, and in many animals the manus is thus placed permanently in the prone position. 

 But in man, in whom the capacity for pronation and supination is highly developed, the hand 

 can assume either position at wiU. In his case the final change is the extension which takes place 

 at the shoulder-joint with the assumption of the upright posture, the limb dropping loosely at 

 the side of the body, and being endowed with the greatest freedom of movement. 



Homological comparison of — 



I. The shoulder and pelvic girdles. — Primarily the lateral half of each girdle consists 

 of a curved bar or rod of cartilage placed at right angles to the longitudinal axis of the trunk and 

 divisible into a dorsal segment, and a ventral segment, the point of division corresponding 

 to the place of articulation with the limb-stalk — i. e., the glenoid and acetabular cavities. In 

 the fore-limb the dorsal segment is the scapula, and the ventral segment the coracoid, whilst 

 in the hind-Umb the dorsal segment is the ilium and the ventral segment the ischium and 

 pubis. 



The dorsal segments of the two girdles — i. e., scapula and ilium — are accordingly regarded 

 as homologous bones, the chief difference being that whereas the scapula is free from articulation 

 with the vertebral column, the ilium is firmly jointed to the rib elements (lateral mass) of the 

 sacrum. But the correspondence is not quite so clear with regard to the ventral segments. In 

 the primitive condition the coracoid articulates with the side of the sternum, an arrangement 

 which persists throughout Hfe in certain animals, such as reptiles and Ornithorhynchus. But in 

 all the higher mammals it undergoes reduction, withdrawing from the side of the sternum, and 

 eventually forming a more or less rudimentary process attached to the scapula. In the more 

 generalised form of shoulder girdle the ventral bar is double, consisting of coracoid and pre- 

 coracoid elements, the latter being situated in front and almost parallel with the coracoid. The 

 pre-coracoid in mammals is largely replaced by the development over it of the clavicle, a dermal 

 or membranous spUnt-bone which eventually invades the underlying cartilage. Parts, however, 

 remain distinct and form the sternal epiphysis of the clavicle, the inter-articular cartilage 

 between it and the sternum, the supra-sternal bones, and the inconstant inter-articular cartilage 

 in the acromio-clavicular joint. 



It has already been noticed that in the hip girdle the ventral segment also consists of two 

 elements, the pubis and ischium. Both take part in the formation of the acetabular cavity, and 

 the pubis meets in the ventral median line the corresponding segment of the opposite side. 



It is generally agreed that the coracoid and ischium are homologous structures. The pubic 

 portion of the ventral segment appears to correspond most closely with the pre-coracoid element 

 of reptiles, so that there is no true homologue of the clavicle in the pelvis. If, however, the 

 clavicle corresponds to the reptilian pre-coracoid, as believed by many anatomists, it then be- 

 comes the representative of the pubis. 



From a consideration of the condition in cranio-cleido-dysostosis, Mr. Fitzwilliams has 

 put forward the following views regarding the homology of the shoulder girdle: — Coracoid bar 

 is represented by (a) medial two-thirds of clavicle; (b) coraco-clavicular liganients; and (c) 

 sub-coracoid centre of coracoid process. The clavicula, a membranous bone, is represented 

 by the lateral third of adult clavicle. The pre-coracoid bar is represented by; — (a) the coracoid 

 process (less the sub-coracoid centre) ; and (b) the costo-coracoid ligament. The epi-coracoid 

 is represented by the meniscus of the sterno-clavicular joint. 



Moreover, it is possible to establish a comparison between the individual parts of the ilium 

 and scapula. A reference to fig. 253 shows that both the scapula and ilium may be resolved into 

 three-sided prismatic rods, each of which has three surfaces and three borders. In the primitive 

 position of the limb one surface — the internal — is turned toward the vertebral column, the 

 remaining surfaces are external, and named pre-axial and post-axial, corresponding to the borders 

 of the limb. The borders separating the internal from the external surfaces are antero-internal 

 (terminating in the acromion or pubis) and poster o-internal (terminating in the coracoid or 

 ischium). The two external surfaces are separated by a ridge, terminating below at the upper 

 margin of the glenoid cavity or acetabulum (glenoid and cotyloid borders) . 



The primitive arrangement is lost by the marked growth of the borders of the rods leading 

 to the formation of fossae and by the rotation of each rod, the scapula laterally and the ihum 

 medially, in association with the rotation which takes place in the free part of the limb, so that 



