296 



OSTEOLOGY. 



With regard to the homologue of the pubic element in the pectoral girdle, there is much difference 

 of opinion ; in reptiles and amphibia it corresponds most closely- to the precoracoid, but it is 

 doubtful what represents it in mammals. According to Goette and Hoffman, the clavicle is a 

 primordial bone, and not, as suggested by Gegenbaur, of secondary or dermic origin. If this be 

 so, it corresponds to the ventral anterior segment of the pectoral girdle, and is therefore homo- 

 logous with the ventral anterior (pubic) segment of the pelvic girdle. On the other hand, if 

 Gegenbaur's view be accepted, the clavicle has no representative in the pelvic girdle. It must, 

 however, be borne in mind that during its ossification it is intimately associated with cartilage, 

 and that that cartilage may represent the precoracoid bar ; nor must too great stress be laid upon 

 the fact that the clavicle begins to ossify before it is preformed in cartilage, since that may be 

 merely a modification in its histogenetic development. 



According to another view (Sabatier), the subcoracoid centre (see Ossification of Scapula) is 

 derived from the posterior ventral segment, and corresponds to the ischium, whilst the coracoid 

 process is the remains of the anterior ventral segment (precoracoid), and is homodynamous with 

 the pubis. 



In no part of the skeleton does function react so much on structure as in the arrangement of 

 the constituent parts of the pectoral or pelvic girdles. In man, owing to the assumption of the 

 erect position and the bipedal mode of progression, the pelvic girdle acquires those characteristics 

 which are essentially human, viz., its great relative breadth and the expansion of its iliac 

 portions, which serve as a support to the abdominal viscera, and also furnish an extensive origin 

 for the powerful muscles which control the movements of the hip-joint. The stability of the 



/Vertebral or internal surfaces 



/'I 



' 



/' ! 



Vertebral or internal surface/ 



A C 



FlG. 285. DIAGRAM TO ILLUSTRATE THE HOMOLOGOUS PARTS OF THK SCAPULA AND ILIUM 



ACCORDING TO FLOWER. 



A, ideal type ; three-sided rod. B, scapula rotated forward through quarter of a circle (90), so that the 

 primitive medial or vertebral surface is now directed anteriorly. C, ilium rotated backwards through 

 quarter of a circle so that the primitive medial surface is now turned posteriorly. In the diagram the 

 primitive medial or vertebral surface of each figure is coloured black, the pre -axial surfaces red, and the 

 post-axial surfaces blue. 



pelvic girdle is insured by the nature of its union with the axial skeleton, as well as by the 

 osseous fusion of its several parts, and their union in front at the symphysis pubis. 



Various attempts have been made to homologise the several parts of the ilium and scapula. 

 All are open to objection ; that by Flower is perhaps the most generally accepted. Assuming 

 that the primitive type is represented by a prismatic rod, of which the dorsal end represents 

 either the epiphysial border of the vertebral edge of the scapula or the iliac crest, whilst the 

 ventral end corresponds to the glenoid or acetabular articular areas respectively, the surfaces of 

 the three-sided rod are disposed so that one is vertebral or medial, another pre-axial, and the 

 third post-axial. These surfaces are separated by borders, of which one is lateral, separating 

 the pre-axial and post-axial surfaces, whilst the antero - medial and postero - medial margins 

 separate the pre-axial and post-axial surfaces respectively from the vertebral or medial aspect. 

 It is a necessity of Flower's theory that this part of the girdle undergoes a rotation along with 

 the rest of the limb. Thus in the fore limb the surfaces of the primitive type are turned so that 

 the vertebral surface looks forward, whilst in the case of the hind limb the vertebral surface is 

 turned backward. A study of the accompanying diagram will enable the reader to realise how 

 the ventral surface of the scapula is thus rendered homologous with the gluteal surface of the 

 ilium, for by reference to the type, both these surfaces will be seen to correspond to the post- 

 axial areas of the primitive condition. In accordance with this view the surfaces and borders of 

 the scapula are homologised by Flower, as shewn in the subjoined table : 



Scapula. 



Supra-spinous fossa 



Infra-spinous fossa 

 Subscapular fossa 



SURFACES 

 Ideal. 



1. Vertebral 



2. Pre-axial 



3. Post -axial 



Pelvis. 



Medial surface of ilium behind linea ar- 

 cuata interna, including the articular 

 surface for the sacrum and the portion 

 of the bone above and below this 



Iliac fossa 



Gluteal surface of ilium 



