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Epiphyseal centres of ossification in the bones forming the elbow joints of pigs between one day and 15 weeks of age were examined radiographically, macroscopically, mesoscopically and microscopically. Thoracic limbs from 39 pigs were perfused with India ink or silicone rubber injection compound and the bones were dissected free of soft tissues. The humerus, ulna and radius were fixed in formalin or ethyl alcohol and then cleared by the modified Spalteholz technique. Bones were radiographed, examined grossly, and then cut into slabs for mesoscopical evaluation. Foci considered to be calcifying within cartilaginous anlage were selected for microscopical examination. It was concluded that the epiphyseal centre of ossification develops at different times in different sites in the bones forming the elbow joint. Centres of ossification are initiated when foci of chondrocytes adjacent to one side of a cartilage canal undergo hypertrophy and the inter-territorial matrix becomes calcified. Osteogenesis then proceeds in the calcified focus, presumably with osteoprogenitor cells that originate within the cartilage canals. Subsequently, each epiphyseal centre of ossification enlarges by one of two methods. Firstly, the layer of cartilage adjacent to the centre undergoes endochondral ossification, thus allowing for the circumferential growth of the epiphyseal centre of ossification. Secondly, foci of calcification develop adjacent to the ends of cartilage canals near the epiphyseal centre of ossification and eventually the focus of calcification coalesces with the developing epiphyseal centre of ossification, thus establishing a new ossification front. Endochondral ossification continues at the periphery of the mass of bone. Mesoscopical examination is more useful than radiographical evaluation for identifying small foci of calcification which precede epiphyseal centres of ossification.

The vasculature of the elbow joint was examined in 39 pigs between one day and 15 weeks of age. Each pig was anaesthetised, exsanguinated and the thoracic limbs were perfused with India ink or a silicone rubber injection compound. The humerus, ulna and radius were dissected free, examined, fixed in formalin or ethyl alcohol, cleared by the modified Spalteholz technique and examined mesoscopically. Features of interest were photographed and then a limb from two pigs in each age group was cut into slabs and examined mesoscopically. The vascular supply of the distal part of the humerus was complex. It was supplied by vessels on both the cranial and caudal aspects and locally each aspect had a dual blood supply. Vessels anastomosed and on the cranial aspect formed a vascular ring. The proximal part of the ulna was supplied by vessels that were on its medial and lateral surfaces. The vessel on the lateral surface continued distally and supplied the lateral aspect of the proximal part of the radius. The proximal part of the radius was also supplied by arteries that were on the cranial and medial surfaces. Blood vessels provided branches to numerous cartilage canals of the articular-epiphyseal cartilage complexes, epiphyseal centres of ossification, and growth plates. The patterns of blood vessels in cartilage canals which were in sagittal or transverse planes were best exemplified by those in the distal part of the humerus. Perforating cartilage canals emerged from the epiphyseal centres of ossification. The pattern of cartilage canals was consistent in a general configuration, but individual variation did occur. Although cartilage canals were abundant in the youngest pigs, with increasing age the distribution of cartilage canals changed and the numbers of cartilage canals decreased.