mm. A nutrient vessel crosses the cortex, through the foramen hole, to supply the bone with necessary nutrients.

       At a macroscopic level, cortical or cancellous bone appears solid. Cancellous

bone is highly porous and appears at the ends of long bones. Cortical or osteonal bone, is found in the diaphyses (middle region) of long bones (like the MC3), and it is of concern in this study. The porosity of cortical bone is usually between 5% and 10%; its pores consist of canals (Haversian and Volkmann's canals) or temporary cavities (resorption spaces). Cortical bone is composed of lamellar layers, either in a parallel order (plexiform bone) or in a circumferential arrangement (osteonal bone). Osteonal bone is composed of osteons, which are concentric layers of lamellar wrap around a central canal (Haversian canal). In addition, osteons are cylindrical in shape, about 200  .im in diameter and usually oriented parallel to the longitudinal axis of long bones. Osteons are surrounded by a weak interfacial layer, called the cement line, which arrests and deflects cracks (physical form of damage). Cortical bone undergoes remodeling (removal and replacement of bone), in part, to repair cracks.

                   Loading Environment and Fracture Incidence The MC3 is one of the most common sites for stress fractures in racing horses, particularly, in young thoroughbreds [38,34,35]. Peak periosteal strains, in other animal species can reach peaks strains of 2,000 [ to 3,000 [ [34]; however, peak compressive strains of nearly 5,000 [g have been measured on dorsal surfaces of the MC3 during galloping [38]. Shear strains approximately measuring 44% less in magnitude were measured during trotting, indicating a torsional loading component. Still, the equine third metacarpus is primarily axially loaded in compression due to gravity and in vivo tests using strain gages suggest dorsopalmar bending [34]