Fibre Architecture of the Deltoid Muscles in the Japanese Macaque, with Special Reference to the Relationship with That of the Pectoral Muscles

Spatial fibre architecture of the deltoid muscles in five Japanese macaques was observed and functionally analysed to compare it with that of the adjoining pectoral muscles. The fibre bundles of the deltoids possess a twisted and radiating architecture, as do those in the pectorals, and are basically arranged symmetrically about the central fibre of the acromiodeltoid. Unlike those of the pectorals, the bundles of deltoids do not exhibit any crossing structure. These structural features of the deltoids are thought to have been formed through evolutionary modifications in the mammal-like reptiles and primitive mammals, which have disposed the humerus more perpendicularly while simultaneously preserving the deltoids as a single-joint muscle. The lateral shift in the cleidodeltoid and the accompanying clavicular origin of the pectoralis in brachiators and human are suggested to have been formed concomitantly with the flattening of the thorax as one of the evolutionary traits in primates.