In anatomical textbooks the divisions of the muscular system invariably are presented from a topographical viewpoint. This is an artificial arrangement and is confusing to the student. For a thorough understanding of myology a knowledge of the morphology is requisite, and the morphological presentation is the one most readily assimilated. For a number of years I have been following this arrangement in my published papers (Howell, '26, '32), but with certain concessions to topography. The present contribution is based purely on phylogeny, and is offered, chiefly with reference to conditions in man, in the belief that it will prove helpful to myologists. For the sake of brevity, points on which there are diverse opinions are not discussed, and hence the contribution is a presentation of my own belicfs. For this reason the tone of the paper necessarily is somewhat dogmatic.
Effective muscular control requires a unit for prime movement and another, having an antagonistic action, for recovery. These units are arranged in dorsal and ventral positions, and this arrangement is primary for axial muscles. Either division, however, is capable of giving rise to muscle slips which evolve for the control of such things as the eyes, gills, and appendages, and the controlling musculature eventually assumes a dorsal-ventral pattern, thus simulating, but secondarily, the primary divisions of the axial musculature ; and to these, different phylogenetic weight must be accorded. For the purposes of proper classification, a number of muscular subgroups must be recognized, but the number of 295