Applied anatomy and kinesiology, the mechanism of muscular movement . and the transverse processes ofthe upper cervical vertebrae. Structure.—For surgical purposes the splenius includes two dis-tinct muscles, but the division is unnecessary here. Like all themuscles acting on the vertebrae at the back it has a series of originsand insertions through its entire length, the fibers from a certainorigin being inserted four to eight vertebrae above, so as to act onall the joints of the spinal column within its range. Action.—The pull of a single strand of the splenius is mainlydownward but slightly
Applied anatomy and kinesiology, the mechanism of muscular movement . and the transverse processes ofthe upper cervical vertebrae. Structure.—For surgical purposes the splenius includes two dis-tinct muscles, but the division is unnecessary here. Like all themuscles acting on the vertebrae at the back it has a series of originsand insertions through its entire length, the fibers from a certainorigin being inserted four to eight vertebrae above, so as to act onall the joints of the spinal column within its range. Action.—The pull of a single strand of the splenius is mainlydownward but slightly backward and toward the median line, sothat when one side acts alone it will rotate the upper both muscles of the pair act together the rotary effects neu-tralize each other, giving pure extension of the head and splenius is especially important for maintaining erect positionof the head and neck. When it is weak or elongated the head andneck droop forward, causing the worst feature of round shoulders. 214 MOVEMENTS OF THE SPINAL COLUMN. Fig. 120.—The splenius and the serratus superior. (Gerrish.) ERECTOR SPINJE. A very large and thick mass of vertically directed fibers that lieon each side of the median line through the whole extent of theback (Fig. 127). Origin.—The posterior one-fifth of the crest of the ilium, theback of the sacrum, the spinous processes of the lumbar and thelast three thoracic vertebra?, and the transverse processes of all thethoracic vertebra?. Insertion.—The processes of the vertebra?, the angles of the ribs,and the base of the skull. Structure.—Beginning as a thick muscle arising directly from thepelvis, the erector spina? has joining it as it passes upward fibersarising from the processes of the vertebra?; as it reaches the levelof the last rib it divides into three parts. The inner part passes ERECTOR SPIN& 215 up close to the median line of the trunk, having a continuous seriesof origins and insertions from the sac
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