. Applied anatomy and kinesiology. chanics to the effect that if a flexible rod is bent firstin one plane and then in another it always rotates on its longi-tudinal axis at the same time. To see why this is true think of theconditions existing in the case shown above. When the subjectbends forward, giving a condition always present in the thoracicregion, it puts a tension on the ligaments at the rear (subfla^a andinterspinous) that makes them resist lateral flexion nu)re than 192 MOVEMENTS OF THE SPINAL COLUMN usual, while the weight, bearing down on the front edges of thebodies, aids in the l
. Applied anatomy and kinesiology. chanics to the effect that if a flexible rod is bent firstin one plane and then in another it always rotates on its longi-tudinal axis at the same time. To see why this is true think of theconditions existing in the case shown above. When the subjectbends forward, giving a condition always present in the thoracicregion, it puts a tension on the ligaments at the rear (subfla^a andinterspinous) that makes them resist lateral flexion nu)re than 192 MOVEMENTS OF THE SPINAL COLUMN usual, while the weight, bearing down on the front edges of thebodies, aids in the lateral bending. The result is that the bodiesof the vertebrae go farther away from the vertical than the pro-cesses during lateral flexion, and this is the rotation shown bythe pointers. The general principle, which is self-evident andwhich helps one to remember in which direction the rotation willbe, is that the concave side of the normal curve, being under press-ure, turns to the convex side of the lateral curve. It follows that. Fig. 115.—The rotation of the vertebra that accompanies lateral flexion of thetrunk. The pointers attached to the back show the direction of the spinous pro-cesses. (Lovett.) in the thoracic region a lateral bend rotates the spinous processesto the same side and in the lumbar region to the opposite side. The principal muscles flexing the spine are the psoas, rectusabdominis, and external and internal oblique. These muscles,excepting the psoas, which has been previously described, are inthe front and side walls of the abdomen and, along with transver-salis, which lies beneath them are commonly called the abdominalmuscles. RECTUS ABDOMINIS 193 RECTUS ABDOMINIS. A rather slender muscle extending vertically across the front ofthe abdominal wall. The right and left rectus are separated by atendinous strip about an inch wide called the linea alba (white line). Origin.—The crest of the pubes. Insertion.—The cartilages of the 5th, 6th, and 7th ribs. St
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