Changes in Length and Position of the Segments of the Spinal Cord with Changes in Posture in the Monkey

There are reasons for believing that the spinal cord, in spite of its sheltered position within the vertebral column, is subject to stresses that change with posture. The changes in posture may involve the trunk or the limbs. In the trunk, flexion must necessarily stretch the cord, since the vertebral canal increases in length. Of postural changes in the limbs, those that put traction on the nerves may be expected to stretch the cord. This is suggested by the observation of Inman and Saunders (1) that the trunks of the lower lumbar nerves, located in the intervertebral foramina, move distally when the hip is flexed and the knee is kept in extension. The present study was undertaken to measure the changes in length and position of the individual spinal cord segments (a) when the trunk is flexed and (b) when the extremities are moved into positions which put traction on the peripheral nerves. Material and Method Normal rhesus monkeys (Macaca mulatta) were killed by anesthetization and evisceration. The left leg and arm, and the left portion of the head were removed by sawing through the body in a sagittal plane close to the left side of the vertebral column. The saw passed in turn through the left hip joint, then close to the left side of the vertebral column, and in the same sagittal plane through the head. The postvertebral muscles were removed from the left side of the vertebral column; the right side of the back was left intact. The spinal cord was exposed by removing the vertebral laminae on the left side and incising the dura longitudinally. The spinous processes were left in place. The lateral aspect of the brain stem was exposed by removing the left margin of the foramen magnum, the left half of the cerebellum, and the left cerebral hemisphere. Pins 1/8 inch long, of fine iron wire, were inserted into the cord along the line of attachment of the dorsal roots, each pin being placed where the lowest rootlet of one nerve met the highest rootlet of the nerve just caudal to it. Because the dorsal rootlets of the first cervical nerve were either absent or rudimentary, the interval between the first and second cervical segments was marked by a pin at the level of the highest rootlet of the second cervical nerve. Three pins were placed in the brain stem in line with those in the cord, one at the level of the obex, the second at the junction of the medulla oblongata and pons, and the third at the junction of the pons and the midbrain. Similar pins were inserted at selected intervals in the large nerves of the lower limb, to lie lengthwise in the central long axis of the nerve. Nails inserted in the bodies of the vertebrae and in the bones of the lower limb served as reference points in locating pins placed in the central nervous system and in the nerves of the lower limb. The change in inclination of the nails in adjacent vertebrae was measured to obtain the range of movement at each of the intervertebral joints.