Tonus is the steady and indefatigable contraction of the muscles required to hold the different parts of the skeleton in their proper relations in the various and constantly changing attitudes and postures of the body. It has been defined by Sherrington1 as postural contraction. A muscle, when functioning for the maintenance of posture, tends to take and hold a given length; and when that length is altered, either by active contraction or by the passive movement of the limb, the muscle again takes and holds the new length. This is particularly evident in decerebrate rigidity. The "lengthening and shortening reactions," so well seen in decerebrate cats, have been described in numerous papers by Sherrington2 and Brown.3 These reactions make the limbs of decerebrate animals moldable, so that they will take and hold any degree of flexion or extension passively imposed on them. To use Sherrington's term, the
RANSON SW. THE RÔLE OF THE DORSAL ROOTS IN MUSCLE TONUS. Arch NeurPsych. 1928;19(2):201–241. doi:10.1001/archneurpsyc.1928.02210080003001
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