POSTURAL COORDINATION 921 



bellum which are concerned with the postural tone of each of these parts. 

 (Black. 30 ) 



Other Clinical Tests of the Proprioceptive Reflex Mechanism. Certain 

 phenomena may be considered at this point which form the basis for 

 practical tests of the integrity of the mechanism by which posture and 

 tone is maintained. The posture of the body is adapted to the position 

 which it occupies in space, i. e., equilibrium is maintained, not only by 

 virtue of afferent impulses arising from the semicircular canals, but 

 as the result of visual sensations and of information supplied to the 

 central nervous system by the receptors of deep sensibility in the limbs. 

 Loss of function in any of these three groups of sense organs may be com- 

 pensated by the other two. As a result sufferers from a destruction of 

 the deep sensibility of the legs have little difficulty in maintaining an 

 upright position, so long as they are aided by the use of their eyes. 

 In the dark, however, their balance is kept with great difficulty. It is 

 consequently possible to test the integrity of the afferent paths from 

 the limbs involved in the maintenance of equilibrium by noting the 

 ability of the subject to stand with the feet close together and the 

 eyes shut. Under these circumstances a normal person will stand quite 

 steadily, but a sufferer from locomotor ataxia, in whom the afferent 

 neurons from the limbs are diseased, will sway violently and tend to 

 fall. This test is known as Romberg's sign. 



The tendon jerks are a group of reactions which result from tapping 

 the tendons of the muscles of the knee, ankle, elbow and wrist. The 

 contraction of the muscle which results is not solely one of postural tone, 

 but it depends for its elicitation on the integrity of the reflex arc between 

 the proprioceptors of the tendon and the muscle. It may consequently 

 be used as a test for the condition of the afferent neurons from the deep 

 structures in the limbs and for the condition of the lower motor neurons 

 to the muscles, i. e., for the integrity of reflex arcs quite similar to 

 those involved in the maintenance and adjustment of tone. When a 

 lesion affects either the afferent neuron, as in locomotor ataxia, or the 

 lower motor neuron, as in anterior poliomyelitis, the tendon jerks and 

 tone alike are wanting. The character of the tendon jerks is also pro- 

 foundly modified by conditions in the central nervous system which affect 

 the tone of the muscles. In the normal individual when the patellar ten- 

 don is tapped, the response of the extensor muscles is prolonged by a 

 tonic contraction which gives way slowly as the flexors draw the leg back 

 into its original position. The tone of the antagonistic muscles checks 

 the limb 011 its return to this position, with the result that there is little 

 tendency for the leg to bounce up and down after the response is over. 

 When, however, the tone of the muscles is reduced as the result of in- 

 jury to remote parts of the central nervous system, as in cerebellar le- 



