254 PHYSIOLOGY FOR DENTAL STUDENT.-. 



imagined that this relaxation is merely a passive act. that is to 

 say, that the uncontracting group of muscles do nothing more 

 than remain quiescent and permit themselves to be stretched. 

 But such is not the case; on the contrary, they become actively 

 extended. This they are enabled to do because of the fact that, 

 even when apparently relaxed, a muscle is really not so, but 

 exists in a condition called tone, that is, in a slightly contracted 

 state. This tone becomes greatly diminished during sleep, and it 

 can be caused almost to disappear by deep anesthesia. It is for 

 this purpose, as well as to abolish pain, that anesthetics are 

 administered before attempting to reduce a dislocation. 



Tone is maintained by the nerve cells of the anterior horn oi 

 the spinal cord. When therefore an afferent impulse brings 

 about flexion at the knee joint, it does so by exercising two 

 diametrically opposite influences on the anterior horn cells: it 

 stimulates those which preside over the flexor muscles and de- 

 presses the tonic influence of those supplying the extensors. 

 This tone-depressing action recalls the inhibitory influence which 

 the vagus nerve exercises over the heart beat (see p. 185), and 

 since it always 'occurs along with a contraction of antagonistic 

 muscles it is called reciprocal inhibition. Certain poisons, par- 

 ticularly strychnine and tetanus toxin, cause this reciprocal 

 action to break down so that all the muscles around a joint con- 

 tract at the same time and produce an extension. Tetanus 

 toxin is the poison produced in the blood by the tetanus bacillus, 

 and its interference with the reciprocal inhibition of the muscles 

 of the lower jaw causes lockjaw. 



Symptoms Due to Lesions Affecting the Reflexes. From 

 what we have learned regarding the functions of the spinal 

 cord, it is easy for us to explain the following symptoms and 

 conditions resulting from pathological destruction or stimula- 

 tion of various parts of it : 



1. In destruction of the continuity of the afferent or efferent 

 fibers of the reflex arc, the reflexes are absent. This occurs in 

 chronic inflammation of the nerves (neuritis) and in the disease 

 called locomotor ataxia, in which the lesion consists of a de- 

 structive pathological process involving the posterior columns 





