602 THE FUNCTION OF THE SPINAL CORD 



during these periods. Conditions of mental excitement and general 

 neurasthenia, on the other hand, increase them very markedly. 



While one or the other of the reflexes enumerated previously may 

 be absent in a perfectly healthy person, their general abolition sug- 

 gests in most cases a pathological lesion of some kind. This defect 

 may be restricted to a particular reflex circuit or may involve more 

 extensive areas of the nervous system. In the first instance, the break 

 must have occurred at some point of the reflex arc which now fails to 

 respond even on reinforcement, while, in the second instance, a more 

 general or central depression of the nervous system must have re- 

 sulted. In illustration of the first condition might be mentioned the 

 loss of a particular superficial or deep reflex of the spinal cord in con- 

 sequence of acute anterior poliomyelitis which infection destroys 

 the motor cells in the anterior horn of the gray matter. Reference 

 might -also be made to tabes dorsalis in which affection the posterior 

 root terminals in the cord are destroyed, thereby causing a break in the 

 central distribution of the analyzer. Among the general depressions 

 of the nervous system producing diminution or abolition of the spinal 

 and other reflexes, might be mentioned increases in intracranial pres- 

 sure, such as result in hydrocephalus or in consequence of cerebral 

 tumors. They are also abolished for a time in comas and epileptic 

 seizures and certain febrile reactions, such as pneumonia. 



Reflexes are said to be exaggerated when the slightest possible 

 stimulus elicits an unusually brisk motor response. This is a common 

 phenomenon in simple neurasthenia and hysteria and other conditions 

 in which the irritability of the nervous system has been increased in 

 consequence of the absorption of various poisons, such as the products 

 of intestinal fermentation, strychnin, caffein, thebein, and others. In 

 many cases the reflexes are then augmented into clonic contractions 

 which are maintained until the tension upon the tendon is again re- 

 leased. Clearly, therefore, the presence of a true clonus^ implies that 

 the reflex arcs are in a state of hyperirritability. In this connection, 

 brief reference should also be made to the fact that the exaggera- 

 tion of the spinal reflexes constitutes a cardinal sign in chronic affec- 

 tions involving the motor neurons of the cerebrum. In general, it 

 may be said that a ''high'' (cerebral) lesion leads to an exaggeration 

 and a ''low" (spinal) lesion to an abolition of the reflexes. It cannot 

 surprise us, therefore, that an affection of the motor areas and pyramidal 

 tracts is generally associated with clonic contractions. As typical 

 examples of this condition might be mentioned hemiplegia from organic 

 brain disease, or paraplegia due to myelitis. Incomplete transections 

 of the cord, as often result in fractures of the spine, produce exagger- 

 ated reflexes, while complete transections are usually followed by a 

 loss of the deep reflexes. 



These differences may be explained in the same way as the phe- 



^ Spurious clonic reflexes are obtained at times in hysterical conditions. They 

 are usually irregular and poorly sustained. 



