THE SPINAL CORD AS A REFLEX CENTER 601 



if the subject is asked to engage in some mental process while the 

 stimulus is brought to bear upon his integument or tendons. The 

 reflexes may also be augmented by asking the subject to make a voh- 

 tional muscular effort at the time the blow is struck. This requires a 

 certain mental concept and it is conceivable that the activation of the 

 cerebrum temporarily abohshes its inhibitory power, and thus dimin- 

 ishes the resistance along the different reflex circuits. Under ordinary 

 conditions, the patellar reflex may be heightened very materially by 

 simultaneously contracting bhe muscles of the hands or by endeavoring 

 to pull the interlocked fingers apart. But, while we are able in this 

 way to intensify a feeble j erk, no effect can be produced after the reflex 

 has been aboUshed by disease. 



This phenomenon which is usually described as reinforcement of 

 reflexes,^ also permits of a second explanation. It is commonly 

 recognized that the functional activity of one part of the nervous 

 system also influences the irritability of others. Thus, it may rightly be 

 assumed that the activation of the cerebrum, accompanying such 

 actions as the interlocking of the hands or fingers, renders this organ 

 more irritable. The motor impulses thus generated in its cortical 

 area, escape through the descending columns of the cord, where they 

 skip to neighboring columns and nuclei and give rise to a general 

 activity of these nervous elements. In other words, the constituents 

 of the spinal reflex circuits are sensitized by an overflow of the cerebral 

 impulses. It is quite impossible at this time to decide definitely 

 which of these two theories is the more correct. Obviously, the first 

 more closely agrees with the common phenomenon of inhibition of 

 reflexes by the cerebral centers, while, the second introduces a rather 

 new factor in the form of an activation of certain parts of the nervous 

 system which he at some distance from the seat of the primary process. 



It should also be noted that the reinforcement does not develop 

 if the interval of time between the simultaneous effort and the excita- 

 tion is too long. Thus, it has been shown by Bowditch and Warren ^ 

 that the knee-jerk suffers its greatest augmentation if the blow upon 

 the tendon precedes the reinforcing action by less than 0.6 to 0.9 

 sec. A greater interval will tend to minimize the reinforcement until 

 it eventually gives way to an inhibition. This diminution of the reflex 

 in consequence of a premature simultaneous eft^ort is designated as 

 negative reinforcement. 



Abolition and Exaggeration of the Reflexes. — With few exceptions, 

 reflexes may be regarded as a safe index of the relative state of irrita- 

 bility of the nervous system, provided, of course, that the method of 

 stimulation is free from error. But even a perfectly normal body 

 undergoes diurnal and seasonal changes which reflect their influences 

 upon reflexes. Thus, we find that they are weakened during sleep and 

 other states of mental rest ; in fact, some of them are abolished entirely 



^ Mitchell and Lewis, Am. Jour, of the Med. Sciences, xlii, 1886, 363. 

 2 Jour, of Physiol., ii, 1890, 25. 



