586 SIGNIFICANCE OF THE NERVOUS SYSTEM 



is also evident that the repeated elicitation of a certain reflex is very 

 prone to lengthen the reflex time and to lessen its conspicuousness, 

 because the structures participating in this reaction become fatigued. 

 We are thus forced to recognize the condition of *' reflex fatigue," and 

 to admit that reflexes also possess a definite '^ refractory period." 

 This implies that they cannot be elicited at shorter intervals than are 

 required for the anaboHc changes in the different elements of the reflex 

 circuit. 



In all these cases, the cell-body, rather than the conducting paths, 

 seems to be the deciding factor. It is to be noted, however, that the 

 reflex time includes several elements, namely, the time of conduction 

 of the impulse through the afferent and efferent paths, its passage 

 through the center and lastly, the latent period of the motor organ. 

 Helmholtz^ has shown that the transfer of the impulses through the 

 gray matter of the spinal cord consumes twelve times as long a time 

 as their passage through the peripheral conducting channels. The 

 total reflex time may thus be regarded as being composed of the 

 ''rough" and ''reduced" reflex phases. The former includes the 

 time elapsing between the moment of stimulation of the receptor 

 and the onset of the response, and the latter, the time consumed 

 by the processes occurring in the center, i.e., the total time less the 

 time of conduction over the afferent and efferent paths and the length 

 of the latent period of the motor organ. Exner,^ for example, states 

 that the closure of the eyelids upon stimulation of the cornea (corneal 

 reflex) occupies 0.0578-0.0662 second. As the conduction requires 

 in this case, 0.0107 second, the central transfer must consume 0.0471- 

 0.0555 second. Listing and Vintschgau^ estimate the time of the 

 reaction of the iris to varying intensities of light (light reflex) at 0.3-0.4 

 second. The reactions accomplished with the aid of smooth muscle, 

 are much slower, a fact which is in keeping with the lesser irritability of 

 this tissue as well as of the nervous elements innervating it. This 

 is especially true of the sympathetic system. 



Spreading or Crossing of Reflexes. — If the stimulus applied to the 

 foot of a reflex frog, is of slight intensity, the leg is withdrawn in a 

 gradual and easy manner, while if the stimulus is severe, the leg is 

 jerked up, and besides, the muscular contractions do not remain 

 confined to this limb, but spread to the muscles on the opposite side 

 and possibly also to those of the trunk and forelegs. This result indi- 

 cates that the primary afferent impulse has been transferred to other 

 reflex circuits, or better, that the primary reflex has led to an activa- 

 tion of those reflex circuits which are in functional relation with the one 

 involved flrst. In order to allow this spreading to take place, certain 

 intermediary neurons must be present, the purpose of which is to 



^ Prot. der Akad. d. Wissensch., Berlin, 1845; also Fano, Arch. ital. de biol., 

 xxxix, 1903, 85. 



2 Pfltiger's Archiv, viii, 1874, 526. 



3 Ibid., xxvi, 1881, 324. 



