SCIENCE 



[N. S. Vol. XXVII. No. 701 



raphy, but with the general facts of the inter- 

 relation of nerve paths. 



The unit of coordination is the simple re- 

 flex, though this is itself an artificially sim- 

 plified unit, " because all parts of the nervous 

 system are connected together and no part of 

 it is probably ever capable of reaction without 

 affecting and being affected by various other 

 parts." Nor does there exist, normally, such 

 a thing as a resting condition, but every reflex 

 supervenes upon a previous condition of reflex 

 activity, and the modifications which it pro- 

 duces in that condition, by reinforcement and 

 inhibition, are part and parcel of itself. The 

 only adequate picture is that of a total reflex 

 " pattern," which may analytically, though 

 somewhat artificially, be considered as made 

 up of a combination of simple reflexes, and 

 which, in response to a stimulus, gives way to 

 a new reflex pattern. 



The simple reflex results from the joint ac- 

 tion of three organs, the receptor or sense 

 organ, the conductor, and the effector organ, 

 which last is usually composed of muscular 

 or glandular tissue. Regarding the receptor, 

 it may be said that its function is to lower 

 the threshold for a particular sort of stimulus 

 — mechanical, chemical, photic — while simul- 

 taneously raising the threshold for other 

 stimuli, so as to make possible different re- 

 actions to different classes of stimuli. The 

 " pain " end organs form an exception to this 

 rule, in that they are not specially adapted to 

 any one physical stimulus, but respond to a 

 stimulus of any sort which threatens injury 

 to the part where the pain organ is located. 

 In regard to the effector organ, it may be noted 

 that iii is not usually a single " muscle " in 

 the anatomical sense, but may be either more 

 or less than that. 



It is, however, the conductor organ, espe- 

 cially that part of it that lies in the nerve 

 centers, which gives to reflex action most of 

 its variety and peculiarity. The character- 

 istics of reflex conduction are brought out by 

 contrasting it with the simpler conduction 

 that is observed in nerve trunks. No less than 

 eleven points of difference are detailed by the 

 author. Some of these, such as the irreversi- 



bility of reflex conduction, as contrasted with 

 the reversibility of conduction in nerve trunks, 

 have long been recognized. The slowness of 

 reflex conduction has also often been empha- 

 sized, but the author shows that the important 

 difl^erence here is rather the great dependence 

 of speed in reflex conduction on the intensity 

 of the stimulus. When the stimulus is strong, 

 the conduction through the centers shows no 

 special slowness, but when the stimulus is 

 weak the reflex may be very much delayed. 

 Again, in contrast with nerve trunlvs, which 

 cease their activity promptly on the cessation 

 of the stimulus, the reflex shows " after-dis- 

 charge," which may be very prolonged when 

 the stimulus is intense. The intensity of the 

 reflex discharge is less closely dependent on 

 the intensity of the stimulus than in the case 

 of the nerve trunk or the muscle. The in- 

 tensity of reaction of the nerve trunk is a con- 

 tinuous, and almost mathematical function of 

 the intensity of the stimulus ; but while some- 

 thing much like this may be said of some re- 

 flexes, in others the reaction remains prac- 

 tically constant for all intensities of the stim- 

 ulus which are strong enough to elicit the 

 reflex at all, and in still others the intensity of 

 reaction remains constant for a considerable 

 range of stimulus, only to make a sudden 

 jump at a certain critical intensity. The 

 threshold value for an effective stimulus is 

 also much more variable in the reflex, since 

 the internal condition of the reflex arc, as 

 dependent on other simultaneous or preceding 

 stimuli, is much more variable than that of 

 the nerve trunk. Summation of subliminal 

 stimuli is in the reflex arc much more likely 

 than in the nerve trunk to produce a response ; 

 some reflexes can scarcely be elicited by a 

 single momentary stimulus. When the stimu- 

 lus is repeated, the rhythm of response in the 

 nerve trunk follows closely that of the 

 stimulus; whereas reflexes are apt to have a 

 rhythm of their own, which is very slightly 

 if at all controlled by that of the stimulus. 

 The rhythm of different reflexes differs; in 

 some it is as high as 13 per second; while in 

 the dog's scratching reflex it is from 5 to 6, 

 and in the " crossed stepping reflex " as low as 



