712 A MANUAL OF PHYSIOLOGY 



latent period of muscular contraction, and the remainder is 

 the time spent in the centre, or the true reflex time. When 

 the conjunctiva or eyelid is stimulated on one side both 

 eyelids blink. This is a typical reflex action reduced to its 

 simplest expression, and the true reflex time is correspond- 

 ingly short only about $ second. An additional y-^ second 

 is consumed in the passage of the afferent impulse along the 

 fifth nerve to the medulla oblongata, of the efferent impulse 

 from the medulla to the orbicularis palpebrarum along the 

 seventh nerve, and in the latent period of the muscle. When 

 a naked nerve, like the sciatic, is stimulated, the true reflex 

 time is reduced to T ^j- to -V second. As estimated by 

 Tiirck's method (p. 768), the uncorrected reflex time is 

 greatly lengthened, it may be to several, or even many, 

 seconds. For here it is evident that the time t; ken by the 

 acid to soak through the skin and reach the nerve-endings 

 in strength sufficient to stimulate them is included. But 

 even when the peripheral factors remain constant, the 

 central factor may vary. With strong stimulation, e.g., the 

 reflex time is shorter than with weak stimulation. Fatigue 

 of the nerve centres delays the passage of impulses through 

 them ; and strychnia, while it increases the excitability of 

 the cord, also lengthens the reflex time. 



3. The Origination of Impulses in the Spinal Cord. 



Automatism. A physiological action is termed automatic 

 when it depends upon a nervous outburst which seems to 

 be spontaneous, in the sense that it is not brought about by 

 any evident reflex mechanism, or, in other words, is not 

 discharged by afferent impulses falling into the centre where 

 it arises. An action known to be caused or conditioned by 

 such afferent impulses is called a reflex action. Automatic 

 actions being thus defined in a negative manner by the 

 defect of a quality, there is always a possibility that some 

 day or other it may be demonstrated that any given action 

 which at present seems automatic in its origin depends on 

 afferent impulses hitherto unnoticed. As a matter of fact, 

 the supposed proofs of spinal automatism have in more than 

 one case vanished with the advance of knowledge, and as 



