9M THE CENTRAL NERVOUS SYSTEM 



Reflex Time. When a reflex movement is evoked, a measurable 

 period elapses between the application of the stimulus and the 

 commencement of the movement. This interval may be called the 

 uncorrected reflex time or the latent period of the reflex. A part of 

 the interval is taken up in the transmission of the afferent impulse 

 to the reflex centre, a part in the transmission of the efferent impulse 

 to the muscles, a part represents the latent period of muscular 

 contraction, and the remainder is the time spent in the centre, or 

 the true reflex time. Ordinarily this time, though absolutely short, 

 is relatively so great that the total latent period of a reflex is much 

 longer than when a similar length of nerve-trunk is interposed be- 

 tween the point of application of the stimulus and the muscle. 

 When the conjunctiva or eyelid is stimulated on one side both eye- 

 lids blink. This is a typical reflex action reduced to its simplest 

 expression, and the true reflex time is correspondingly short only 

 about J ff second (50 a*). An additional T ^y second (10 a-) is con- 

 sumed 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 y^ to -^ second. As 

 estimated by Tiirck's method (p. 996), the uncorrected reflex time 

 is greatly lengthened, it may be to several, or even many, seconds. 

 For here it is evident that the time taken 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 stimula- 

 tion. With weak stimuli the latent period of the flexion reflex in 

 the dog is usually 60 a- or 120 a-. It may even be as long as 200 a. 

 With strong stimuli it may be as little as 30 cr. Even 22 a has been 

 seen, which is little more than for nerve-trunk conduction. Fatigue 

 of the nerve-centres delays the passage of impulses through them ; 

 and strychnine, while it increases the excitability of the cord, also 

 lengthens the reflex time. 



Reflexes in Disease. In order that a reflex action may take place, 

 the reflex arc afferent nerve, central mechanism, and efferent nerve 

 must be complete; and, in fact, a whole series of simple reflex move- 

 ments exists, the suppression, diminution, or exaggeration of which 

 can be used in diagnosis as tests of the condition of the reflex arc. It 

 is customary to divide these into superficial reflexes, elicited from 

 receptive fields on the, surface of the body (extero-ceptive fields), and deep 

 reflexes, elicited from receptors in the depth of the organism (proprio- 

 ceptiue fields), especially in the muscles and the tendons and joints con- 

 nected with them. The extero-ceptive reflexes are normally excited 

 by extraneous stimuli acting on the surface from the environment. 

 The proprio-ceptive reflexes are normally excited by changes (muscular 



* <r=o - ooi second. 



