8io A MANUAL OF PHYSIOLOGY 



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 between 

 the point of application of the stimulus and the muscle. 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 correspondingly short 

 only about ^ second (50 a*). An additional TT j<y second (10 a) 

 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 

 r<y<j to -jfV second. As estimated by Tiirck's method (p. 886), 

 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 stimulation. With weak stimuli the 

 latent period of the flexion reflex in the dog is usually 60 a to 

 120 cr. It may even be as long as 200 a. With strong stimuli 

 it may be as little as 30 a . 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 movements exists, the suppression, diminution, or exaggera- 

 tion 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-ceptive fields), especially in the 

 muscles and the tendons and joints connected 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 contractions) 

 occurring in the body itself, which changes are in turn usually 

 initiated by excitation of surface receptors by the environment. 

 * (T = o'ooi second. 



