274 THE NERVOUS SYSTEM 



iibre and the anterior fibre. Such intermediary fibres, since 

 they are situated entirely within the cord, are known as 

 proprio-spinal. They serve especially to connect the 

 posterior fibre of one segment with the anterior fibre of 

 another. 



Conduction in the Reflex Arc 



Conduction in a reflex arc differs from conduction along 

 a nerve-fibre in the following respects : — ■ 



1. In its slower speed. Frog's nerve at 15° C. conducts 

 at the rate of 3 cm. per o {o ^ -001 sec). The flexion 

 reflexion in the frog occupies 30 a. Moreover, the rate of 

 transmission varies with the intensity of the stimulus and 

 differs in different reflexes. 



2. In the tendency to after-discharge. In the reflex the 

 effect often continues long after the cessation of the 

 stimulus, this period of after-discharge increasing with 

 intensity of stimulus. 



3. In its irreversibility. Conduction occurs only from 

 receptor to effector. 



4. In its liability to fatigue. 



5. In its greater dependence upon oxygen. 



6. In its greater susceptibihty to the action of ancesthetics. 



7. (When the reflex response is rhythmic), in the want of 

 correspondence between the rhythm of stimulation and the 

 rhythm of effect. The rhythm of the scratch-reflex, for 

 instance, is the same whatever the mode of stimulation. 



8. In its greater habihty to summation. Summation we 

 have already seen in nerve-fibre — it is the effectiveness of 

 frequently repeated stimuh each of which is ineffective 

 singly. 



9. In the greater length of the refractory period. 



Tendon or Deep Reflexes 



The question arises here whether the contraction of a 

 muscle which occurs when its tendon is struck — the knee- 

 jerk, for instance— is reflex or not. Clearly the spinal 



