THE NERVOUS SYSTEM. 87 



2. Law of symmetrical reflection. A stronger irritation is reflected, 

 not only on one side, but also along the corresponding motor nerves of 

 the opposite side. Thus, if the spinal cord of a man has been severed by 

 a stab in the back, when one foot is tickled both legs will be drawn up. 



3. Law of intensity. In the above case, the contractions will be more 

 violent on the side irritated. 



4. Laiv of radiation. If the irritation (afferent impulse) increases, it 

 is reflected along the motor nerves which spring from points higher up 

 the spinal cord, till at length all the muscles of the body are thrown into 

 action. 



Simple and Co-ordinated Reflex Actions. In the simplest form 

 of reflex action a single nerve cell with an afferent and an efferent fibre is 

 concerned, but in the majority of actual actions a number of cells are 

 probably concerned, and the impression is as it were distributed among 

 them, and they act in concert or co-ordination. This co-ordinating 

 power belongs to nerve centres. 



Primary and Secondary or Acquired Reflex Actions. We 

 must carefully distinguish between such reflex actions which may be 

 termed primary, and those which are secondary or acquired. As examples \ 

 of the former class we may cite sucking, contraction of the pupil, drawing 

 up the legs when the toes are tickled, and many others which are performed \ 

 as perfectly by the infant as by the adult. \ 



The large class of secondary reflex actions consists of acts which re- 

 quire for their first performance and many subsequent repetitions, an 

 effort of will, but which by constant repetition are habitually though not 

 necessarily performed, mechanically, i.e., without the intervention of con- 

 sciousness and volition. As instances we may take reading, writing, 

 walking, etc. 



In endeavoring to conceive how such complicated actions can be per- 

 formed without consciousness and will, we must suppose that in the first 

 instance the will directs the nerve-force along certain channels causing 

 the performance of certain acts, e.g., the various movements of flexion 

 and extension involved in walking. After a time, by constant repetition, 

 these routes become, to use a metaphor, well worn : there is, as it were, 

 a beaten track along which the nerve-force travels with much greater ease 

 than formerly: so much so that a slight stimulus, such as the pressure of 

 the foot on the ground, is sufficient to start and keep going indefinitely 

 the complex reflex actions of walking during entire mental abstraction, 

 or even during sleep. In such acts as reading, writing, and tlje like, 

 it would appear as if the will set the necessary reflex machinery going, 

 and that the reflex actions go on uninterruptedly until again interfered 

 with by the will. 



Without this capacity possessed by the nervous system of "organizing 

 conscious actions into more or less unconscious ones," education or training 



