THE NERVOUS SYSTEM. 469 



fluids, those which unite the various generative functions, and many 

 pathological phenomena. 



Relations between the Stimulus and the Resulting Reflex 

 Action. Certain rules showing the relation between the resulting reflex 

 action and the stimulus have been drawn up by Pfliiger, as follows: 



1. Law of unilateral reflection. A slight irritation of sensory nerves 

 is reflected along the motor nerves of the same region. Thus, if the 

 skin of a frog's foot be tickled on the right side, the right leg is drawn 

 up. 



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. Law 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. 



Varieties of Reflex Actions. 



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 care- 

 fully distinguish between such reflex actions which may be termed pri- 

 mary, 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 per- 

 formed 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 consciousness and volition. As instances we may take reading, writ- 

 ing, 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, 



