556 



HAKDBOOK OF PHYSIOLOGY. 



Fig. 349. Showing the arrangement 

 of the reflex mechanism, with a neu- 

 ron intercalated between the sensory 

 iind motor neurons. 



fig. 349. (b) All reflex actions are essentially involuntary ', though most 



of them admit of being modified, controlled, or prevented by a voluntary 



effort. 



(c) Reflex actions performed in health have, for the most part, a dis- 

 tinct purpose, and are adapted to secure 

 some end desirable for the well-being of the 

 body; but, in disease, many of them are 

 irregular and purposeless. 



(d) Muscular contractions produced by 

 reflex action are often more sustained than 

 those produced by the direct stimulus of 

 motor nerves themselves. The irritation 

 of a, muscular organ, or its motor nerve, 

 produces contraction lasting only so long as 

 the irritation continues; but irritation ap- 

 plied to a nervous centre through one of its 

 centripetal nerves may excite reflex and 

 harmonious contractions, which last some 

 time after the withdrawal of the stimulus. 



Relations letiveen the Stimulus and the 

 Lffect produced. Certain rules showing 



the relation between the resulting reflex action and the stimulus havu 



been drawn up by Pfluger as follows: 



1. Law of unilateral reflection. A slight irritation of the surface 

 supplied by certain 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. 



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

 more violent on the side irritated, but it must not be assumed that the 

 effect is always in proportion to the strength of the stimulus. 



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

 it is reflected along other motor nerves till at length all the muscles of 

 the body are thrown into action. 



In the simplest form of reflex action a single sensory and single motor 

 neuron may be supposed to be concerned, but in the majority of actual 

 actions many neurons are probably engaged. The impulse is carried by 

 collaterals up and down to different levels of the spinal cord, and thus 

 a number of groups of cells are affected (fig. 349A). 



The reflex effect produced by a stimulus applied to a sensory surface 

 depends, however, not only upon the strength of the stimulus, but also 

 upon other circumstances, the most important of which is the condi- 

 tion of the nerve-centre itself. Looking upon the effect produced as 



