582 SIGNIFICANCE OF THE NERVOUS SYSTEM 



and their connecting paths, it is commonly regarded as one of the 

 chief realms of reflex action. This statement, however, is not meant 

 to convey the idea that the cerebrum and other complexes of the 

 nervous system are composed exclusively of reaction circuits, and are 

 devoid of reflex circuits. Such an assumption could easily be proved 

 to be incorrect, because many of the most common reflexes invade the 

 cerebrum and neighboring parts. For example, if the intensity of the 

 light is increased, the pupil is constricted, or if the cornea is touched, 

 the eyelids are closed. Similarly, we react to sound impressions quite 

 frequently by movements of the head, and to visual impressions by a 

 hyperproduction of saliva and gastric juice. In all these instances, as 

 well as in many others that might still be mentioned, at least a section 

 of the reflex circuit is situated in the realm of the cerebrum and parts 

 immediately adjoining. Nevertheless, these actions are thoroughly 

 reflex in their nature. As additional proof it might be mentioned 

 that a group of reflexes, known as the association reflexes, actually 

 necessitate the formation of distinct sensory concepts, otherwise 

 the motor response invariably fails to develop. This is true, for 

 example, of the act of yawning elicited by observing somebody else 

 yawning, and of the flow of saliva and gastric juice following the sight 

 of attractive food. In all these cases, volition does not play a part and 

 hence, it must be concluded that reflex circuits may be found in all 

 parts of the nervous system and even in the domain of the cerebrum, 

 where they are brought into relation with the processes of conscious- 

 ness. It is to be noted, however, that the impulses conveyed by them 

 do not lose their reflex character unless dominated finally by volition. 

 Whenever this change takes place, the reflex becomes an associated 

 act or a volitional reaction. 



The conditions found in the lower forms are most closely simulated 

 in the sympathetic system, because this system consists of a series of 

 ganglia which are connected with one another by closely interwoven 

 nerve fibers. While these ganglia are generally situated in the im- 

 mediate vicinity of the structures innervated by them, they may also 

 be placed directly within their substance. If we direct our attention 

 for a moment to the stomach and intestine, we find that these organs 

 may be made to contract and to secrete even outside the body, pro- 

 vided that they are kept under proper conditions of moisture and 

 temperature. They are thus proved to possess a remarkable independ- 

 ency of function which is made possible by the fact that they are 

 amply equipped with reflex circuits which in all probability are con- 

 tained in the plexuses of Meissner and Auerbach. But even if these 

 organs are left in situ, it is not difficult to divide the bridges connecting 

 them with the cerebrospinal system. In this way, volition may be 

 absolutely excluded from them as well as from all other sympathetic 

 organs. Since their functions are not seriously disturbed thereby, it 

 must be concluded that they are typically reflex in their nature. 



Very similar conditions are met with in the spinal cord, the reflex 



