142 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



a rapid reflex movement. This paradoxical result may depend pos- 

 sibly upon the variety of nerve fiber stimulated by the two kinds 

 of stimuli or may be connected with the fact that the acid 

 stimuli may bring about inhibitory as well as excitatory processes 

 in the cord. 



Reflexes from Other Parts of the Nervous System. Nu- 

 merous typical reflexes are known to occur in the brain. The 

 reflex effects upon the important centers in the medulla, such 

 as the vasomotor center, the respiratory center, and the cardio- 

 inhibitory center, the winking of the eye, sneezing, the light reflex 

 upon the sphincter muscle of the iris, and many other similar cases 

 might be enumerated. All of these reactions will be described 

 and discussed in their proper places. The conscious reactions of 

 the brain are not included among the reflexes by virtue of the defi- 

 nition which lays stress upon the involuntary characteristic of the 

 reflex response, but it should be remembered that, so far as the 

 nervous mechanism is concerned, these conscious reactions do not 

 differ from the true reflexes. When we voluntarily move a limb 

 the movement is guided and controlled by sensory impulses from the 

 muscles put into action. The fibers of muscle sense from these 

 muscles convey sensory impulses through a chain of neurons to 

 the cortex of the brain and there the impulses doubtless affect and 

 set into action the motor neuron through which the movement is 

 effected. So far as we know, the discharges from the efferent 

 neuron of the brain are not really automatic, but are conditioned 

 or originated by stimuli from other neurons ; so that the activities 

 of the brain are carried on by a mechanism of one neuron acting 

 on another, just as in the case of the reflex arc. The added feature 

 of a psychical factor, a reaction in consciousness, enables us to draw 

 a line of distinction between these activities and those of so-called 

 pure reflexes; but the distinction is perhaps one of convenience 

 only, for, although the extremes may be far enough apart to suit 

 the definition, many intermediate instances may be found which 

 are difficult to classify. All skilled movements, for instance, such 

 as walking, singing, dancing, bicycle riding, and the like, although 

 in the beginning obviously effected by voluntary co-ordination, 

 nevertheless in the end, in proportion to the skill obtained, become 

 more or less entirely reflex, that is, involuntary. In learning 

 such movements one must, as the saying goes, establish his reflexes, 

 and the result can hardly be understood otherwise than by suppos- 

 ing that the continual adjustment of certain sensory impulses to 

 certain co-ordinated movements results in the formation of a more 

 or less complex reflex arc, a set of paths of least resistance. 



Reflexes through Peripheral Ganglia Axon Reflexes. 

 Many attempts have been made by physiologists to ascertain 



