150 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



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 neurons through which the movement is 

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

 neurons of the brain do not arise independently within these cells, 

 they 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 in- 

 stance, 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 ob- 

 tained, 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 supposing that the continual adjustment of certain sensory im- 

 pulses 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 

 whether or not reflexes can occur through the peripheral nerve 

 ganglia, lying outside the central nervous system. With regard 

 to the posterior root ganglia, it has usually been supposed that 

 they cannot exhibit reflexes. When the posterior root con- 

 necting such a ganglion to the cord is severed, then, according 

 to our usual conception, the cells in the ganglia are cut off 

 from all connections with the peripheral tissues by efferent 



