148 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



may give an augmentation or reinforcement of the reflex. A 

 striking example of this augmenting effect is given below in the 

 paragraph upon the knee-kick. 



Influence of the Condition of the Cord on its Reflex Ac- 

 tivities. — The time and extent of the reflex responses may be 

 altered greatly by various influences, particularly by the action 

 of drugs. The effect in such cases is usually upon the nerve centers 

 — that is, upon the cells themselves or upon the synapses — the 

 process of conduction within the sensory and motor fibers being 

 less easily affected. A convenient method of studying such in- 

 fluences is that employed by Tiirck. In this method the reflex 

 frog is suspended, and the tip of the longest toe is immersed to a 

 definite point in a solution of sulphuric acid of a strength of 0.1 

 to 0.2 per cent. If the time between the immersion and the reflex 

 withdrawal of the foot is noted by a metronome, or by a record upon 

 a kymograph, it will be found to be quite constant, provided the 

 conditions are kept uniform. If the average time for this reflex 

 is obtained from a series of observations it is possible to inject 

 various substances — such as strychnin, chloroform, potassium 

 bromid, quinin, etc. — under the skin, and after absorption has 

 taken place to determine the effect by a new series of observations. 

 So far as drugs are concerned the results of such experiments belong 

 rather to pharmacology than to physiology. 



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- 

 inhibitor}^ 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 hmb 

 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. 



