792 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



control reflex action, before mentioned, as produced after a mechanical 

 irritation of the conjunctiva. Tickling of the feet leads to an inclination 

 to movement ; this also is a reflex action, and, as is well known, such a 

 movement may, by an exertion of the will, be suppressed, such a sup- 

 pression being an inhibition of reflex action. This voluntary control of 

 reflex action may be educated to a certain extent, but only within certain 

 limits. 



If the stimulus be severe and frequently repeated reflex action 

 occurs in spite of the effort of the will to prevent it. On the other hand, 

 numerous reflex actions are entirely beyond the control of the will ; thus 

 the contraction of the iris and parturition are reflex actions over which 

 the will has no control. 



It may be demonstrated by experiment that a spinal nervous mech- 

 anism exists for the purpose of keeping reflex action in control. If the 

 cerebrum be removed from a frog on a line with the anterior edges of the 

 t3'mpanic membrane reflex action may be readily produced. 



The best method of studying the influence of different agents on the 

 production of reflex action is that of Tiirck, of Vienna. 



The frog, from which the cerebrum has been removed, should be 

 suspended vertically by the nose, and if after the shock of the operation 

 lias passed away the tip of one toe be dipped in a solution of sulphuric 

 acid it will be rapidly withdrawn ; the duration of immersion before the 

 foot is withdrawn may be taken as indicating the degree of reflex 

 .activity of the spinal cord. After each immersion the foot should be 

 dipped into distilled water, so as to wash off the excess of acid and pre- 

 vent constant corrosion of the skin. If the time be determined which 

 elapses before the foot is withdrawn from the acid in the frog from whom 

 the cerebrum has been removed, and the cerebro-spinal axis be then again 

 divided on a line tangent to the posterior borders of the tympanic 

 membranes and the toe be again immersed in acid, it will now be found 

 that the foot is withdrawn after a much shorter interval than in the 

 previous experiment. This result would indicate that in some portion 

 of the cerebro-spinal axis between the lines of the two incisions is located 

 a mechanism which has for its function the controlling of reflex action. 



If in another frog the cerebrum be removed and the time of immer- 

 sion in the acid determined before reflex action takes place, and now one 

 of the optic lobes be exposed and irritated, as by placing a crystal of com- 

 mon salt in contact with it, it will be found that the frog will retain its 

 foot in the acid for a much longer time than before, or may even entirely 

 fail to remove it. That this result is due to the stimulation of the 

 inhibitory apparatus and not to a paralysis of reflex mechanism is 

 proved by the fact that if the spinal cord be now divided below the 

 medulla oblongata the foot will be as promptly withdrawn, or even more 



