THE CENTRAL NERVOUS SYSTEM 807 



removed from a frog, and one leg of the animal dipped into dilute 

 acid, a certain interval, the (unconnected) reflex time, will elapse 

 before the foot is drawn up (p. 886). If, now, a crystal of common 

 salt be applied to the optic lobes or the upper part of the spinal cord, 

 and the experiment repeated, it will be found either that the interval 

 is much lengthened or that the reflex disappears altogether. Strong 

 stimulation of an afferent nerve may abolish or delay a reflex move- 

 ment which is being elicited through other receptors. 



That the brain exerts more than a merely inhibitory influence on 

 the production of reflex movements is suggested by many facts. 

 The knee-jerk, for example, is increased or ' reinforced ' if an instant 

 before the tendon is struck the patient makes a voluntary movement 

 or is acted on by a sensory stimulus (Bowditch and Warren). In 

 health it varies in strength with many circumstances which affect 

 the activity of the central nervous system, as a whole (Lom- 

 bard, etc.). It often disappears in pathological lesions, situated 

 high up in the cord in man, and is markedly impaired after high 

 section of the cord in dogs. In hemiplegia (paralysis of one side of 

 the body, caused by disease in the brain) the cutaneous reflexes 

 on the paralyzed side may sometimes be absent for years. Some 

 observers have even gone so far as to say that, under normal con- 

 ditions, the so-called spinal reflexes are really cerebral in other 

 words, that the afferent impulses run up to the brain and there 

 discharge efferent impulses, which pass down to the motor cells of 

 the anterior horn and cause their discharge. It may be admitted 

 that there is no physiological ground for supposing that the afferent 

 impulses which have to do with the reflex contraction of the muscles 

 of the leg when the sole is tickled, stop short at the motor cells of 

 those spinal segments from which the efferent nerves come off, while 

 the afferent impulses which have to do with the sensation of tickling 

 pass up to the brain. The probability is that under ordinary 

 circumstances such aiferent impulses pass up the cord in long afferent 

 paths, as well as directly towards the motor cells along those fibres 

 of the posterior roots and their collaterals which bend forward into 

 the anterior horn at the level of their entrance into the cord. And 

 the only question is whether, as a matter of fact, the spinal motor 

 cells are most easily discharged by the impulses that reach them 

 directly, or by the impulses that come down to them by the round- 

 about way of the brain and the efferent fibres that connect it with 

 the cord. It is evident that the answer to this question need not 

 be the same for all kinds of animals. It may well be that in the 

 higher animals, in which the cortex has undergone a relatively great 

 development, the spinal motor mechanisms are more easily dis- 

 charged from above than from below, while in lower animals the 

 opposite may be the case. When the cord is cut off from the brain, the 

 afferent impulses may overflow more easily into the spinal motor 

 cells since their alternative path is blocked. In the frog, where 

 there is already a beaten track between the posterior root-fibres 

 and the cells of the anterior horn, this overflow may be established 

 immediately after section of the cord, and may of itself lead to an 

 exaggeration of the reflexes. In animals like the dog a longer time 

 may be necessary before the unaccustomed route from the end 

 arborizations of the afferent axons and their collaterals to the 

 dendrites or the bodies of the motor cells becomes natural and easy ; 

 in man a still longer interval may be required. Moore and Oertel 

 have made a careful comparative study of reflex action after com- 



