706 A MANUAL OF PHYSIOLOGY 



gone so far as to say that, under normal conditions, the so-called 

 spinal reflexes are really cerebral, in other words, that the afferent 

 impulses run up to the cortex of 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 afferent 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 roundabout way of the 

 cortex 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 discharged 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 recently made a 

 careful comparative study of reflex action after complete section of 

 the cord in the cervical or upper dorsal region, and conclude that 

 the spinal reflexes in the higher animals are far more dependent on 

 the upper portions of the central nervous system than in the frog. 



In order that a reflex action may take place, the reflex 

 arc afferent nerve, central mechanism, and efferent nerve 

 must be complete; and in fact a whole series of simple reflex 

 movements exists, the suppression, diminution, or exaggera- 

 tion of which can be used in diagnosis as tests of the con- 

 dition of the reflex arc. Such are the plantar reflex (the 

 drawing-up of the foot when the sole is tickled), the cremasteric 

 reflex (retraction of the testicle when the skin on the inside 

 of the thigh just below Poupart's ligament is stroked, espe- 



