804 A MANUAL OF PHYSIOLOGY 



receive motor fibres iron one and the same root, and stimulation 

 of the root must simultaneously excite antagonistic muscles. 

 ' The collection of fibres in a motor spinal root does not represent 

 a reflex figure i.e., a number of simple reflexes occurring 

 simultaneously nor does the receptive field of a reflex corre- 

 spond with the distribution of an afferent root/ 



(5) It follows from (i), (2), and (4) that the spinal reflex 

 movement which can be elicited in and from any one spinal 

 region will exhibit much uniformity even when the exciting 

 stimulus is applied at different and distant points within the 

 receptive field. The flexion reflex of the hind-limb, e.g., will 

 have the same general character i.e., flexion of each of the 

 three main joints whatever part of the surface of the limb is 

 stimulated. Yet the flexion movement will be strongest at the 

 joint whose flexors are innervated by motor cells situated in a 

 spinal segment near the entrance of the afferent fibres from the 

 stimulated skin area. 



For the long spinal reflexes it is less easy to deduce definite 

 rules, for they can be less easily and constantly evoked than the 

 short reflexes. The so-called laws of spread formulated by 

 Pfliiger for the long spinal reflexes, and based mainly on observa- 

 tions made in the brainless frog and on clinical records in cases 

 of spinal lesion in man, need not be stated here. For Sherrington 

 has shown that they require serious modification. Especially 

 is this true of Pfliiger 's fourth law that the reflex irradiation 

 spreads always more easily up in the direction of the medulla 

 oblongata, so that stimulation of a fore-limb does not cause 

 reflex contraction of a hind-limb, although excitation of a hind- 

 limb may cause movement of one or both fore-limbs. This law 

 does not hold in the mammal. As a rule, indeed, irradiation 

 takes place more easily down than up the cord. Excitation of 

 the skin of the pinna easily causes reflex movements of the limbs, 

 while the reverse is rare. Reflex movements of the hind-limb 

 in the spinal animal are more easily evoked by stimulation of the 

 fore-limb than movements of the fore-limb by stimulation of the 

 hind. It is easier for the irradiation to cross the cord from 

 hind-limb to hind-limb than to pass up from hind- to fore-limb ; 

 but it is often easier for irradiation to occur down the cord from 

 fore- to hind-limb than across the cord from one fore-limb to the 

 othei . Afferent channels from the skin of the shoulder, through 

 which the scratch-reflex is discharged (in the dog), are freely con- 

 nected with efferent paths to the muscles of the hip, knee, and ankle 

 by an uncrossed path descending the lateral column (Sherrington) . 

 In cats, after temporary occlusion of the cerebral circulation, 

 which throws the brain out of gear, it is easy to elicit movements 

 of the hind-legs by pinching the fore-paws or the skin of the 



