FUNCTIONS OF THE SPINAL CORD 8y9 



traction, but inhibition. (4) The groups of motor cells contempor- 

 aneously discharged by spinal reflex action innervate synergic 

 muscles (muscles which act in the same direction in effecting a 

 harmonious movement), and not antergic muscles (which antagonize 

 each other). 



This disproves the old idea that the movements, caused by ex- 

 citation of an efferent spinal root, are co-ordinated synergic move- 

 ments, since at many joints the flexors and extensors both receive 

 motor fibres from 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 correspond with the distribu- 

 tion of an afferent root.' 



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

 ment 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 char- 

 acter 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 arid constantly evoked than the short 

 reflexes. The so-called laws of spread formulated by Pfliiger for 

 the long spinal reflexes, and based mainly on observations 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. Excita- 

 tion 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 

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



