THE CENTRAL NERVOUS SYSTEM 803 



clearly when other reflexes are studied in a similar way ; and 

 certain laws or rules which define the spread of the impulses in 

 spinal reflexes have been deduced. For descriptive purposes, 

 in dealing with reflex action, it is convenient to consider each 

 lateral half of the cord as divisible into regions each related on 

 the one hand to a certain area of the receptive surface (skin), and 

 on the other to certain muscles. Such regions are those of the 

 neck, including the pinna (cervical), the fore-limb (brachial), the 

 trunk (thoracic), the hind-limb (crural), and the tail (caudal). 

 According to their relation to these regions the spinal reflexes 

 can be classified as ' short ' or ' long.' The short spinal reflexes 

 are those in which the muscular response takes place in the same 

 region as the application of the stimulus. The long reflexes are 

 those evoked when the stimulus is applied to the receptive field 

 of one region, and the response occurs in the musculature of 

 another region. For the short reflexes Sherrington has given 

 a number of rules, which may be stated as follows : (i) The closer 

 together their spinal segments, the easier is it for stimulation of 

 a given afferent root to excite reflex contractions of muscles 

 supplied by a given efferent root. (2) For each afferent root 

 there exists in its own spinal segment (of course, on its own side 

 of the cord) a reflex motor path of as low a threshold (i.e., as 

 easily set into action) and of as high potency (i.e., producing 

 as great a reflex effect) as any open to it anywhere. It has been 

 shown that the afferent nerves of a skeletal muscle are derived 

 from the spinal ganglion corresponding to the segment of the 

 cord containing its motor cells. (3) Motor mechanisms for the 

 skeletal musculature lying in the same region of the cord, and in 

 the selfsame spinal segment, show markedly unequal accessibility 

 to the local afferent channels as judged by the reflex contractions 

 produced. For example, the reflex contraction of the flexors of 

 the knee on the stimulated side, and of the extensors of the 

 opposite knee, is in many animals much more easily elicited than 

 contraction of the extensors of the homonymous and the flexors 

 of the contralateral (i.e. opposite) side. This, however, is not 

 because the last-named extensors and flexors are really incapable 

 of being reflexly affected through the afferent fibres of the 

 corresponding spinal segments, but because the reflex effect 

 produced by them is in this case not contraction but inhibition. 

 (4) The groups of motor cells contemporaneously 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 

 excitation of an efferent spinal root are co-ordinated synergic 

 movements, since at many joints the flexors and extensors both 



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