878 THE CENTRAL NERVOUS SYSTEM 



will cause, in addition to flexion of the same hind-foot, extension of 

 the opposite hind-limb, then in the homonymous fore-limb (i.e., the 

 limb on the same side) extension at the elbow and retraction at the 

 shoulder, then certain definite movements, the details of which need 

 not detain us here, in the opposite fore-limb, and ultimately also 

 definite movements of the head and tail (Sherrington). Obviously 

 there is a certain orderliness in the spread of the reflexes; they 

 follow a certain regular march; the irradiation in the tangle of the 

 spinal paths is not an indiscriminate one. The same fact emerges 

 quite as 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 rela- 

 tion 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, thd easier is 

 it for stimulation of a given efferent root to excite reflex contractions 

 of muscles supplied by a given afferent 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 con- 



