658 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



system, there would be found the following arrangement : Afferent fibres run- 

 ning from the skin, muscles, and tendons, and forming the dorsal nerve-root 

 with its ganglion. The central mass of the cord in which these roots end, each 

 root marking the middle of a segment. From each segment of the cord go 

 the ventral root-fibres passing to the muscles lying beneath the skin to which 

 the sensory nerves are distributed, as well as to the ganglia of the sympa- 

 thetic system. The mechanism demanded for a reflex response is an afferent 

 path leading to the cord ; cells in the cord by which the incoming impulses 

 shall be distributed ; and a third set of efferent elements to carry the outgoing 

 impulses. It is important to consider in detail what occurs in each portion 

 of this reflex arc. 



In a frog thus prepared, stimulation of the skin in any part supplied by 

 the sensory nerves originating from the spinal cord causes a contraction of 

 some muscles. 



Influence of Location of Stimulus. The muscles which thus contract 

 tend to be those innervated from the same segments of the cord that receive 

 the sensory nerves that have been stimulated. Thus stimulation of the skin 

 of the breast causes movements of the fore limbs, and stimulation of the rump 

 or legs corresponding movements of the hind limbs. It is noticeable, how- 

 ever, that wherever the stimulus is applied, the hind limbs have a tendency 

 to move at the same time that the muscles most directly concerned contract. 



Segmental Reactions. In attempting to explain this associated contrac- 

 tion of the leg muscles, it must be remembered that the hind limbs are, par 

 excellence, the motile extremities of the frog, and therefore all general move- 

 ments involve their use. We infer from this, moreover, that the arrangement 

 in the spinal cord of the frog is not such that the sensory impulses coming 

 into any segment tend to rouse exclusively the muscles innervated by that 

 segment, but that these incoming impulses are diffused in the cord unevenly 

 and in such a way as to easily involve the segments controlling the legs. As 

 reflex co-ordinating centres, therefore, the several segments of the cord have 

 not an equal value. 



When the stimulus is applied on one side of the median plane, the re- 

 sponses first appear in the muscles of the same side, and if the stimulus is 

 slight they may appear on that side only. The incoming impulses are there- 

 fore first and most effectively distributed to the efferent cells located on the 

 same side of the cord as that on which these impulses enter. Such a state- 

 ment is most true, however, when the stimulus enters the cord at the level 

 where the nerves to the limbs are given. off. At other levels the diffusion to 

 the limb centres may take place more readily than to the cells in the opposite 

 half of the same segment. When the muscles of the side opposite contract 

 it is found that those there contracting correspond to the group of muscles 

 giving the initial response. The diffusion then tends to be across the cord 

 and to involve the cells located at the same level as that at which the 

 incoming impulses enter it. 



There is some reason to think that the path by which the diffusion takes 



