210 AN AMERICAN TEXT- BOOK OF PHYSIOLOGY. 



its collaterals, or a central cell is excited and serves to carry the impulse to a 

 distance. 



Segmental Reactions. — In attempting to explain this associated con- 

 traction of the leg muscles, it must he remembered that the hind legs are. 

 •par excellence, the motile extremities of the frog, and therefore all general 

 movements involve their use. We inter from this, moreover, that the arrange- 

 ment in the spinal cord of the frog is not such that the sensory impulses com- 

 ing 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 responses 

 firs! appear in the muscles of the same side; and if the stimulus is slight they 

 may appear on that side only. The incoming impulses arc therefore first and 

 mosl effectively distributed to the efferent cells located on the same side of 

 the cord as that on which these impulses enter. Such a statement 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 on the side opposite to the point of stimulation 

 contract it is found 'hat they correspond to the group of muscles giving the 

 initial response on the side of the stimulus. The diffusion then tends to cross 

 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 when the impulses enter the cord 

 toward the lumbar end the path by which the diffusion takes place with 

 least resistance is not the shortest one between the two groups of cells, but 

 a path toward the cephalic end of the cord, so that the impulses tend to pass 

 up the cord on one side and down on the other. 1 



Strength of Stimulus. — In a reflex response the strength of the stimulus 

 influences the extent to which the muscles are contracted, the number of 

 muscles taking part in the contraction, and the length of time during which 

 the contraction continues. That the strength of the stimulus influences the 

 extent to which the contraction of a given group of muscles takes place is 

 easily shown when, for example, the toe of a reflex frog which has been sus- 

 pended i> stimulated by pinching it or dipping it in dilute acid. In this case, 

 if the stimulus hi' slight, the leg is hut slightly raised, whereas, if the stimu- 

 lus he strong, it is drawn up high. In the same way by altering the stimulus 

 the muscles which enter into the contraction may he only those controlling 

 the joints of the foot, whereas, with stronger stimuli, those for the knee and 

 hip are successively affected, thereby involving a much larger number of 

 muscles. Here, too, we infer a spread of the incoming impulses which is 

 orderly, since the several joints of the limb are moved in regular sequence. 

 1 Rosenthal and Mendelssohn : Neurologisches Centralblatt, 1897, Bd. xvi. S. 978. 



