620 MANUAL OF PHYSIOLOGY. 



until the whole body is tossed about by coordinated motions. 

 The movements seem to spread from the local nerve-cells to their 

 neighbors, and then to reach those governing the corresponding 

 muscles of the other side, in which, however, they are always less 

 marked than in those of the side stimulated. This spreading of 

 movement from one set of muscles to another, as the strength of 

 the stimulus is increased, of course, must be preceded by a spread- 

 ing of the impulse from one group of nerve-cells in the cord to 

 another by a kind of radiation from the focus of excitation. 



Very slight stimulation, though not sufficient to produce imme- 

 diate response, may, after a time, give rise to definite reflex action, 

 as if the weak impulses arriving at the nerve-cells in the cord were 

 stored up until their sum sufficed to produce a definite reflex 

 movement. This may also indeed, much better be seen in ani- 

 mals whose nerve centres are intact, for the cells of more remote 

 parts exercise a kind of checking influence on those in the region 

 receiving the stimulus, and thus the accumulative action (summa- 

 tion) comes more commonly and more effectively into play. This 

 is seen in the human subject where slight visceral stimulations 

 exist for a long time. In some of these cases, even without any 

 really sensory appreciation of any local excitation, an amount of 

 energy may be accumulated along the gray tract of the cord from 

 the prolonged income of impulses, that will bring on the most ex- 

 tensive forms of reflex muscular movement, and give rise to serious 

 results. These movements are generally different from the regular 

 coordinated motion resulting at once from an adequate skin stim- 

 ulation, and have usually a tendency to assume a convulsive form. 

 As an example of this may be named the convulsions that com- 

 monly occur in young children, from the prolonged irritation of 

 intestinal worms, or during the painful period of dentition. Epi- 

 lepsy and hydrophobia may possibly be explained in the same 

 way. 



In certain conditions of the nervous system these irregular 

 movements or spasms (convulsions) can be excited much more 

 readily than is normally the case. As most striking among these, 

 may be named poisoning with the alkaloid of nux vomica (strych- 

 nia) and the state of the blood which is produced by cessation of 



