1884.] Reflex Action of the Cardiac Xerves in Fishes. 



33 



whether the current may not (if a strong current be used) spread to 

 the sympathetic nerves lying along the aorta on the ventral aspect 

 of the vertebral column. Any such source of error can be avoided 

 by employing mechanical stimulation of the cord by means of a wire 

 passed some distance up the spinal canal, so as to destroy the cord at 

 that part. 



Moreover, if an interrupted current of the same strength as before 

 be applied to the empty spinal canal at the point where the cord was 

 formerly stimulated, it is found to cause no inhibition at all, though 

 it is quite as free as before to pass into the sympathetic nerves and 

 other neighbouring structures. It is clear then that the interrupted 

 current which caused inhibition when applied to the cut end of the 

 spinal cord caused inhibition by stimulating the cord itself, and not 

 by its effect upon any neighbouring structures into which it might 

 have spread. 



When direct stimulation of the cord is employed as a means of 

 inducing reflex cardiac inhibition, the accompanying movements of 

 the skeletal muscles occur so spasmodically that it is impossible to 

 observe any order of succession in their generation. 



But when the animal appears to be in a sluggish and depressed condi- 

 tion, stimulation of the tail is often followed by a struggling movement 

 which begins in the region of the stimulated point, and slowly passes 

 along the body towards the head. When this wave of movement 

 passing along from the tail reaches no further than the middle of the 

 animal's length, no cardiac inhibition occurs, but when it traverses 

 the whole length of the body, along to the head, cardiac inhibition 

 occurs, and removal of the brain above the medulla does not obviate 

 this result. The wave of muscular movement passing along the body 

 from tail to head (as a result of stimulation of the tail) is not simply 

 a contraction wave propagated by the muscular tissues ; it indicates a 

 wave of activity passing over the motor cells of the spinal cord. 

 Section of the spinal cord at once arrests the transmission of the 

 movement in question. It would seem that when this phase of activity 

 passing along the cord reaches the medulla, the vagus centre is thrown 

 into action together with the neighbouring motor centres, e.g., the 

 respiratory centre, the centre for movement of the pectoral fins, &c. 

 It must be remembered that in these experiments the animal is made 

 fast on a board. In such circumstances, the struggling movements 

 possibly attain an intensity much greater than is commonly the case 

 in the normal condition of the animal. 



When the wave of action passing along the spinal cord fails to 

 reach the medulla, the vagus centre would seem to be unaffected, 

 since no change in the heart's action is discernible. And when irrita- 

 tion of the tail fails to produce any reflex movements, it also fails to 

 bring about cardiac arrest. The result of the application of a stimulus 



VOL. XXXVIII. - D 



