284 CIRCULATION OF THE BLOOD. 



lows : In a rabbit, the trachea is connected with the apparatus 

 for artificial respiration, and the vagi are exposed in the neck. 

 Thereupon the spinal cord is divided immediately below the 

 medulla oblongata. On the cessation of breathing, artificial 

 respiration is commenced. The cervical sympathetics are 

 then divided, and a needle is inserted in the heart. A succes- 

 sion of observations of the frequency of the heart's action is 

 then made, and both vagi are divided. No acceleration of the 

 pulse rate occurs. 



The purpose of the experiment is to show that when the affer- 

 ent sympathetic nerves which are known to be in reflex relation 

 with the vagus heart nerves are severed, the same effect is pro- 

 duced on the vagus as if it were itself divided. There is no way 

 of accomplishing this directly, without such interference with 

 other nerves as would affect the heart, and thereby render the 

 result ambiguous. The most complete method would be to 

 remove the whole, ganglionic cord on both sides. Without 

 reference to the extreme difficulty of such an operation, it is 

 clear that it would involve the accelerator nerves (see 80), 

 and thereby perhaps produce an effect the opposite of that which 

 we intended a slowing instead of an acceleration of the pulse. 

 So also, when the spinal cord is divided immediately below the 

 medulla oblongata, the effect is modified not only by the de- 

 struction of the accelerator nerves, but by the general paralysis 

 of the vasomotor system. Consequently no answer to the 

 question is to be obtained by direct observation of the changes 

 which are produced by any such operation in the rate of pulsa- 

 tion of the heart, so that the end we have in view can only be 

 accomplished indirectly. We already know that both vagi are 

 in constant action, i. e.. that the heart is constantly under their 

 inhibitory control ; and that when this control is removed by 

 dividing them, the frequency of the pulse increases. It is ob- 

 vious that this effect can only be witnessed so long as the con- 

 trol is in actual exercise ; in other words, that if the vagi arc 

 not acting, it would make no difference as regards the heart 

 whether they are divided or not. The consideration of this 

 fact suggests the method which is employed in the experiment 

 above described, which shows that in an animal in which the 

 spinal cord has been divided below the medulla, the rate of the 

 pulse is the same before and after section of the vagi. 



Bernstein has further shown that the same thing happens 

 after destruction of the whole ganglionic cord, or of the cervi- 

 cal part, provided that the spinal cord is at the same time 

 severed at the seventh vertebra. In the dog, section of the 

 cord generally diminishes the frequency of the pulse. There 

 is no such effect in the rabbit. The difference can only be ex- 

 plained by supposing that in the former the activity of the 

 accelerator nerves is less, as compared with that of the nerves 



