BY DR. BURDON-SAXDERSON. 281 



fact is, that the effect does not attain its maximum till several 

 seconds after the commencement of the excitation. 



[In this and all other experiments in which it is desired to 

 note the time which elapses between the application of a 

 stimulus and its effect, we use the electrical indicator. It is 

 an arrangement exactly similar to an electrical bell, with the 

 exception that the hammer, instead of striking a bell, writes 

 on the recording cylinder of the kymograph. By a simple 

 mechanical arrangement, the same act which opens the Du 

 Bois' key closes another circuit, of which the electro-magnet 

 of the indicator forms part, and vice versa. This being the 

 case, the instrument makes vertical strokes on the cylinder at 

 the moment that the excitation of the nerve begins and ends.] 



75. 2. Demonstration of the Influence of the Vagus 

 Nerve on the Heart in Mammalia. In mammalia, the 

 inhibitory nerves contained in the vagi are in constant action, 

 consequently division of both vagi produces acceleration of 

 the contractions of the heart. In the dog, this effect is much 

 more considerable than in the rabbit, and is attended with an 

 increase of the arterial pressure, which in the latter is absent 

 (see fig. 238). On the other hand, electrical excitation of the 

 vagus, whether previously divided or not, retards the contrac- 

 tions of the heart in all animals, and, if the induced current is 

 strong enough, arrests the organ in diastole. (See fig. 239 a, 6.) 



To show these facts in the rabbit, all that is necessary is to 

 narcotize the animal, to insert a needle in the heart at the 

 upper part of the prrecorclia (i. e., about an inch to the left of 

 the middle line, at the level of the third cartilage), and to ex- 

 pose the vagi on both sides of the neck. If, now, either 

 nerve is placed between the electrodes, and the key opened, 

 the movement of the needle either stops, becomes irregular, 

 or is merely retarded and diminished in extent, according to 

 the strength of the current. To observe the effect of section, 

 loose ligatures must be placed round both nerves, and the 

 animal then left to itself, while the number of pulsations per 

 fifteen seconds is carefully counted. The two nerves are then 

 divided at once, and the countings repeated. The increase of 

 frequency usually amounts to about twenty percent. Finally, 

 the peripheral end of one nerve is excited, and the same effects 

 produced as by excitation of the undivided trunk. 



In demonstrating the influence of the vagus on the heart in 

 the dog, it is desirable to connect the carotid or crural artery 

 with the kymograph ; for the most important effects are those 

 which relate to the changes in the arterial pressure. The pre- 

 liminary steps of the experiment are those described in 34. 

 Loose ligatures having been placed round both vagi, and a 

 kymographic observation made, to determine the normal arte- 

 rial pressure and frequency of the pulse, both nerves are 



