282 CIRCULATION OF THE BLOOD. 



divided simultaneously. The mercurial column at once rises, 

 and the contractions of the heart become so frequent, that the 

 oscillations can no longer be followed by the eye, all that can 

 be distinguished being a vibratile movement of the column. 

 On exciting the peripheral end of either vagus, the same effects 

 are produced as in the rabbit. If the current is sufficiently 

 strong to stop the heart, the mercurial column sinks rapidly, 

 inscribing a parabolic curve on the paper (fig. 239ft), the exact 

 form of which depends on the condition of the arterial system ; 

 the rate of descent varing inversely as the arterial resistance 

 encountered by the blood in its progress towards the veins. 

 On discontinuing the excitation, the heart begins to beat 

 again, at first at long intervals, subsequently more frequently, 

 the pressure rapidly increasing until (for a few moments) it 

 exceeds that observed before excitation. In man, the trunk 

 of the vagus may in some persons be excited by pressure, and 

 results produced which correspond with those of electrical ex- 

 citation in animals. Prof. Czermak, of Leipsic, is able, by 

 making pressure at the proper spot on the right side of the 

 neck, to nrrcst the action of his heart for a few moments. 1 



76. 3. Demonstration of the Influence of certain 

 Afferent Nerves, in reflex Relation with the Inhibi- 

 tory Nerves contained in the Vagus, on the Heart. 

 Bernstein's Experiment. The inhibitory heart nerves 

 contained in the vagus are in intimate relation, through the 

 heart centre in the medulla oblongata, with certain afferent 

 fibres contained in the sympathetic system ; so that when 

 these fibres are excited, the same effects are produced as if 

 the vagus itself was directly acted upon. This may be shown 

 in the frog as follows: A frog is secured in the supine position. 

 The pleuro-peritoneal cavity is then opened, and the intestines 

 and other viscera are removed, great care being taken not to 

 injure the mesentery or the vessels and nerves which it con- 

 tains. Nothing now remains excepting the heart resting upon 

 the oesophagus. By carefully dividing the double layer of 

 serous membrane which forms the lateral wall of the cisterna 

 magna on both sides (see. Chap II.), the ganglionic chains 

 (fig. iMO) are brought into view along with the rami com- 

 )iinni<'antes by which the ganglia are severally connected with 

 the anterior roots of the corresponding spinal nerves. In the 

 thoracic part of the visceral cavity the two aortoe are seen 

 converging downwards, till at the level of the sixth vertebra 

 the)' meet to form one trunk, from which at its origin the me- 

 senteric artery is given off, to be distributed to the stoma-ch 

 and intestines. If now the two aortse are raised near their 

 junctions, with the point of the forceps, it is seen that one of 



1 Populare Vortrage, p. 27. 



