ix CAKDIAC MUSCLE AND NEEVES 335 



resistance which the heart has to overcome is reduced. The 

 retarded rhythm tends to the same result. 



Some interesting experiments of Sewal and Steiner illustrate 

 this theory. On ligaturing the two carotids they observed a rise 

 of arterial pressure, which was obviously nervous in origin, since it 

 failed when the vagi had previously been divided. When, on the 

 contrary, the depressors alone had been cut before ligaturing the 

 carotids, a much greater pressor effect was obtained than with the 

 intact depressors. They concluded that the depressors are highly 

 sensitive to any increase of mechanical resistance presented to the 

 functions of the heart. 



In studying blood pressure during asphyxia, Konow and 

 Stenbeck saw that it was far more irregular when the depressors 

 had been previously divided than when they were left intact, a 

 proof of their importance in the regulation of the circulation. 



Direct proof that the rise in aortic pressure stimulates the 

 depressors, independent of cardiac influence, was obtained by 

 Koster and Tscherniak (1902). They showed that even in the 

 isolated heart the passive distension of the walls of the aorta, on 

 filling it rapidly with artificial fluid, produced stimulation of the 

 depressors, as exhibited in a negative variation (action current) of 

 the demarcation current led off from two points of the nerve. 



Spallitta and Consiglio endeavoured to ascertain whether the 

 depressor fibres run with the vagus or the accessory. After 

 dividing the ramus intern us of the accessory, they found that 

 excitation of the depressors produced a fall of blood pressure, but 

 no slowing of cardiac rhythm. The depressors accordingly contain 

 two kinds of fibres : one set cause reflex dilatation of the vessels, 

 and belong to the vagus ; the other reflexly excites the inhibitory 

 centre of cardiac rhythm, and runs in the accessory. Mirto and 

 Pusateri confirmed these results by histological methods. After 

 intracranial section of the roots of the accessory nerve they found 

 degenerated fibres in the depressor. 



The afferent fibres from the heart are incapable of conveying 

 clear or conscious sensations. This was shown by Harvey's 

 experiments on the exposed heart of Viscount Montgomery. The 

 indefinite malaise complained of in the cardiac region by cardio- 

 paths (v. Ziemssen) may result from compression of the sensory 

 nerves beyond the heart. Although incapable of arousing 

 conscious sensations, the centripetal cardiac nerves are able 

 reflexly to produce more or less diffuse movements of the skeletal 

 muscles. Budge, Goltz, Gurboki, on pinching the heart in rabbit 

 and frog, obtained these reflex movements, which failed when 

 the vagi had been divided. 



Even such afferent fibres of the vagus as do not belong to the 

 heart reflexly modify cardiac rhythm. If the central end of one 

 divided vagus is stimulated while the other is intact, a delayed 



