THE CARDIAC NERVES 177 



ganglion cells. In a frog, these differences can be easily shown. 

 Stimulation of the sinu-auricular groove will produce an action both 

 before and after the application of nicotine, but not after atropine. 

 Both drugs prevent the effect of stimulating the vagus. In mammals, 

 a local application of the drugs to the sinu-auricular node abolishes 

 the inhibitory action of the vagus. Muscarin, obtained from poisonous 

 fungi, slows and finally arrests the heart probably by acting upon 

 the vagus nerve endings; atropine antagonizes this action (Fig. 81). 



A great many of the cardiac vagal fibres convey impulses to the 

 spinal bulb (centripetal), and reflexly influence the heart-frequency, 

 the breathing, and the tonus of the bloodvessels. In particular, 

 certain fibres, termed depressor, cause dilatation of the arterioles, and 

 a fall of arterial pressure, by inhibiting the tonic action of the vaso- 

 motor centre in the spinal bulb. The depressor fibres arise front the 

 root of the aorta, and overdistension of this part excites them, as 



FIG. 83. AORTIC PRESSURE. EXCITATION OF DEPRESSOR. (Bayliss.) 



The drum was stopped in the middle of the curve, and the excitation maintained for 



seventeen minutes. 



evidenced not only by the above effect, but also by the electrical 

 variation (action current) which has been observed passing up the 

 depressor nerve. In some animals, such as the rabbit, it is found in 

 the neck as a slender nerve running close to the sympathetic. It 

 can be recognized in the rabbit by the fact that it joins the vagus 

 and its superior laryngeal branch, dividing into two shortly before 

 its junction with these (see Fig. 82). Stimulation of its peripheral 

 end has no effect. 



The fall of blood-pressure (Fig. 83) induced by excitation of the 

 depressor results chiefly from vaso-dilatation in the splanchnic area. 

 After section of the splanchnics, this fall of blood-pressure naturally 

 is not marked. Its action is increased by the secretion of the thyroid 

 gland, induced by stimulation of the superior laryngeal nerves, or 

 by the injection of thyroid extract. 



In the vagus nerve there are also sensory fibres which when excited 

 cause reflexly through the vasomotor centre a rise of pressure (Fig. 84). 



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