CHAPTER XXV. 



THE CIRCULATION OF THE BLOOD AND LYMPH, AND 

 THE RESPIRATION. 



The Reflex Nerve Control of the Circulation and of the 

 Respiratory Movements. 



In Exp. 35, p. 79, it has been demonstrated that the heart 

 beat is controlled largely through the vagus nerve and that the 

 degree of resistance to the blood flow through the splanchnic 

 vessels depends on impulses carried by the splanchnic nerves. 

 The vagus and vaso-motor nerves arise in the medulla oblongata. 

 The efferent nerves of the respiratory muscles (phrenic and inter- 

 costal nerves) arise in spinal nerve centres, which are dominated 

 by a higher centre. This chief respiratory centre, as it is called, 

 is also situated in the medulla. The present experiment is devised 

 to throw light on the afferent pathways through which the above 

 centres become stimulated. 



Demonstration 1. Simultaneous tracings are taken of the 

 carotid blood pressure and of the respiratory movements (p. 97), 

 in an etherised and tracheotomised dog. The sciatic nerve in 

 one leg and the vagus nerves on both sides of the neck are exposed 

 and loose ligatures placed around them. A hot iron is then applied 

 to the skin of the foot while the recording drum is revolving at 

 such a speed that the cardiac pulsations can readily be counted. 

 Observe the effects on: (1) the pulse rate (2) the mean arterial 

 blood pressure and (3) the rate and depth of the respirations. 

 What general conclusions may be drawn concerning the influence 

 on the centres of over-stimulation of skin nerves? 



After moderately tightening the ligature, the sciatic nerve is 

 cut peripheral to it and the hot iron reapplied to the skin. What 

 conclusion do you draw from the results? 



The central end of the cut sciatic is then stimulated with the 

 faradic current, of a strength which just produces some change in 



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