2/2 ESSENTIALS OF PHYSIOLOGY. 



to the resistance offered by the high blood pressure, partly to the direct 

 effect of lack of oxygen upon the nutrition of the heart itself, its output 

 gradually diminishes, it becomes more and more distended, and finally 

 ceases to beat. When the vagus nerves are intact, the vaso-constriction 

 leads to reflex slowing of the heart, and the rise of blood pressure is 

 smaller; the slowing of the heart lessens the strain thrown upon it. 

 and life is prolonged for a minute or so longer. During asphyxia the 

 conductivity of the bundle of His is often diminished, producing a 

 condition of heart block, so that the auricles may beat twice or thrice 

 as frequently as the ventricles. 



The increased activity of both the respiratory and vaso-motor centres 

 in asphyxia is, undoubtedly, the result of increased H ion concentration 

 of the blood ; it must be noted, however, that the respiratory centre is 

 the more sensitive, and may be excited by an excess of carbonic acid 

 too slight to affect the vaso-motor centre. 



THE NERVOUS REGULATION OF RESPIRATION. 



The respiratory centre can be influenced by nervous impulses 

 reaching it (1) along the vagus, (2) from the higher parts of the brain, 

 and (3) from other afferent nerves ; the impulses affect primarily the 

 frequency of the respiratory movements. 



(1) The Vagi. The influence of the vagus nerves is most easily 

 studied in the rabbit, by recording the movements of an isolated slip 

 of the diaphragm (p. 265). When a record of the respiratory movements 



is obtained by this method, it may 

 be seen that division of the vagus 

 nerve is followed by a decrease in 

 the frequency of respiration, al- 

 though each breath is deeper than 

 before. Electrical stimulation of 



the central end of one vagus may 



FIG. 106. Stimulation of central end , , . L. 



of one vagus between X and X. then Produce one of two effects 

 The slip of diaphragm remains in upon the respiratory movements, 

 the relaxed, expiratory condition. , . . ,, .,, 



Sometimes, especially with a weak 



stimulus, or with a constant ascending current, the inspiratory move- 

 ments are inhibited (fig. 106), and the slip of diaphragm remains in 

 the expiratory position, that is, it is relaxed. More often the inspiratory 

 movements are increased (fig. 107), as is shown by the fact that the 

 diaphragm contracts and may remain in the inspiratory position. 



These experiments make it clear, first, that the vagus contains 

 afferent fibres carrying impulses to the respiratory centre, which 



