'204 HANDBOOK OF PHYSIOLOGY. 



send out rhythmical impulses, by which undulation of blood -pressure fe 

 produced independently of the mechanical effects of respiration. 



The action of the vaso-motor centre in taking part in producing 

 rhythmical changes of blood-pressure which are called respiratory, is 

 shown in the following way: In an animal under the influence of urari, 

 .a record of whose blood-pressure is being taken, and where artificial res- 

 piration has been stopped, and both vagi cut, the blood-pressure curve 

 rises at first almost in a straight line, but after a time new rhythmical 

 undulations occur very like the original respiratory undulations, only 

 somewhat larger. These are called Traube's or Traiibe-Hering* s curves. 

 These continue whilst the blood-pressure continues to rise and only cease 

 when the vaso-motor centre and the heart are exhausted, when the 

 pressure speedily falls. These curves must be dependent upon the vaso- 

 motor centre, as the mechanical effects of respiration have been elimi- 

 nated by the poison and by the cessation of artificial respiration, and the 

 effect of the cardio-inhibitory centre by the division of the vagi. It may 

 be presumed therefore that the vaso-motor centre, as well as the cardio- 

 inhibitory, must be considered to take part with the mechanical changes 

 of inspiration and expiration in producing the so-called respiratory 

 undulations of blood-pressure. 



Cheyne- Stokes' breathing. This is a rhythmical irregularity in res- 

 pirations which has been observed in various diseases, and is especially 

 connected with fatty degeneration of the heart. Respirations occur in 

 groups, at the beginning of each group the inspirations are very shallow, 

 but each successive breath is deeper than the preceding until a climax is 

 reached, after which the inspirations become less and less deep, until they 

 cease after a slight pause altogether. 



Apnoea. Dyspnoea. Asphyxia. 



As blood which contains a normal proportion of oxygen sufficiently 

 excites the respiratory centre (p. 199) to produce normal respiration, 

 and, as the excitement and consequent respiratory muscular movements 

 are greater (dyspnoea) in proportion to the deficiency of this gas, so an 

 abnormally large proportion of oxygen in the blood leads to diminished 

 breathing movements, and, if the proportion be large enough, to their 

 temporary cessation. This condition of absence of breathing is termed 

 Apncea, 1 and it can be demonstrated, in one of the lower animals, by 

 performing artificial respiration to the extent of saturating the blood 

 ivith oxygen. 



When, on the other hand, the respiration is stopped, by, e.g., inter- 

 ference with the passage of air to the lungs, or by supplying air devoid 



1 This term lias been, unfortunately, often applied to conditions of dyspnoea or as- 

 phyxia ; but the modern application of the term, as in the text, is the more convenient. 



