172 A MANUAL OF PHYSIOLOGY 



diminution of arterial pressure. But this is not strictly correct, 

 for at an early period in the resuscitation of the brain after anaemia 

 excitation of the rabbit's ' depressor ' causes a slight rise of 

 pressure not followed by any fall. This, perhaps, indicates the 

 presence in the ' depressor ' of a small number of pressor fibres, 

 which are resuscitated sooner than the depressor fibres proper. 

 The same phenomenon, only more marked, may be seen when 

 the central end of the cat's vagus, containing the depressor 

 fibres, is excited at intervals during resuscitation (Fig. 71). 

 Or the result may depend upon a change in the response of the 

 altered vaso-motor centres to impulses reaching them along the 

 depressor fibres. If specific ' depressor ' fibres exist in other 

 nerves, they are so mingled with ' pressor ' fibres that their action 

 is masked when both are stimulated together. The state of the 



FIG. 72. RISE OF BLOOD-PRESSURE IN ASPHYXIA : RABBIT. 



Respiration stopped at i . Interval between 2 and 3 (not reproduced ) 44 seconds , 

 during which the blood-pressure steadily rose. At 4,'respiration resumed. Time 

 trace, seconds. 



vaso-motor centre is unquestionably a factor which has some 

 importance in determining the result of reflex vaso-motor stimula- 

 tion. For instance, in an animal deeply anaesthetized with 

 chloroform or chloral, excitation of pressor fibres (in an ordinary 

 sensory nerve) causes, not a rise, but a fall of blood-pressure ; 

 while in an animal fully under the influence of strychnine stimula- 

 tion of the depressor nerve causes not a fall, but a rise. 



These facts enable us to some extent to understand the manner 

 in which the distribution of the blood is adjusted to the require- 

 ments of the different parts of the body, so that to a certain 

 degree of approximation no organ has too much, and none too 

 little. The blood-supply of the organs is always shifting with 

 the calls upon them. Now, it is the actively-digesting stomach 

 and thelactively-secreting glands of the alimentary tract which 

 must be fed with a full stream of blood, to supply waste and to 



