186 THE CIRCULATION OF THE BLOOD AND LYMPH 



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 resus- 

 citation (Fig. 83). 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 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 chloro- 

 form or chloral, excitation of pressor fibres (in an ordinary sensory 



Fig. 84. 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. 



nerve) causes, not a rise, but a fall of blood -pressure; while in an 

 animal fully under the influence of strychnine stimulation of the 

 depressor nerve causes not a fall, but a rise. 



The vaso-motor reflexes in man can be conveniently studied by 

 the calorimetric method described on p. 220. One of the most 

 important of the vaso-motor reactions is that by which the vessels 

 of the skin respond to the temperature of the environment so as 

 to regulate the loss of heat from the body (p. 674). When one 

 hand, e.g. the left, is immersed in cold water (say at about 8 C), 

 the blood-flow in the right is at once reduced owing to reflex vaso- 

 constriction. Other parts of the body are also affected, but not so 

 readily as the contra-lateral hand, since the segments of the cord 

 into which the afferent fibres from a given skin area run are at the 

 same time the segments from which the efferent vaso-motor fibres 

 for the symmetrically-placed area on the opposite side of the body 

 arise. The reflex diminution in the flow persists for a time which 



