168 



APPLIED PHYSIOLOGY 



to make room for the increased quantity. In conditions 

 of true plethora, and in chlorosis, the total volume of 

 the blood may be increased to this amount, and the 

 vasomotor centre accommodates the vascular area to 

 it without affecting the pressure. 



On the other hand, if the vasomotor centre be 

 paralyzed the vascular bed is so much widened that 

 the blood simply accumulates passively in the most 

 dependent part of it, or in the large splanchnic area. 

 In these circumstances the return of blood to the heart 

 becomes less and less, and finally the circulation may 

 come to a standstill, even although the heart is still 

 quite fit to maintain it were it only supplied with blood. 

 This is what happens in surgical shock and in the 

 ' collapse ' which often comes on in acute disease ; and 

 the indication in such cases is not to stimulate the 

 heart, but artificially to increase the peripheral resistance 

 (see footnote, p. 162). 



In the distribution of the blood as a whole there is 

 a reciprocal action between the splanchnic system and 

 the more peripheral parts of the circulation. When 

 blood is wanted at the periphery the splanchnic vessels 

 are contracted by the vasomotor centre and the normal 

 level of the blood-pressure maintained. This happens, 

 for instance, when the cutaneous vessels are dilated by 

 heat, and if the compensating mechanism be not in 

 good working order, the blood-pressure may fall so low 

 that the individual faints. Many cases of ' collapse ' in 

 hot weather are brought about in this way. On the other 

 hand, when more blood is wanted in the splanchnic system, 

 the peripheral vessels contract to make up for it; hence 



