THE EFFECT OF CHANGE OF POSTURE. 



95 



-ThW 



AW 



When the compensatory mechanism is abolished by destruction, 

 exhaustion, or inhibition of the bulbar centres, the circulation fails 

 and becomes inadequate to maintain life in the vertical feet-down 

 posture. The blood passes into the capacious reservoirs of the toneless 

 abdominal veins, the heart empties, and the cerebral circulation ceases. 

 There can be no doubt that the control 

 of this compensatory mechanism is one 

 of the most important and absolutely 

 necessary functions of the group of 

 bulbar centres, a function which must 

 have been evolved to its highest point 

 as the anthropoids slowly assumed the 

 erect posture. 



During the course of each day the 

 compensatory mechanism becomes ex- 

 hausted, and especially is this so after- 

 severe muscular exertion. 1 By sleep 

 the compensatory power is restored. 

 In conditions of neurasthenia, weakness 

 and exhaustion after disease, shock after 

 severe injury or haemorrhage, this power 

 may be almost entirely lost. 



By sudden fright in the standing 

 posture the respiration is often arrested, 

 the vasomotor tone inhibited, and syn- 

 cope induced by the rush of blood from 

 the brain to the abdomen. Eecovery FlG - 65.-Scheme of the circulation.- 



. , , . . » Hill and Barnard. 



from syncope is brought about by the ThW thoracie . AW , abdon)inal wall . CrW , 



assumption Of the horizontal position. rigid cranial wall ; CO, cerebral capillaries ; 

 -, TT1 L . , i . JV, jugular vein ; CA, carotid artery ; SVC, 



When the compensatory mechanism is vena cava superior; tvci and" avci, 



ptiHvpIv lrmr Hip nirnnlatinn is nnlv thoracic and abdominal vena cava; FY, 

 entile!) lOSt, Hie CllCUiailOn lb Oil!) femoral vein; RH, LH, right and left 



possible in the recumbent position, aild heart; L, pulmonary capillary area; A 



r . . 11* i aorta ; FA, femoral arterv ; FC, femoral 



life IS at ltS lowest ebb. Among the capillary area ; IC, HC* intestinal and 



,i ,• i i e hepatic capillary areas ; PA", portal vein. 



anaesthetics in common use, chloroform 



stands prepotent as a drug which has the power to abolish the com- 

 pensatory mechanism. Chloroform causes cardiac and vascular dilatation, 

 weakens the respiration, and abolishes the tone of the abdominal muscles. 2 



A useful clinical guide to the condition of the compensatory mechanism 

 in man is afforded by the rate of the pulse on change of posture. If the heart 

 becomes greatly accelerated on rising from the horizontal to the vertical 

 position, the vasomotor tone is deficient. 



The alteration in the distribution of the blood by the influence of gravity 

 has been studied on man by Mosso. 3 He balanced the body on a horizontal 

 board, turning on a transverse axis. The head-end soon becomes the heavier, 

 owing to the increased outflow from the legs in the horizontal posture. If the 

 legs have previously been dilated by standing in a hot bath, this effect becomes 

 more marked. The change in weight may be as much as 260 c.c. of 

 blood. At each inspiration the foot-end becomes heavier on account of the 

 compression of the vena cava inferior by the descent of the diaphragm. The 

 volume of one arm is increased if the other be raised up, and similarly in 

 the case of the legs. 



1 George Oliver, " Pulse Gauging," London, 1895. 



2 Leonard Hill, Brit. Med. Journ., London, 1897, vol. i. p. 957. 

 3 Arch. ital. dchioh, Turin 1884, vol. v. p. 130. 



