THE EFFECT OF CHANGE OF POSTURE. 



95 



crW 



-TKW 



-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 * IG 

 from syncope is brought about by the Thw? thoracic;AW5 

 assumption of the horizontal position. 

 When the compensatory mechanism is 

 entirely lost, the circulation is only 

 possible in the recumbent position, and 

 life is at its lowest ebb. Among the 

 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 tho 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. dc UoL, Turin ; 1884, vol. v. p. 130. 



wall ; CrW, 

 rigid cranial wall ; CC, cerebral capillaries ; 

 JV, jugular vein ; CA, carotid artery ; SVC, 

 vena cava superior ; TVCI and AVCI, 

 thoracic and abdominal vena cava; FV, 

 femoral vein ; RH, LH, right and left 

 heart ; L, pulmonary capillary area ; A, 

 aorta; "FA, femoral artery ; FC, femoral 

 capillary area ; 10, HC, intestinal and 

 hepatic capillary areas ; PV, portal vein. 



