Il 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



sure decreases (8, 103), whereas occlusion of the 

 arterial supply to the bone decreases both mean and 

 pulse pressures (8, 60, 103). Fracture of both sides of 

 the femur causes the intramedullary pressure to fall 

 to zero (8). Kaiser and his co-workers (74) demon- 

 strated a direct correlation between altitude and a 

 fall in marrow pressure in dogs, and experimentally 

 confirmed the fact that the marrow cavity acts as a 

 semiclosed cavity. 



This conception is supported by the study of 

 Petrakis (95) who showed that the Valsalva maneuver 

 in man produces a rise in systemic venous pressure, 

 thus reducing the venous outflow from the marrow 

 cavity and causing an increase in marrow pressure 

 and a decrease in pulse pressure. Human marrow 

 pressure varies with respiration and body position. 

 Thus, sternal pressures were found to be near or 

 below atmospheric pressures in recumbent, nonleu- 

 kemic patients, and to vary with the respiration. The 

 more distal iliac sites did not respond to the respira- 

 tory effects of forced breathing, but required the more 

 strenuous effects of the Valsalva effort or of coughing 

 to evoke changes. The higher pressures obtained in 

 the iliac crest are presumably a result of die erect 

 position of man. The effects of changes in body posi- 

 tion and of alterations in respiration on marrow 

 pressures indicate that, under conditions of normal 

 activity, intramedullary pressure varies considerably 

 and is passively aflected by changes in venous pres- 

 sure resulting from these activities. 



In this connection, it is interesting to note that in- 

 creasing the blood volume by the intravenous infu- 

 sion of large quantities of saline causes a gradual in- 

 crease of marrow pressure (8). On the other hand, a 

 decrease in blood volume produced by hemorrhage 

 causes a slow fall in medullary pressure (60). 



Although marrow pressure does not ordinarily re- 

 flect changes in mean systemic arterial pressure (74), 

 decapitated cats and cats with acute spinal injury 

 have low femoral arterial pressures and do show low 

 marrow pressures (60). Chronic "spinal" cats, in 

 which the systemic pressure has returned to the levels 

 seen before the spinal cord was cut, had bone marrow 

 pressures similar to those found in unoperated animals. 

 Stimulation of the cut peripheral end of either vagus 

 nerve produced the usual slowing of the heart and fall 

 in svstemic pressure. This was associated with a re- 

 duction in bone marrow pressure. Changes in marrow 

 pressure induced by stimulation of the central ends 

 of the cut vagi or of the central end of the cut femoral 

 or sciatic nerve, or by making an incision in the ab- 

 dominal wall, were small and unpredictable. The 



increase in systemic arterial pressure produced bv 

 occluding both carotid arteries was associated with a 

 rise in marrow pressure. 



Stimulation of the cut peripheral end of the ab- 

 dominal sympathetic chain isolated from its connec- 

 tions with the spinal cord produced a fall in marrow 

 pressure within the femur. A similar reduction in 

 marrow pressure of the mandible occurs when the 

 peripheral end of the cut cervical sympathetic cord is 

 stimulated. The fall in marrow pressure caused by 

 excitation of sympathetic nerve fibers has been used 

 to trace the pathway by which such fibers reach 

 specific bones (1 12). 



To determine whether the sympathetic innervation 

 of the marrow vessels is constantly influenced by tonic 

 impulses, the abdominal sympathetic chain of one 

 side was removed with strict aseptic precautions. 

 Several days after recovery from the operation, simul- 

 taneous measurement of the marrow pressures 

 showed no difference. However, when the experiment 

 was repeated using cats made decerebrate by ligating 

 both common carotid arteries and the basilar artery, 

 the pressure in the denervated femur was found to be 

 25 mm Hg higher than its innervated control (60). 



Stimulation of the peripheral end of the cut splanch- 

 nic nerve produced the usual prolonged rise in femoral 

 arterial pressure, whereas the marrow pressure of the 

 femur was greatly reduced (fig. 6) and recovered 

 only as the systemic pressure returned to the control 

 value (60). The same phenomena can be reproduced 

 by the intravenous injection of Adrenalin (fig. 7) 

 (8, 60, 82). The Adrenalin effect can be reversed by 

 the prior injection of Hydergine (60). Other drugs 

 which produce an increase in systemic pressure and a 

 reduction in marrow pressure are norepinephrine 

 (60), Pituitrin (8, 82), Neo-Synephrine (8), Syneph- 

 rine (8), and tyramine (8). 



A rise in systemic blood pressure with a simulta- 

 neous increase in marrow pressure is produced by the 



fig. 6. Effect of stimulating the peripheral end of the cut 

 splanchnic nerve on the femoral arterial pressure (upper record) 

 and the marrow pressure (lower record) in the femur of the 

 cat. [From Herzig & Root (60).] 



