THE VASOMOTOR MECHANISM. 



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6. Observation of the lateral pressure in the peripheral end of an artery, 

 that is to say, at a point peripheral to the origin of anastomotic branches, 

 which are sufficient to maintain the circulation through the organ. Hiirthle 

 employed this method in a research on the brain. He recorded the pressure 

 in the peripheral end of the internal carotid artery. Dastre and Moral 

 similarly observed the lateral pressure in the peripheral end of the femoral 

 artery, below the origin of the profunda femoris. 



7. Simultaneous records of the lateral pressure in the afferent artery and 

 the efferent vein of any organ. This method has been used in researches on 

 the portal system, the brain, and the lungs. 



8. Estimation of the capillary pressure. The intra-cranial and intra-ocular 

 pressures have been studied by this method. 



The greatest accuracy and scientific acumen is required in the 

 execution of researches on the more delicate vasomotor mechanisms. 

 It must constantly be borne in mind that changes in the vascularity of 

 an organ may depend on central as well as peripheral alterations in the 

 circulatory system. Thus, for example, during the rise of arterial 

 pressure which occurs during asphyxia, the kidneys decrease in volume, 

 and the limbs increase ; the vessels of the rabbit's ear may either 

 contract or dilate ; the vessels of the intestines contract ; in either the 

 cerebral or the pulmonary veins the pressure rises, while the pressure in 

 the portal vein may either rise or fall. From the mere observation of 

 the circulatory changes in any of these organs, we cannot tell whether 

 the above effects are passive or active. The question of peripheral 

 change is complicated in each case by the passive effects produced by 

 general changes. Owing to this fact, it is usually necessary to record 

 the aortic and vena cava pressures simultaneously with the observations 

 of the organ which are made by one or other of the above methods. 



If the general venous and arterial pressures remain constant, while 

 the circulation in the organ varies, then unequivocal evidence is obtained 

 of the existence of a vasomotor mechanism in that organ. Such is the 

 case also if the circulation in the organ vary in the opposite sense to 

 the changes which take place simultaneously in the general circulation. 



In many cases it is necessary to abolish, as far as possible, all 

 passive effects produced by changes in the general circulation. 



Thus, to exclude the effect of the respiratory pump, the animal can 

 be curarised and a constant artificial respiration supplied. In the case 

 of the brain or fore-limbs, the circulation, by ligature of the thoracic 

 aorta and vena cava inferior, can be limited to these parts; thus all 

 changes occurring in the splanchnic circulation can be eliminated. The 

 same result can be arrived at by section of the spinal cord above the 

 origin of the splanchnic vasomotor nerves. The splanchnic vasomotor 

 mechanism is by far the most powerful in the body, and in investigations 

 on the limbs, lungs, or brain, it is absolutely necessary to eliminate its 

 action in order to obtain unequivocal evidence for or against a vasomotor 

 supply to these parts. Attempts have been made to avoid all these 

 difficulties by the establishment of an artificial circulation through the 

 particular organ under investigation. Vasomotor tone is, however, so 

 easily abolished by alterations in the temperature and quality of the 

 blood, that this method has as yet neither proved trustworthy nor of 

 any particular value. 



To distinguish the existence of vaso-constrictor and vaso-dilator 

 fibres, running in one common nerve trunk, special means must be 



