Abstract
In experiments upon the cardio-vascular responses following repeated occlusions and restorations of the circulation of the head arteries in cats, it has been repeatedly observed that the spinal level of blood-pressure, that is, the level of pressure maintained by the activity of the spinal cord alone, when the functional activity of the medulla has been eliminated, may vary between 35 and 75 millimeters of mercury.
Certain criteria are necessary to determine whether blood-pressure within this range is actually spinal pressure, since in some animals, a pressure as low as 50 or 60 mm. is sufficient to restore medullary activity, whereas in other animals, a pressure to 70 to 75 mm. may be ineffectual in bringing this about. Blood-pressure, then, may be considered spinal when
(1) its level is unaffected by occlusion or release of the head arteries.
(2) no signs of any change in level of pressure or return of any medullary reflex is observable from half to three quarters of an hour after circulation has been restored to the head arteries.
With these conditions in mind, a series of experiments was carried out, with varying numbers of successive occlusions, from 1 to 14, in normal cats. In these animals, at some stage in the occlusion series, the head arteries were permanently ligated following an anaemic rise and fall of blood pressure. Blood pressure recorded on the chart in each case, is the mean pressure.
In these experiments, it was found that the height of the spinal pressure 'has a definite relationship to the number of occlusions which have been done on the cat, as the accompanying curve shows. That is, when the first or second occlusion is permanent, spinal pressure is about 70, whereas, when the 13th or 14th occlusion is permanent, the spinal pressure is about 20 mm.
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