For some time we have made intra-cranial pressure the subject of an enquiry. The present paper deals with the effect upon the circulation and respiration of an increase of such intra-cranial pressure. Our object is to indicate how the degree of the intracranial tension may be estimated by changes in the pulse and breathing, whether a prognosis may be formed as to the termination of the condition, and whether the effect is due to gradual loss of function of the lower part of the medulla. Until lately, the pulse and breathing have yielded signs, the meaning of which was uncertain; thus the pulse, it is said, may be slow and of low tension, or again, in severe cases, quicker than normal and of high tension. The respiration also, is sometimes noted as being stertorous, or of the “Cheyne-Stokes” rhythm. But although death frequently comes by arrest of respiration in tumours of the brain, whilst the heart may continue to beat for five, ten, or even thirty minutes after, little has been recorded concerning the abnormal rhythm of respiration preceding its arrest; we therefore arranged our experimental method so as to investigate these phenomena in their entirety. It is impossible to give a full explanation of all the results we have obtained, because the functions of the medulla are not yet fully understood, and therefore, whatever hypotheses we have advanced on these points in this paper, have been inserted merely for the sake of clearness, and are fully open, of course, to future revision. However, the recorded tracings of all the experiments show so clearly that a diminished activity of the medulla occurs as a definite sequence of events contemporaneously with increase in the intra-cranial pressure, that we regard this fundamental fact to be established; therefore the value of the results, as regards the estimation of the effect of increased pressure, is independent of exact knowledge of the mode of their production. Further, we believe that our experiments have an important general bearing, in that they show how the three “centres” regulating the heart rate, the blood pressure, and the respiration, can be impeded or arrested, either together, or almost separately, and consequently the varying influences they have the one on the other can be estimated with an approximation to accuracy.