This caused considerable interest in the observation of the pulse, and the sphygmograph was supposed to be of the greatest importance for medical diagnosis. Burdon Sanderson,[40] Landois, Lorain,[41] Ozanam,[42] Pfungen,[43] Riegel,[44] Roy and Adami,[45] and others have studied the sphygmographic curve under abnormal conditions, and wellnigh all diseases have been studied by these observers with the sphygmograph. The results were ambiguous and did not seem to justify the amount of work spent on these observations. The enthusiasm for the sphygmograph subsided, and it was no longer expected to obtain a diagnosis, or even, indeed, a prognosis of a disease from mere inspection of a pulse curve. Later investigators, in fact, confined their research to the proof of the ambiguity of the sphygmograms, which could be valuable only in connection with other observations. It could not be hoped that an explanation of the abnormalities of the pulse curve would be found before an understanding of the normal form was attained. It, therefore, seemed necessary to decide between two theories of the origin of the normal pulse curve, which had opposed each other almost since the discovery of the existence of the dicrotic elevation. Both theories chiefly refer to the origin of the dicrotic, and they agree on this, that the dicrotic elevation is due to a wave travelling in the blood, but they disagree on the direction in which this wave is moving. These two theories may be called the theory of the peripheral, and the theory of the central origin of the dicrotic wave. The theory of the peripheral origin of the dicrotic wave assumes that the change of pressure which is indicated by the dicrotic elevation originates somewhere at the periphery and travels through the arteries towards the heart.[46] Commonly it is assumed that the dicrotic originates in the arterioles. This theory has been mentioned first, because it is the simpler in every respect, though the less probable. The origin of the dicrotic wave according to this theory is similar to the origin of the echo. Buisson was the first who gave an explanation of the dicrotic elevation by assuming a central origin of this wave. His theory was adopted by Marey, who stated it in this way. The action of the heart causes the blood to be pumped into the aorta with considerable strength. The blood leaves the aorta by its inertia and expands the arterial system. In the arterioles it finds an obstacle and being reflected it flows back to the aorta. But there it finds the semilunar valves closed and a new wave is produced by reflection. This wave has an effect similar to the first, and this reflection of waves lasts until the valves are thrown open again. The existence of several secondary waves is explained by the great velocity with which the blood travels through the arterial system.[47]