The blood-pressure is usually estimated in animals by connecting a large artery, such as the carotid or femoral, with a bent tube containing mercury by means of a connecting tube, which is filled with a solution of carbonate of sodium to prevent coagulation. The pressure is estimated by the height at which the mercury stands in the outer limb of the tube. The height may either be read off with the eye, or, what is much better, it may be registered on a revolving cylinder by means of a long float which rests upon the surface of the mercury, and bears on its upper end a brush or pen. This method, which is important both in itself and as being the introduction of the graphic method into physiology, we owe to C. Ludwig. The apparatus is known as the kymograph.

Tracings may be taken upon paper with a varying speed : it is usual to take them upon paper travelling rapidly, so that quick and small oscillations due to the cardiac beats may not be lost or obscured by fusion. The great disadvantage of this is that it is impossible to use the curves directly: they must be reduced, and this is a work requiring much time and labour. When taken on a slowly revolving cylinder we get the general results of the action of a drug on the blood-pressure shown us at a glance; and its effects on the form and rapidity of the pulse may by a little arrangement be recorded from time to time on another cylinder revolving more rapidly.

This method gives us both the blood-pressure and the oscillations which it undergoes on account of the cardiac pulsations and respiration. If we wish to get the mean blood-pressure unaffected by these oscillations, it is done by simply narrowing at one point the calibre of the tube containing the mercury, either by a stopcock, or by reducing the tube to a capillary bore.