The relation between secretory and capillary pressure. I.—The salivary secretion

Since Ludwig made the discovery that the secretory pressure of a gland may double that of the arterial pressure when the outflow of saliva is obstructed, no one, so far as we know, has investigated the circulatory conditions, in the gland under these circumstances. This has been the object of the present research. Our method is as follows:—We place a cannula in the duct of the submaxillary gland of the cat or dog and prepare the chorda tympani nerve for excitation. A second cannula is placed in the carotid artery of the opposite side of the neck. Each cannula is connected, either with a mercurial manometer or, as in our latest experiments, with two Leonard Hill pocket sphygmometer gauges. This gauge consists of a thick-walled glass tube with a fine capillary lumen closed at one end where the lumen expands into a small air chamber. Half an inch from the open end there is a side hole. On placing this end in a solution of potash a fluid meniscus rises to the side hole, which marks the zero of the instrument. (Potash is used to keep the tube free from grease.) One end of a piece of rubber is slipped over the open end of the gauge so as to cover the side hole, and the other end then connected with the cannula. The pressure of the saliva or blood forces the meniscus up the gauge, which is graduated in millimetres of mercury and acts as a spring manometer. We find these gauges very convenient to use as they can be placed side by side and the readings compared at a glance. Before making the connections with the gauges we expose the veins which course over the submaxillary gland and contribute to the formation of the external jugular vein. Having found the vein which issues from the gland we tie all the other veins, leaving this one free so that at the right moment we can clip the external jugular and open it so as to observe the outflow of blood from the gland. When all is thus prepared we excite the chorda tympani nerve. As soon as the secretory pressure rises above the arterial pressure we open the vein and observe the flow of blood. We find that under these conditions the blood continues to flow and issues from the vein in a fairly ample stream of a colour more arterial than venous. The gland itself feels tense to the touch. By squeezing the gland we find that we can further raise the pressure of the secretion, while at the same time we impede the outflow of blood. On allowing the secretory pressure to fall to atmospheric pressure we find that the outflow from the vein becomes much ampler; in one experiment it was approximately doubled,e. g. 40 drops in 15 seconds as compared with 27 drops in 20 seconds at the highest secretory pressure (240 mm. Hg.). In some experiments we observed the venous outflow from the time we began to excite the chorda tympani. Under these circumstances we find that the outflow from the gland at rest is very slow and the blood venous in colour. On stimulating the chorda, the outflow becomes very ample, the blood arterial in colour, and remains so during the rise of secretory pressure until this pressure rises higher than the arterial, when the outflow becomes lessened and the blood less arterial in colour.