All these physiological variations in the number of the blood corpuscles, are dependent, according to Cohnstein and Zuntz, on vasomotor influences. Stimuli, which narrow the peripheral vessels, locally diminish the number of red blood corpuscles; excitation of the vasodilators brings about the opposite effect. Hence it follows, that the normal variations of the number contained in a unit of space are merely the expressions of an altered distribution of the red elements within the circulation, and are quite independent of the reproduction and decay of the cells. =Climatic conditions= apparently exercise a great influence over the number of corpuscles. This fact is important for physiology, pathology, and therapeutics, and has come to the front especially in the last few years, since Viault's researches in the heights of the Corderillas. As his researches, as well as those of Mercier, Egger, Wolff, Koeppe, v. Jaruntowski and Schroeder, Miescher, Kuendig and others, shew, the number of red blood corpuscles in a healthy man, with the normal average of 5,000,000 per mm.^{3}, begins to rise immediately after reaching a height considerably above the sea-level. With a rise proceeding by stages, a new average figure is reached in 10 to 14 days, considerably larger than the old one, and indeed the greater the difference in level between the former and the latter places, the greater is the difference in this figure. Healthy persons born and bred at these heights have an average of red corpuscles which is considerably above the mean; and which indeed as a rule is somewhat greater than in those who are acclimatised or only temporarily living at these elevations. The following small table gives an idea of the degree to which the number of blood corpuscles may vary at higher altitudes from the average of five millions. -------------------+---------------+-------------------+------------- Author | Locality | Height above sea- | Increase of | | level | -------------------+---------------+-------------------+------------- v. Jaruntowski | Goerbersdorf | 561 metres | 800,000 Wolff and Koeppe | Reiboldsgruen | 700 " | 1,000,000 Egger | Arosa | 1800 " | 2,000,000 Viault | Corderillas | 4392 " | 3,000,000 -------------------+---------------+-------------------+-------------