CIRCULATION. 79 



Rapidity of the Circulation. — By experiment the shortest time has been 

 measured which is taken by a particle of blood in passing from a point in the 

 external jugular vein of a dog to and through the right cavities of the heart, 

 the pulmonary vessels, the left cavities of the heart, the commencement of 

 the aorta, and the arteries, capillaries, and veins of the head, to the starting- 

 point, or to the same point of the vein of the other side. This time has 

 been found to be from fifteen to eighteen seconds. Naturally, the time would 

 be different in different kinds of animals and in the different circuits in the 

 same individual. 



Order of Study of the Mechanics of the Circulation. — The significance 

 and the fundamental facts of the circulation have now been indicated. Its 

 phenomena must next be studied in detail. 1 . As the blood moves in a circle, 

 we may, in order to study the movement, strike into the circle at any point. 

 It will, however be found both logical and instructive to study first the move- 

 ment of the blood in the capillaries, whether systemic or pulmonary. It is 

 only in passing through these and the minute arteries and veins adjoining that 

 the blood fulfils its essential functions; elsewhere it is in transit merely. 

 Moreover, it is only in the minute vessels that the blood and the nature of 

 its movement are actually visible. 



After the capillary flow shall have become familiar, it will be found that 

 the other phenomena of the circulation will fall naturally into place as indi- 

 cating how that flow is caused, is varied, and is regulated. 



B. The Movement of the Blood in the Capillaries and in the 

 Minute Arteries and Veins. 



Characters of the Capillaries. — Each of the vessels which compose the 

 immensely multiplied capillary network of the body is a tube, commonly of 

 less than one millimeter in length, and of a few one-thousandths only of a 

 millimeter in calibre, the wall of which is so thin as to elude accurate measure- 



Fig. 10.— A capillary from the mesentery of the frog (Ranvier). 



ment. The calibre of each capillary may vary from time to time. These 

 facts indicate the minute subdivision of the blood-stream in the lungs, and 

 among the tissues — that is, at the two points of its course where the essential 

 functions of the blood are fulfilled. These facts also show the shortness of 



1 The following is a very valuable book of reference: Kobert Tigerstedt: Lehrbuch der 

 Physiologie des Kreislaufes, 1893. 



