662 SPECIAL PHYSIOLOGY. 



from observations made upon animals. Two kinds of instruments 

 have been employed in such observations. One, the hcemadromometer 

 of Volkmann, consists of a bent U-shaped glass tube, having its ends 

 fitted into a short, straight, hollow metallic mounting, placed at right 

 angles to it. By means of stopcocks, a free passage can be main- 

 tained, either through the straight portio-n of the apparatus only, or 

 through the bent U-shaped part. When the two ends of the straight 

 portion of this instrument are fastened into the cut ends of the 

 divided carotid artery of a dog, the arterial blood current may either 

 be allowed to flow through the straight portion, or it may be suddenly 

 diverted, by changes in the stopcocks, through the bent or U-shaped 

 part. The rate of motion of the blood through the latter, being ob- 

 served, and the length of this tube being known, the velocity of the 

 blood-current is ascertained. Another instrument, the hcematochometer 

 of Vierordt, is composed of a small square box or cell, made with glass 

 sides, filled with water, and having an aperture of entrance and one of 

 exit, each fitted with a tube ; to these tubes, the cut ends of a divided 

 artery are attached. Within the box is a fine pendulum, carrying, in 

 order to aid the observation of its movements, a disc of silver, which, 

 when the pendulum is at rest, hangs close to the aperture of entrance. 

 A curved graduated scale is marked on the side of the vessel. When 

 the arterial blood is permitted to flow into this box, it raises the pen- 

 dulum with a velocity corresponding with that of its own motion, and 

 which is at once measured by the graduated scale. According to 

 Yierordt, the mean velocity of the blood in the carotid of a horse is 

 11.7 inches, of the dog 10 inches, and of the calf 9 inches per second ; 

 the calculated velocity in the aorta of the horse is 12.5 inches, and in 

 the human carotid, rather more than 10 inches per second. Accord- 

 ing to Volkmann, the velocities for the carotid artery in these animals, 

 are a little higher ; but in the metatarsal artery of the horse, only 2.2 

 inches per second. By means of the tachometer of Chauveau, a mod- 

 ification of Vierordt's instrument, it is shown that a great difference 

 in the velocity of the blood-current, exists during the systole and 

 diastole of the left ventricle ; for during the systole, in the horse, the 

 velocity is about 20J inches, and during the diastole only 8-f- inches 

 per second. The velocity of the blood in the arteries is, moreover, 

 diminished during inspiration, and increased during expiration. 



From the preceding figures, it appears that the rate of motion of 

 the blood in the arteries is quickest near the heart, gradually becom- 

 ing slower in the more distant vessels. First, the effect of the heart's 

 action is diminished by the resistance offered, by friction and adhe- 

 sion, to the passage of the blood through the arteries and capillary- 

 vessels ; this fractional resistance, though rendered as slight as possi- 

 ble by the smooth lining membrane of the arteries, is increased by the 

 curvature, by the angular bending, and by the frequent subdivision, 

 of the arteries, by an unusual rigidity of the walls of the arteries, and 

 by any alteration in the viscidity of the blood, or in its nutritive 

 attractions for, ov relations with, the capillary walls and the tissues 

 beyond them. All these conditions, therefore, tend to retard the 

 velocity of the blood-current, by an increase of resistance and friction. 



