CHAP, iv.] THE VASCULAR MECHANISM. 127 



In the horse, Volkmann found the velocity of the stream 

 to be in the carotid artery about 300 mm., in the maxillary artery 

 165 mm., and in the metatarsal artery 56 mm. in the second. 

 Chauveau determined the velocity in the carotid of the horse to 

 vary from 520 to 150 mm. per sec. at each beat of the heart, flow- 

 ing at the former rate during the height of each pulse-expansion, 

 and at the latter in the interval between each two beats. Ludwig 

 and Dogiel found the velocity in the dog and in the rabbit to vary 

 within very wide limits, not only in different arteries, but in the 

 same artery under different circumstances. Thus while in the 

 carotid of the rabbit it may be said to vary from 100 to 200 mm. 

 per sec., and in the carotid of the dog from 200 to 500 mm. per sec., 

 both these limits were frequently passed. 



3. The Flow in the Veins. 



When a vein is severed, the flow from the distal cut end (i.e. the 

 end nearest the capillaries) is continuous, the blood is ejected with 

 comparatively little force, and with no great velocity. 



When a vein is connected with a manometer, the lateral pressure 

 is found to be very small ; it is greater in the veins farther from 

 the heart than in those nearer the heart,. In the former it is much 

 less than that of the small arteries, and in the latter amounts only 

 to a few millimetres of mercury. Indeed in the immediate neigh- 

 bourhood of the heart the pressure may (during the inspiratory 

 movement) become negative, i.e. when the manometer is brought 

 into connection with the interior of the vein, the mercury in the 

 distal limb falls, instead of, as in the case of an artery, rising. 



In the case of most veins, under ordinary circumstances the 

 mercury of a manometer connected with a vein does not shew any 

 of those pulse -oscillations which are so striking in the arteries. As 

 a general rule the pulse is seen on the arterial side only of the j J^ 

 capillaries, though in special cases, under conditions which we shall 

 study presently, it may make its way through the capillaries from 

 the arteries to the small veins ; and it is probable that in general 

 a slight impulse does make its way right through the capil- 

 laries, but so feeble that it cannot be recognised by ordinary in- 

 struments save in special cases. Moreover, in the great veins near 

 the heart, under certain circumstances at all events, the movements 

 of that organ may make themselves felt as a so-called 'venous pulse' 

 transmitted in a backward direction along the veins from the heart. 

 But these exceptional instances and these recurrent oscillations do 

 not invalidate the truth of the general statement that the pulse is 

 absent from the veins. The exact determination of venous pressure 

 is attended with great experimental difficulties, and our knowledge in 



