208 A TEXTBOOK OF PHYSIOLOGY 



question. Thus the effect on the blood-flow of vaso-motorial excite- 

 ment or functional activity can be investigated. 



By means of the dromograph it has been shown that, during 

 systole, the blood is checked by the compressive action of the cardiac 

 muscle in its flow from the aorta towards the coronary arteries; 

 that the velocity of flow in the carotid is five or six times greater 

 when a horse is actively masticating than when at rest. The normal 

 velocity in the carotid artery during systole was found to be 540 milli- 

 metres per second. After section of the cervical sympathetic, how- 

 ever, in consequence of the peripheral dilatation, the velocity becomes 

 equal to 750 millimetres per second. After section of the spinal cord 

 in the upper dorsal region, the velocity in the arterial system becomes 

 greatly accelerated during systole, and greatly diminished during 

 diastole. Owing to the lowering of the peripheral resistance from 

 the loss of vascular tone, the heart is able to discharge the blood with 

 greater rapidity into the venous system, and in consequence, during 

 diastole, the arterial system is emptied of blood to a great extent. If 

 this condition be pushed to an extreme, and the frequency of the heart 

 be small, the blood becomes discharged into the veins intermittently. 

 Comparative records have been obtained of the velocity of flow 

 in the carotid and facial arteries. At the end of the diastole the 

 velocity is small in the carotid, and relatively great in the facial 

 artery. In the beginning of the systole the primary wave of velocity 

 rises rapidly in the carotid, and is proportionatel} 7 small in the facial 

 artery. The secondary increase of velocity, which is produced by 

 the dicrotic wave, is far more evident on the carotid than on the facial 

 curve. These results show how the intermittent energy of the heart is 

 stored up by the elasticity of the large arteries, and expended in 

 maintaining a continuous flow through the small arteries. . 



The flow in the large veins is approximately equal to that in the 

 large arteries. In the jugular vein of a dog the mean velocity was 

 found to be 225 millimetres, and in the carotid 260 millimetres, per 

 second. The velocity in the capillaries has been measured by direct 

 observation with the microscope. It is very small e.g., 0-5 to 1 milli- 

 metre per second. This variation of velocity in different parts of the 

 vascular system is explained by the difference in the width of bed 

 through which the stream flows. The vascular system may be com- 

 pared to a stream which, on entering a field, is led into a multitude 

 of irrigation channels, the sum of the cross-sections of all the channels 

 being far greater than that of the stream. The channels gradually 

 unite together again, and leave the field as one stream. If the flow 

 proceeds uniformly for any given unit of time, the same volume must 

 flow through any cross-section of the system. Thus the greatest 

 velocity is where the total bed is narrowest, and slowest where the bed 

 widens to the dimensions of a lake. 



Any general change in velocity at any section of this circuit tells 

 both backwards and forwards on the velocity in all other sections, 

 for the average velocity in the arteries, veins, and capillaries, these 

 vessels being taken respectively as a whole, depends always on the 

 relative areas of their total cross-sections. 



