64 A MANUAL OF VETERINARY PHYSIOLOGY 



be seen, for instance, at post-mortem examinations. The veins 

 as they pass from the capillaries towards the heart become 

 reduced in number and increased in size, and they terminate 

 in the right auricle of the heart by means of two trunks, the 

 united areas of which greatly exceed that of the aorta. 



In the veins valves are found. These are well marked in the 

 veins of the head, neck, and extremities. The valves look 

 towards the heart, and supply a simple and essential means of 

 insuring the return flow of the blood along the veins to the heart. 

 In certain places the veins have no valves, such as the large veins 

 entering the heart, those of the bones, the abdominal veins, and 

 the veins of the foot and brain. 



Veins are normally pulseless, but under certain conditions a 

 pulse-wave may pass through the capillaries into the veins, pro- 

 ducing a venous pulse. The best physiological example of this 

 form of pulse can be experimentally produced by stimulation 

 of the chorda tympani, a nerve supplying the submaxillary gland 

 with fibres which cause the bloodvessels to dilate. Under 

 stimulation the vessels dilate, the veins pulsate, and even the 

 blood coming from them is arterial in colour. Another form of 

 venous pulse is met with in the great veins at the root of the 

 neck ; the mechanism of the pulsation in these has already been 

 explained (see p. 38). It is abnormal for pulsations to extend 

 any distance up the jugular vein ; when this occurs the explana- 

 tion is pathological, not physiological. 



Mechanics of the Circulation. — At each systole of the ventricle 

 a certain amount of blood is forced under great pressure into 

 an already full aorta, and imprisoned there by the closure of the 

 aortic valves. The aorta dilates to receive this extra blood, 

 because, owing to the friction in the smaller vessels, or, as we 

 shall speak of it, the peripheral resistance, it is impossible for the 

 amount pumped into the aorta at each systole to pass out at 

 once at the periphery ; in this way high blood-pressure is pro- 

 duced in the arteries. The increase in the size of the aorta to 

 accommodate this extra blood commences near the heart, and 

 runs as a wave to the periphery ; this wave is the pulse. 



The two important points in the circulation which we have 

 now to consider are blood-pressure and pulse, and to understand 

 these it is necessary that we should study briefly the laws which 

 govern the flow of fluids through tubes.* 



If water be pumped through a rigid tube or pipe, at every 

 stroke of the pump as much fluid passes out at the further end 



* The subject of the movement of fluids in tubes is not only an extremely 

 difficult branch of physics, but one still imperfectly understood. We have 

 introduced less of it into this chapter than appears in most works on 

 physiology, and, in fact, have only touched on those general principles 

 which have a direct bearing on the circulation. 



