116 THE HORSE IN HEALTH AND DISEASE 



the aorta is equal to that of a column of blood 10 feet in height. 

 Smith states that at each stroke about 2.25 pounds of blood are 

 pumped into the aorta. From these figures we may compute that 

 the work of the left ventricle with each systolic movement is 

 equivalent to raising 22.5 pounds one foot. This should give some 

 idea of the energy expended by the body in simply sustaining the 

 circulation in a state of repose. 



The pressure and rate of flow of the blood in the veins is very 

 slight as compared with that in the arteries. They are aided by 

 the following factors: 



(1) Respiratory movements, as was explained in the chapter on 

 Respiration, result in expansion and contraction of the thorax. 

 During inspiration the blood is sucked into the large intrathoracic 

 veins in much the same way that the air is into the lungs. The 

 greater thickness of the walls of the arteries prevents in large 

 measure this effect on them. During expiration the reverse action 

 would take place, and the blood would be forced back into the 

 tributary veins if they were not provided with valves. As there 

 are no valves in the pulmonary veins and those opening into the 

 right auricle, their contents run into the upper chambers of the 

 heart. 



(2) Muscular contractions result in a shortening of muscles. 

 This brings about compression of the veins which lie between 

 them and forces the blood from the smaller into the larger veins, 

 the valves, as already stated, preventing any backward flow. 

 The more rapidly a muscle contracts, the greater is the amount 

 of blood circulating through it. 



From these facts it is shown that good circulation is made 

 possible only by exercise. When this is provided, the horse must 

 breathe deeply and flex and extend the skeletal muscles, so that 

 the blood is pumped out of the veins and the lymph is made to 

 flow more freely in the lymph-vessels. 



THE PROPERTIES OF THE BLOOD AND LYMPH 



The blood is a red fluid, alkaline in reaction, with a peculiar 

 odor thought to be due to a volatile fatty acid. Its specific 

 gravity is 1060. When drawn from an artery the blood has a 

 bright scarlet color. Venous blood is dark red in color, but on 

 standing in the air readily becomes scarlet, due to the absorption 

 of oxygen. Blood clots almost immediately when exposed to 



