82 DISEASES OF CATTLE, 



The circulation is. considered as two systems: The pulmonary, the 

 lesser, is that from the right heart to the lungs, and back to the left 

 In-art. The systemic, or greater, is from the left heart to all parts of 

 the body, and back to the right heart. To simplify the subject it niay 

 be fiaid at once that the blood of the body is of two colors 

 bright red, or pure blood, and dark red, or impure blood. All the; 

 arteries, except the pulmonary artery and its branches, carry bright 

 red ; and all the veins, except the pulmonary veins, carry dark-red blood. 

 The impure dark-red blood is collected from the capillary vessels and 

 carried to the right auricle by the veins 5 it pusses through the auriculo- 

 ventricular openings into the right ventricle and thence into the pul- 

 monary artery, and through its branches to the capillaries of the lungs, 

 where the carbonic acid gas and other impurities are given up to the 

 air in the air-cells of the luogs (through the thin walls between the 

 capillaries and air-cells), and where it also absorbs from the air the 

 oxygen gas necessary to sustain life, which changes it to the bright red, 

 pure blood. It passes from the capillaries to the branches of the pul- 

 monary veins, which convey it to the left auricle of the heart; it then 

 passes through the auriculo- ventricular opening into the left ventricle, 

 the contraction of which forces it through the common aorta into the 

 posteiior and anterior aortas, and through all the arteries of the body 

 into the capillaries, where it parts with its oxygen and nutritive ele- 

 ments, and where it absorbs carbonic acid gas and certain other impuri- 

 ties and becomes dark colored. (See theoretical diagram of the circu- 

 lation, Plate vn.) 



The muscular tissue in the walls of the arteries, under the stimulus 

 of the vaso-niotor nerves, regulates the caliber of the vessels and the 

 amount of blood supplied to the different parts, and, on account of its 

 contractibiUty, it assists in controlling a hemorrhage when an artery is 

 cut completely across, by causing the retraction of the severed ends of 

 the vessel. 



The flow of blood through the capillaries is very slow, not much more 

 than an inch a minute. The power which forces it through them is the 

 heart; and many physiologists maintain that the changes in the blood 

 which take place in the capillaries favor and compel the blood to flow 

 through them, and as a proof they refer to the fact that arteries are 

 found empty after death, because the capillaries have sent the blood 

 into the veins. On this theory, capillary circulation may be compared 

 to oil rising in the wick of a lamp to burn as the demand requires. 



The blood is caused to flow through the veins to the heart by several 

 different forces. The contraction of muscles in proximity to veins 

 causes a pressure on them which assures -an onward movement to the 

 heart, since the valves in the veins prevent its backward movement. 

 At each inspiration (or taking in a breath, there is a suction-like action 

 of the chest, which induces the flow to the right auricle. The heart 

 probably exerts P, force from behind which assists the flow in the veins. 



