THE CIRCULATORY SYSTEM 131 



drains away quite gradually, eventually losing its identity. 

 The larger lymph channels are provided with simple valves to 

 prevent a backward flow of the lymph. These channels conduct 

 the lymph from nearly all parts of the body to the great reservoir 

 located in sublumbar region known as the receptaculum chyli. 

 The thoracic duct conducts the lymph from this reservoir to the 

 anterior vena cava at the junction of the jugular veins near the 

 base of the heart (see Fig. 31). This duct in the horse is about 

 half an inch in diameter but is not of uniform caliber throughout 

 its length. 



The lymph-glands lie along the course of the lymph channels. 

 Each organ of the body is provided with one or more lymph- 

 glands. They act as filters for the lymph, and are very important 

 structures in maintaining the health. The bronchial lymph- 

 glands are located at the bifurcation of the bronchi, and are 

 usually discolored from the large amounts of very minute parti- 

 cles of carbon and dust which they have caught. In tuberculosis 

 and some other diseases, the lymph-glands early become infected 

 with the microorganisms which they have enmeshed, and become 

 greatly enlarged. 



The circulation is the term applie'd to the entire circuit which 

 the blood must traverse in its course from a given point and re- 

 turn. In reality the blood makes a double circuit; that from the 

 right ventricle to the left auricle being known as the pulmonary 

 or lesser circulation, and that from the left ventricle to the right 

 auricle the systemic or greater circulation. The general arrange- 

 ment of the organs and course of the circulation is shown in Fig. 

 42. All the blood in the body circulates through a man's lungs 

 once every twenty- three seconds, in the horse it takes a little 

 longer. More than three-quarters of all the blood is probably 

 in the systemic circulation at any given moment. 



THE PHYSIOLOGY OF CIRCULATION 



The method by which the blood circulates was not discovered 

 until 1621, when Harvey showed how the blood flows and the 

 heart functions. Before that time it was thought that the blood 

 ebbed and flowed like the tide. Through the constant contrac- 

 tion and relaxation of the heart muscle the circulation of the 

 blood is maintained. 



The heart movements may be best studied by removing the 

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