36 THE AEMY HORSE IN ACCIDENT AND DISEASE. 



and part of the chest. It is located in the front pait of tlie thorax, 

 below the trachea, and between the right and left pleuras, and empties 

 into the right auricle. 



The jugular veins (right and left) are the largest branches of the 

 anterior vena cava and collect the blood from the head and neighbor- 

 ing parts. Just below and back of the lower jaw they approach the 

 carotid arteries and run down the neck in their company. Each 

 jugular is outside of the corresponding carotid and the two are sepa- 

 rated by a thin muscle. The jugular veins in their descent follow 

 grooves at the side of the neck (jugular furrows), and at first are 

 close to the surface and easily felt; they soon take a deeper course, 

 running beneath the panniculus carnosus muscle. They enter the 

 front part of the thorax, where they empty into the anterior vena 

 cava just in front of the heart. 



The posterior vena cava is the main vein returning the blood from 

 the hind parts and from the abdominal and pelvic organs. It corre- 

 sponds to the posterior aorta, which, as has been seen, is the main 

 artery carrying the blood to these parts. This vein is formed at the 

 front part of the pelvis and runs forward under the lumbar vertebra, 

 accompanying the posterior aorta, which is at its left. When it 

 reaches the upper border of the liver it inclines downward and passes 

 through a notch or fissure of that organ. Thence it passes through 

 the diaphragm into the thoracic cavity; here it follows a groove on 

 the upper surface of the right lung and then enters the right auricle 

 of the heart. 



The important veins of the foot will be discussed later. 



ANATOMY AND PHYSIOLOGY OF THE LYMPHATIC SYSTEM. 



The lymphatic or absorbent system resembles the system of blood 

 veins with which it is connected. The main part of the system col- 

 lects surplus lymph (to be described later) and returns it to the blood ; 

 a smaller part has the same fimction, but, in addition, absorbs and 

 collects chyle and adds it to the blood. 



When the blood in its circuit reaches the capillaries the serum. 

 oozes through their thin walls into the minute spaces in the surround- 

 ing tissues and there receives the name of lymph. This colorless 

 fluid bathes and nourishes the tissues and takes up worn-out material. 

 The spaces in the tissues assemble into minute, delicate, and trans- 

 parent vessels (lymphatics), which are remarkable for their knotted 

 appearance, due to numerous valves. The vessels join and increase 

 in size, like veins, and through them flows the surplus lymph with 

 its collected waste material. 



The vessels of the right fore extremity, the right side of the head, 

 neck, and thorax, form tubes uniting in a main trunk, called the 

 right lymphatic vein, which leads into the anterior vena cava; the 



