244 STRUCTURE OF THE VERTEBRATES 



The lymphatic sj^stem also functions in collecting the fats 

 absorbed by the digestive tract. Each villus which projects into 

 the lumen of the intestine is supplied with a minute lymphatic 

 duct in addition to the vein and artery. The fats pass through 

 the lining epithelium into the lacteals, or Ij^mphatic ducts of 

 the villi, and through them into the main lymphatic stream. The 

 whitish, fat-ladened fluid is known as chyle and the lymphatics 

 from the intestine are frequently called chyle ducts. As a result 

 of these two functions of the lymphatic system, the lymph enter- 

 ing the veins has a high content of fats and waste products, and 

 a low content of oxygen, carbohydrates and proteins. 



Both in phylogeny and ontogeny there is a reduction in the 

 extent of the lymphatic system. In the primitive vertebrate and 

 in the embryo there are large irregular lymph sinuses, and these 

 become reduced to a definite system of vessels, the system oc- 

 cupying relatively less space in the more specialized animal. 



In the cyclostomes and fish the lymphatic capillaries collect 

 into definite vessels which empty into large sinuses connecting 

 with each other. The largest is below the vertebral column, lying 

 around the dorsal aorta. Others are in the ventral body wall and 

 underneath the skin. A single lymphatic vessel drains each of 

 the sinuses, and these efferent ducts fuse to form four main 

 ducts which empty into the veins. The anterior two join the 

 veins at the junction of the jugulars with the subclavians, and 

 the posterior two enter either the caudal or the iliac veins. At 

 the point where the lymphatic vessel leaves the sinus there is 

 a pulsating lymph heart forcing the lymph toward the veins. 

 Each is protected by valves w^iich keep the fluid flowing in the 

 same direction. 



The lymph hearts are retained by the amphibia, particularly 

 the urodeles. In the anura (frogs and toads) the hearts are 

 definitely smaller and the anterior hearts disappear. Almost the 

 same condition is found in the lower reptiles. The crocodiles and 

 alligators retain the two posterior hearts, but the large dorsal 

 sinuses are reduced to thoracic lymph ducts. The birds lose the 

 lymph hearts entirely in the adults, although there are both 

 anterior and posterior connections with the venous system. In 

 both reptiles and birds there is an increase in the number of the 

 smaller lymphatic vessels, and the system takes on the appear- 



