g-I Florence E. Sabin. 



capillaries gradually invade the body in orderly sequence in definite 

 and characteristic zones and layers. The growth 'is always in the 

 capillary bed, that is, all lymphatics develop as capillaries, and the 

 earliest ones to develop become the first lymph trunks or ducts. Third, 

 the method of growth throughout is by the sprouting of the proto- 

 plasm and the nuclear division of the endothelial of the capillary wall. 

 The ultimate capillaries are distinguished by the absence of valves. 

 In a word, there is a continuous growth and invasion of the body by 

 lymphatic capillaries from the primary sprouts which bud off from 

 the veins to the ultimate peripheral lymphatic capillaries. 



IV. PRIMARY LYMPHATIC SYSTEM IX DIFFERENT 



FORMS. 



1. Primary Lymphatic Sacs ix the Pig. 



A. general nummary of the sacs. 



I shall begin the section on the special anatomy of the lymph sacs 

 by a detailed description of the origin of the lymphatics in the pig, 

 since that is the best known form. I can now describe the origin of 

 all of the lymphatic sacs in the pig and give the relation of the main 

 ducts to all of these primitive sacs. On the basis of our knowledge of 

 the lymphatic system in this form I shall give what is known of the 

 lymphatic system in other mammals, in birds, reptiles and amphibia, 

 and shall then be in a position to compare the primary lymph sacs in 

 mammals with the amphibian lymph hearts and sacs. 



In the pig the lymphatics bud off from the veins in two places, from 

 the anterior cardinal veins and from the veins of the Wolffian body. 

 There are two sets of paired sacs, the jugular and the iliac; and two 

 unpaired sacs, the retroperitoneal and the cisterna chyli. In the most 

 general terms the jugular sacs drain the anterior half of the body; 

 the iliac sacs drain the posterior half of the body, while the retroperi- 

 toneal or prse-aortic sac drains the viscera. The cisterna chyli with 

 the thoracic duct connects the jugular and renal lymphatics. In more 

 specific terms the jugular lymphatics bud off from the anterior cardi- 

 nal veins, form large sacs in the neck, from which lymphatics grow 

 to the head and neck, the foreleg and thorax, and to the heart and 

 lungs. From the left sac arises the jugular stem of the thoracic duct. 

 All the rest of the lymphatic sacs bud off from the vena cava, where it 

 is a part of the median mesonephritic vein, from the median mesone- 



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