764 THE CIRCULATORY SYSTEM 



Two general views are held as to the origin of the lymphatic system. One 

 view holds that lymphatic vessels develop independently of blood vessels and 

 originate as small spaces in the mesenchyme, the mesenchymal cells flattening 

 and forming an endothelial lining for the space (Huntington, '14). Such 

 primitive lymph spaces fuse with nearby lymph spaces to form discrete channels 

 (McClure, '21). A second view maintains that the certain, small lymph sacs 

 arise from small endothelially lined channels which are a part of the primitive 

 venous plexuses in certain areas (Sabin, '12, p. 709). Both views agree, how- 

 ever, that once formed, the primitive lymph vessels grow and spread by sprout- 

 ing new channels from previously established vessels (Clark and Clark, '32). 



The first lymphatic capillaries appear to develop along the main veins. In 

 certain regions, these capillaries give origin to the lymph sacs. Right and left 

 jugular lymph sacs arise in the mammal along the anterior cardinal veins at 

 the base of the neck (fig. 343D) . These lymph sacs grow, expand, and coalesce 

 with smaller adjoining lymph spaces. Various other lymph sacs arise, such as 

 the subclavian lymph sac which is associated with the subclavian vein in the 

 axillary region, the cisterna chyli which arises from the retroperitoneal, median 

 lymph sac in the lumbar area, and the iliac lymph sacs which arise posterior 

 to the retroperitoneal rudiment of the cisterna chyli. From these central lymph 

 sacs, the peripheral lymph channels arise and grow rapidly in a distal direc- 

 tion. The thoracic duct comes into existence as a longitudinal vessel along 

 the middorsal area of the body and together with the left jugular lymphatic 

 trunk opens into the venous system near the junction of the internal and ex- 

 ternal jugular veins. The right jugular lymphatic trunk opens into the venous 

 system similarly on the right side. From these main lymphatic areas, smaller 

 peripheral channels arise as endothelial outgrowths. Valves develop within. 



Fig. 343. Branches of dorsal aorta; lymphatic structures. (A) Diagram illustrating 

 various branches of dorsal aorta. (B) Arteries of brain area, appendages, body wall 

 and umbilical cord of human embryo of seven weeks. (Redrawn from Patten, 1946. 

 Human Embryology, Blakiston, Philadelphia, after Mall.) (C and C) Two stages in 

 development of forelimb arteries of pig: C, embryo of 4.5 mm.; C, embryo of 12 mm. 

 (Redrawn from WooUard, 1922. Carnegie Contribution to Embryology, No. 70. Vol. 

 14.) (D) Formation of primitive lymph sacs in the mammal (cat). (Redrawn from 

 F. T. Lewis, 1906. Am. J. Anat. 5.) (E and E') Four stages in the development of 

 a lymph node. (Redrawn from Bremer, 1936. A Text-book of Histology, Blakiston, 

 Philadelphia.) Diagram E, to the left. Lymphatic vessels come to surround a mass of 

 primitive lymphoid tissue composed of mesenchymal tissue and lymphocytes. Primitive 

 connective tissue surrounds the mass. Diagram E, to the right. The ingrowing lymphatic 

 channels break up the lymphoidal tissue with the subsequent formation of lymph smuses. 

 Observe that a peripheral lymph channel is established, and also that the surrounding 

 connective tissue is beginning to form a surrounding capsule from which trahcculae are 

 growing into the lymphoidal mass. Diagram E', to the left. Further development of 

 growth changes shown in E, to the right. Diagram E', to the right. A loose meshwork of 

 lymph channels and sinuses appears in the central portion or luedulla of the lymph 

 node, whereas the periphery or cortex is composed of secondary nodules separated into 

 compartments by the ingrowth of trabecuiae from the peripheral capsule. 



