

DEVELOPMENT OF THE LYMPH VASCULAE SYSTEM. 1043 



The cephalic vein of the upper extremity appears at a later period and, in the first instance, 

 joins the external jugular vein, the primitive termination being occasionally retained in adult 

 life. At a later period its terminal extremity is transferred to the axillary vein. 



The deep veins which accompany the arteries are the latest venous formations of the 

 extremities. 



The primitive vein in the lower extremity becomes the small saphenous vein, which is con- 

 tinued proximally, as the inferior glutseal vein, to that part of the posterior cardinal vein which 

 later becomes the hypogastric vein. The great saphenous vein and the femoral vein appear later 

 and are continued to the posterior cardinal vein as the external iliac vein. After the external 

 iliac vein joins the posterior cardinal vein the part of the latter vessel which lies caudal to the 

 i point of union is called the hypogastric vein. 



THE DEVELOPMENT OF THE LYMPH VASCULAR SYSTEM. 



Very little is known regarding the origin of the vessels of the lymph vascular system in man, 

 but during recent years numerous investigations have been made with the object of discovering 

 the various phases of the development of the lymph vascular system in other mammals ; some of 

 the main points are however still subjects of dispute. 



It is generally admitted that the terminal parts of the great lymph vessels, that is, the 

 terminal part of the thoracic duct and the right lymphatic duct, are derived from venous channels, 

 which, for a time, lose their connexion with the larger veins and become modified into terminal 

 lymph sacs which obtained a secondary union with the great veins at the root of the neck j but 

 regarding the origin of the thoracic duct and the cisterna chyli and the peripheral lymph vessels 

 there is, as yet, no agreement. According to Florence Sabin, and those who agree with her, the 

 peripheral lymph vessels are outgrowths from a series of lymph sacs, themselves of venous origin, 

 and from veins. Of the lymph sacs the two cervical, a retro-peritoneal, a cisterna chyli, and two 

 posterior sacs, lying along the inferior glutaeal veins, are recognised in human embryos of about 

 24 mm. length. The origin of these sacs, in the human embryos, has not been proved, but it 

 appears probable that the cervical sacs are derived as in other mammals from venous capillary 

 vessels. There is no agreement, however, concerning the origin of the other sacs, for whilst, on 

 the one hand, Florence Sabin appears to believe that they have an origin like that of the 

 jugular lymph sacs and that the thoracic duct is developed in the same manner, Huntington, on 

 the other hand, believes that, with the exception of the jugular or cervical lymph sacs, all the 

 other lymph vessels, including the thoracic duct, are developed neither as outgrowths from 

 lymph sacs nor by the modification of venous capillaries, but by the formation of endothelial 

 lined spaces in the mesodermal tissues. The spaces are at first entirely closed but afterwards 

 attain union with one another and with the terminal lymph vessels. 



According to the view upheld by Florence Sabin and her supporters, the lymph vessels are 

 outgrowths from the venous system, and are, therefore, lined with endothelium. which is 

 genetically the same as that in the veins. According to Huntington this is not the case ; for 

 his observations lead him to believe that the endothelium of the lymph vessels is formed, in 

 situ, from the mesodermal cells, and it has, therefore, no direct genetic connexion with the 

 endothelium of the veins, which is derived from the original angioblast. The evidence brought 

 forward by the supporters of the opposite views is interesting and instructive upon many 

 points, but the question must still be regarded as an open one. 



The Development of Lymph Glands. Lymph glands are developed from plexuses of lymph 

 vessels. The transformation is brought about by the aggregation of numerous lymphocytes in 

 the connective tissue strands of the plexuses and the transformation of the lumina of the vessels 

 into .the peripheral and central portions of the lymph sinus. The stroma, the capsule, and the 

 proper substance of a lymph gland are therefore formed from the fibro-cellular reticulum of the 

 lymphatic plexus, and the cavity of the lymph sinus is formed from the lumina of lymph 

 capillaries. The rudimentary lymph glands possess blood vascular as well as lymphatic 

 capillary networks, and if the blood vascular network preponderates over the lymph vascular the 

 developing gland has a reddish appearance and is known as a haemal gland. Such glands are 

 found in man (Schafer) as well as in other mammals, and it would appear from the recent observa- 

 tions of S. v. Schumaker that haemal glands are merely rudimentary forms of true lymph glands 

 (Arch.f. mikr. Anat. H. 2., 1912). 





MOEPHOLOGY OF THE VASCULAE SYSTEM. 



In conformity with the general plan of the vertebrate body, the vascular system is essentially 

 segmeutal in character. This is obvious, even in the adult, in the intercostal and lumbar 

 vessels. It is distinguishable, though less obvious, in the vessels of the head and neck and of 

 ! the pelvis. 



The intersegmental arteries and veins form a series of bilaterally symmetrical vessels, each 

 which is united to the vessels of adjacent segments by segmental channels, which anastomose 



, 



