940 



HUMAN ANATOMY. 



veins. The various channels finally unite to form a continuous system which acquires 

 new openings with the venous system near the termination of the subclavian veins, 

 the condition found in the adult being thus established. 



More recently Huntington and McClure/ working with cat embryos, have also 

 found the earliest traces of the lymphatic system in a series of spaces which appear in 

 the tissue surrounding the intima of the anterior cardinal veins, but they found that 

 these spaces have at first no connection with the veins, nor are they outgrowths 

 from them. The anterior cardinal vein of each side is early divided longitudinally 

 into two portions by the passage through it of the cervical nerves, and the dorso- 

 lateral portion of the vein later 

 Fig. 793. undergoes retrogression, the 



ventro-medial portion persisting 

 as the internal jugular. As the 

 dorso-lateral portion shrinks, the 

 lymphatic spaces along its course 

 rapidly enlarge, fuse together, 

 and form a large lymphatic stem, 

 which subsequently makes con- 

 nection with the subclavian vein, 

 and thus forms the primary 

 lymphatic trunk of the body 



(Fig- 793)- 



Later, spaces develop along 



the course of the anterior cardinal 

 veins below the point where the 

 subclavians open into them, but 

 it is noticeable that those occur- 

 ring in association with the left 

 vein, which undergoes retrogres- 

 sion, develop more rapidly than 

 those accompanying the same 

 portion of the right vein and form 

 the thoracic duct (Fig. 794), this 

 structure thus belonging essen- 

 tially to the left half of the body, 

 since the principal persistent veins 

 occur on the right side. Similar 

 spaces appear in the peri-intimal 

 tissue of other veins, and in all 

 cases those associated with retro- 

 gressive veins are the most rapidly 

 developed. While most of the 

 principal lymphatic trunks unite with the thoracic duct, yet they may also form 

 temporary or even permanent communications with other veins than the subclavian, 

 certain of the adult anomalies being results of these connections. 



From these observations it seems that the lymphatics arise from spaces which 

 are primarily independent of, although associated with, the veins, and that, while this 

 mode of origin of the lymphatics applies to those following the primitive systemic 

 veins, yet the more peripheral portions of the system are developed by a process of 

 budding from the main stems, just as is the case with the smaller branches of the 

 blood-vessels. By this budding process the system gradually extends throughout the 

 body, invading the various tissues, the invasion, however, failing to af!ect certain of 

 the tissues, such as cartilage and the central nervous system. 



The development of the lymph-nodes has been recendy studied by Kling^ and 

 by Sabin."' According to the latter investigator, the lymph-nodes may be regarded 

 as formed by two fundamental parts — the lymphoid element, consisting of lympho- 

 cytes in a reticulum surrounding the terminal artery and its capillaries within the 



' Amer. Jour, of Anatomy, vol. vi., 1907. ^ Archiv f. mikros. Anat., Bd. 63, 1904. 

 ^ Amer. Jour, of Anatomy, vol. v., 1905. * Amer. Jour, of Anatomy, vol. v., 1905. 



Developing lymphatics in rabbit embryo of ii mm. (14 days); 

 X 9. Lymphatic vessels arc heavily shaded ; veins are light. In.J., 

 £".r.y., internal and external jugular veins; /*>. 6'., primitive ulnar; 

 Ex.M., external mammary; Az., azygos; VCf., inferior vena cava ; 

 C, gastric; S.M., superior mesenteric; V.. vitelline; Sc, subcar- 

 dinal; P.A., renal anastomosis of subcardinals ; Pr.Fi., primitive 

 fibular; c.*., connecting branch ; y^w. 7"., anterior tibial ; c, caudal; 

 i, 4, 5, 6, position of corresponding cervical nerves. {F. T. Lewis.*) 



