THE THORAX 1017 



plexus in the root of the mesentery becomes converted into a lymph-sac, 

 without any venous connection, jind this constitutes the retro-peritoneal or 

 pre-aortic sac, which establishes a communication with the receptaculum 

 or cistema chyh, and through it with the thoracic duct. It is ultimately 

 replaced by a plexus of l>Tnphatic vessels, and from this plexus chains of 

 h-mphatic glands are developed which he along the ventral aspect of the 

 abdominal aorta. From the retro-peritoneal sac lymphatic vessels pass 

 within the mesentery along the branches of the superior mesenteric artery, 

 and these form another lymphatic plexus, from which the mesenteric lym- 

 phatic (lacteal) glands are developed. Subsequently lymphatic (lacteal) 

 vessels enter the wall of the small intestine. 



Seiatic Saes. — ^The sciatic lymph-sacs, right and left, are developed from 

 capillar>' venous plexuses in connection with the two primitive sciatic veins. 

 On either side the sac extends from near the caudal end of the receptaculum 

 chyU along the outer side of the primitive sciatic vein. It ultimately becomes 

 converted into lymphatic glands. 



Reeeptacalum Chyli.— The receptaculum or cistema chyli is a single sac, 

 situated at the caudal end of the thoracic duct. It is developed from, and 

 replaces, a venous plexus. The thoracic duct connects the receptaculum 

 chyh with the left jugular lymph-sac, and the receptaculum chyU communi- 

 cates with each sciatic sac. From each jugula? sac a vessel grows caudal- 

 wards. On the right side this vessel constitutes the right lymphatie tronk, 

 aad on the left side it forms the thoraeie daet. As the thoracic duct ap- 

 proaches the developing aortic arch it divides and gives rise to two thoracic 

 ducts, which embrace that arch and then pass to join separately the recep- 

 taculum chyh opposite the WoLQ&an bodies. Subsequently these two ducts 

 fuse, and one duct is formed. 



The thoracic duct is developed from a series of venous capillaries, originally 

 having a connection with veias. 



Lymphatie Vessels. — ^The formation of these vessels, along with that of 

 Ij-mph-glands, constitutes the second stage in the development of the Ijrm- 

 phatic system. According to Sabin, the lymphatic vessels are formed from 

 the endothelial lining of the lymph-sacs, whilst the Ijmph-glands are developed 

 from plexuses of lymphatic capillaries. The development of the thoracic 

 daet, as stated, takes place from several groups of venous capiUciries. It is 

 the largest lymphatic vessel, and it connects the receptaculum chyli with 

 the left jugular sac. 



The IjTuphatics derived from the jugular Ijonph-sacs pass to the head, 

 neck, and fore-Umb buds; those from the sciatic lymph sacs extend to the 

 hind-limb buds; and those from the retro-peritoneal (mesenteric) Ij-mph- 

 sac enter the mesentery', within which lymph-glands (mesenteric) are devel- 

 oped in connection with them. From these mesenteric glands, in succession, 

 fresh Ijmiphatic vessels pass to the intestinal tube. 



The other view of the development of the lymphatic system, held by Hunt- 

 ington amongst others, is that the connection of the Ijonphatic vessels with 

 the venous sj'stem is secondary, and not primary-, as Sabin holds. According 

 to this other \-iew the Ij-mphatic vessels spring from lymph-spaces, which are 

 formed in the mesoderm (mesenchyme), and are linedwitii mesathelium. The 

 endothehal cells of the lymphatic vessels are derived from this mesothehum, 

 and not from the venous endotheUum according to Sabin, and the vessels 

 estabhsh a connection with the venous s>^tem at a later period. 



Lymph-Glands. — Some of these are formed in connection with the lymph- 

 sacs, whilst others are formed in connection with peripheral lymphatic vessels. 

 There are three stages in the development of a h-mph-gland. The first stage 

 consists in the formation of a plexus of lynnphatic capillaries pervaded by 

 connective-tissue septa. The second stage consists in the invasion of these 

 septa by blood-capillaries, stirrounded by lymphocytes. The third stage 

 consists in the formation of a lymph-sinos from the original plexus of Ijtu- 

 phatic capillaries. 



The essential elements of a lymph-gland are thus threefold — namely: 



