THE VASCULAR SYSTEM 



79 



system. Some fishes have lymph hearts serving to assist the circulation 

 of the lymph. But fishes do not have lymph glands. 



A thoracic duct is present in .Amphibia. Larger subcutaneous sinuses 

 occur in this group possibly as an adaptive arrangement which prevents 

 dessication. Lymph hearts may occur in various parts of the body. 

 Reptiles have large paired lymphatic trunks. Lymph hearts also are 

 found in this group. 



Most mammals have paired thoracic ducts. In man the left duct 

 persists throughout life in connexion with the left subclavian vein, while 



EXT. JUGULAR 



EXT JUGULAR_ 



A ^^B ' C D 



Fig. 252 . — Diagrams illustrating the chief 1 ymphatic trunks and their relations to the 

 veins in mammals and man. A, South American monkeys; B, Hiammals (Lepus) in 

 which postcaval-renal communications are wanting; C, mammals in general; D, man. 

 In all mammals lymph enters the veins at the point of junction between the jugular and 

 the subclavian veins. In most mammals there is also communication between the 

 lymph vessels and the postcaval and renal veins. In man the right thoracic duct 

 degenerates in part and the only communication with veins is at the root of the jugulars. 



the right duct is rudimentary, having a length of only a few centimeters. 

 One lymphatic sinus, the cistema or receptaculum chyli, also persists in 

 man. The lymph is returned to the veins at the point of least pressure, 

 where subclavian and jugular veins meet. Lymph glands are numerous in 

 man and mammals and generally occur in clusters in the axillary region 

 and in the groin and neck. Lymph hearts are wanting in mammals and 

 man. Lymphoid or adenoid tissue is found in all vertebrates. Lymph 

 nodes however make their first appearance in reptiles. 



Development of the Heart. The heart makes its appearance as a 

 two-layered tube ventral to the pharynx, so that the early embryo has its 

 heart in its throat. Of the two layers, the inner becomes the endothelial 

 lining of the heart, while the outer forms the epicardium and the muscular 



