THE VASCULAR SYSTEM 



363 



ducts of mammals. Lymph sinuses surround ihe heart and nervous 

 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 are also 

 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. JUGULAI?. 



UBCLAVIAN 



INTERCOSTAL 



GLANDS- 



THORACIC DUCT. 



AZYGOS V. ^"i 



POSTCAVAL- 

 RENAL 

 COMMUNICA- 

 TIONS 



RECEPTACULUM 

 CHYLI 



LYMPH GLANDS\ 



A ^-B - C D 



Fig. 302. — Diagrams illustrating the chief lymphatic trunks and their relations to the 

 veins in mammals and man. A, South American monkeys; B, Mammals (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 chyli, also persists in man. The 

 lymph is returned to the veins at the point of least pressure, where sub- 

 clavian 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. 



Evolution of Hemopoietic Tissues. Hemopoietic or blood-forming 

 tissues are found in several phyla of invertebrates. Blood-cell formation 

 consequently appears not to be a novelty in chordates. Identical highly 



