IOI2 



VASCULAR SYSTEM 



combined trunk being then known as the v. cava inferior. But the 

 left V. hepatica magna receives also the v. umbilicalis, a long unpaired 

 vein arising in adult birds from the walls of the abdomen and its 

 air-sacs, and frequently anastomosing with the epigastric veins. Its 



United ste^n of the 

 vena hepatica 

 dextra ai^d sinistra 



V. hep. sin istra 



■.Left Liver lobe 



Stem runs nearly midway along 

 the visceral surface of the abdo- 

 minal wall, passing first to the 

 right of the stomach and then 

 between the two lobes of the 

 liver, finally joining the left hep- 

 atic vein. This peculiar vessel is 

 port, the remnant of one which jilayed 

 an important part in the embryo, 

 for it originally collected all the 

 blood of the yolk-sac (p. 211) 

 into one stem Avhich passed along 

 the left side of the gut ; and, 

 after receiving the mesenteric 

 vein, entered the right auricle 

 of the heart as the v. ornphalo- 

 mesaraica. This stem, however, 

 soon became surrounded by the 

 liver, and began to form the 

 Hepatic Portal' System by partly 

 breaking up into capillaries, while 

 the mesenteric develoj^ed more 

 and more, until the primitive ves- 

 sel persists only as the umbilical 



Diagram of a Bird's Portal Venolts 

 System. Ventral Aspect. 



A^ein. 



Owing to the numerous anas- 

 tomoses set up by veins on the confines of their several districts, 

 some of these connexions are erf ten used by the blood as " short 

 cuts," and then become wider channels, while the original vessels 

 suffer atrophy, so that quite new modifications are brought about. 

 Such variations are so common, especially in the cervical and 

 pelvic regions, that they deprive the Venous System of much of 

 its taxonomic value.^ 



B. Lymphatic System. 



The Lacteal or Absorbent vessels arise in the villi of the 

 intestine, whence they convey the chyle into the Lymphatics 

 (p. 139), together with the white blood or lymph corpuscles 

 (jD. 43), which are produced in the follicles at the base of the villi 



^ A careful, minute and amply illustrated description of the venous .system 

 of numerous birds — "Systema venosiim Avium cum eo Mammalium et inprimis 

 Hominis collatum" — was published by Neugebauer in 1844 {A'ov. Act. L.-C. 

 Acad. xxi. pp. 517-698, tabb. xxxvi.-l.). 



