VERTEBRATA. 



353 



the latter the caudal vein. A renal-portal circulation or supply of venous 

 blood to the kidneys exists in all Amphibia, in Reptilia except Chelonia, and 

 possibly in Aves, in addition to the arterial supply from the aorta. The 

 blood from the hind-limbs passes in Amphibia partly through the renal- 

 portal system, and partly through an epigastric vein, as it does in Crocodilia, 

 through the renal-portal alone in Lacertilia, through an epigastric alone in 

 Chelonia. The epigastric vein lying in the ventral wall of the abdomen 

 exists in Amphibia and Reptilia, and in the embryoes of Aves and Mam- 

 malia where it is lost in the adult. Primitively double, it rarely remains so 

 (Chelonia, Crocodilia), but becomes single by the atrophy of one of the two 

 veins. Its anterior end is connected with the hepatic portal system. Its 

 posterior extremity is variously connected to the renal-portal system and 

 hind-limbs (supra), but in the embryoes of Sauropsida and Mammalia to 

 the allantois, a foetal membrane, which is developed as a ventral out- 

 growth of the posterior extremity of the mesenteron. And it may be 

 noted that veins from the allantoid bladder, the homologue of the allantois, 

 fall into the epigastric in Amphibia. 



The arteries and veins are connected peripherally by the capillary 

 vessels which form networks in the various tissues, some few, e.g. epidermis, 

 epithelium, cartilage, &c. excepted. The capillaries have a simple epithelial 

 wall, the smaller arteries and veins add a coat of transverse, i. e. circularly 

 disposed non-striated muscle cells, to which the larger vessels superadd 

 coats of connective tissue. The larger vessels are formed apparently from 

 solid cords of cells, the central cells being set free and becoming blood 

 corpuscles, the peripheral being converted into the walls of the vessels. 

 Capillaries are formed in many instances by the development of a vacuole 

 in a branched cell, the branches and vacuoles of adjoining cells becoming 

 connected. Certain of the veins in higher Vertebrata possess valves which 

 ensure a flow of blood in a constant direction. The blood consists of a 

 plasma containing in suspension ' white ' corpuscles or leucocytes which 

 possess the power of amoeboid motion. The red haemoglobin-containing 

 corpuscles or haematids are oval and nucleated in all Vertebrata except 

 Mammalia, where they are circular and non-nucleated. They have a fixed 

 outline. The leucocytes are formed for the most part in the lymphatic 

 system (infra}. 



A spleen a structure with a lacunar blood-vascular circulation is 

 found in all Vertebrata in connection with the mesogastrium. It possesses 

 a blood-making function. There is also a lymphatic system of spaces and 

 vessels formed in the mesoblastic tissues. The lymphatic system communi- 

 cates with the coelome, and with the blood-vessels at certain spots either 

 anteriorly (Mammalia) or anteriorly and posteriorly. Contractile lymph 

 hearts are sometimes found close to the points of communication except in 

 Mammalia. The lymphatic channels of the intestine and mesentery con- 



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