346 Vertebrata. 



second, on tlie contrary, is especially well developed, and supplies 

 most of the blood for tlie aorta ; tlie first arcL. practically supplies the 

 head only ; the fourth, the lungs. The second often loses its connec- 

 tion with the other arches, so that it alone forms the aorta. In other 

 Vertebrata, the third arch is wanting ; and it must also be noticed 

 that the primitively simple arterial trunk is divided into two or three 

 branches, of which one is exclusively connected with the last arch. 

 In the Reptilia the arrangements are otherwise essentially siinilar 

 to those of the Amphibia; in Birds and Mammalia, on the 

 other hand, a further reduction has occurred, for the aorta is formed 

 from n e of the second arches only ; from the right in Birds, from 

 the left in Mammals. The connections between the first and second, 

 and the second and fourth arches, which may still occur in the 

 Reptiles, are absent from Birds and Mammals. (For details see the 

 various groups.) 



In rish, the embryo develops six simple arterial arches on eitter side, whicli 

 run along tie six primary visceral arches, and unite to form the aorta ; the first 

 of these, and in Teleostei, etc., the second also, degenerates, whilst the others 

 split into afferent and efferent branchial arteries. Further, in almost all other 

 Yertebrata these six ai-terial arches are present in the embryo ; then some of 

 them gradually atrophy, and thus are obtained the adult conditions aheady 

 described. The appearance of ai-terial arches and visceral clefts in the embryos 

 of higher Vei-tebrata (Reptilia, Aves, Mammalia) seems to prove that these have 

 been derived from forms with branchial respiration. 



With regard to the venous system, it must be mentioned that 

 the venous blood from the alimentary canal, spleen, and other viscera 

 does not go direct to the heart, but is collected into a large trunk, the 

 portal vein, which enters the liver, and branches to form a 

 capillary net-work. The blood is collected again into hepatic 

 veins, which carry it to the heart. In Fish, Amphibia, and Reptiles 

 such an arrangement also obtains for the kidney, the renal portal 

 system: veins from the tail and hind limbs proceed to the kidney, 

 and break up into capillaries, from which other veins arise and go to 

 the heart. In Fish and Amphibia it not infrequently happens that 

 several large veins are pulsatile f venous hearts); at these 

 points, just as in the heart, transversely striped muscle cells are 

 present. The veins, but not the arteries, are provided throughout 

 with valves, which direct the current of blood. In the Verte- 

 brata a well-developed capillary network is present, connect- 

 ing the smallest arteries and veins. The blood corpuscles are 

 of two kinds : amoeboid white corpuscles, few in number ; and 

 discoid red corpuscles, constant in form, usually oval and nucleate, 

 but in the Mammalia circular, biconcave, and without a nucleus. They 

 impart the red colour to the blood, the plasma itself being colourless. 



Occasionally an arteiy or vein breaks up suddenly into numerous branches, 

 lying close together, and frequently anastomosing, and uniting again to form a 

 single vessel. Such a network is termed a rete mirabile (swim-bladder of Fish, 

 kidney, etc.). 



