292 COMPARATIVE ANATOMY. 



The two anterior cardinal veins give rise to the right and 

 left precavals and to the jugular veins. In Monotremes and 

 Marsupials, as well as in many Rodents and Insectivores, both 

 precavals persist throughout life ; but in other Mammals the main 

 part of the left disappears, all the blood from the head and anterior 

 extremities passing into the right : this explains how it is that 

 the left azygos gives up its connection with the left precaval and 

 unites by means of transverse anastomoses with the right precaval. 



Most of the veins are provided with valves, which are 

 adapted to prevent the reflux of the blood : they have the form 

 of semilunar folds of the internal coat, and each is usually made 

 up of two folds, placed opposite to one another. 



RETIA MIRABILIA. 



By this term is understood the sudden breaking-up of a venous 

 or arterial vessel into a cluster of fine branches, which, by anasto- 

 mosing with one another, give rise to a capillary network ; the 

 elements of this network may again unite to form a single vessel. 

 The earlier condition maybe described as a unipolar, the later as 

 a bipolar rete mirabile. If it is made up of arteries or of veins 

 only, it is called a rete mirabile simplex; if of a combination 

 of both kinds of vessels, it is a rete mirabile duplex. 



The retia mirabilia serve to retard the flow of blood, and thus 

 cause a change in the conditions of diffusion. They are extremely 

 numerous throughout the Vertebrate series, and are found in the 

 most varied regions of the body, as, for instance, in the kidneys 

 (glomeruli), where their above-mentioned function is most clearly 

 seen ; on the ophthalmic branches of the internal carotid ; on 

 the pseudobranchia of Fishes ; along the intercostal arteries of 

 Cetacea ; on the mesentery of Man ; on the portal vein ; on the 

 vessels of the air-bladder of Fishes ; and along the caudal portion 

 of the vertebral column in Lizards and Blindworms. In the last- 

 mentioned case they are relatively very large, and probably have 

 to do with the power these animals have of reproducing the tail 

 when it is lost (comp. p. 43). A well-developed bipolar rete 

 mirabile may also be seen on the dorsal wall of the pharynx in 

 the Frog. 



LYMPHATIC SYSTEM. 



In the Anamnia and Reptilia the lymphatic vessels 

 occur mainly alongside the great blood-vessels, as well as on the 

 bulbus arteriosus and ventricle, and lie in the connective-tissue 

 surrounding these structures. Numerous independent lymphatic 

 vessels are also found in Fishes, arising from a capillary network 



