2O8 COMPARATirE ANATOMY AND PHYSIOLOGY. 



vertebrate series as the jugular veins. In the pen- 



taclactyle vertebrates the chief afferent function for 

 the hinder part of the body is undertaken by the just 

 mentioned vena cava inferior, the posterior cardinals 

 becoming the pelvic or hypogastric veins, while 

 the Cuvieriaii ducts come to be the terminations of the 

 jugular veins, and so lead into the sinus venosus 

 whether that remains distinct from the atrium, or, as in 

 higher forms, becomes a portion of the right auricle. 



In addition to the important pulmonary veins 

 which open into the left auricle, the vessel that brings 

 the blood from the fore-limb (the sutoclaviaii) comes 

 to be almost as important as the jugulars. 



In the Mammalia the relations of the superior veins 

 become considerably altered ; thus the left jugular, after 

 having entered into an anastomosis with the right, 

 becomes atrophied in various Ungulates and Rodents ; 

 in Man and the other Primates, in the Carnivora, and 

 in Cetacea, the left jugular ceases to have, as a rule, 

 any rudiment left in the adult condition. Rudimentary 

 left ducts of Cuvier have, however, been occasionally 

 observed in the human subject (Quain, Marshall). 



Although there is a general body of truth in the 

 statement that arterial vessels gradually become 

 smaller, and veins larger, as the one leave and the 

 other approach the central heart, yet, as we have 

 already seen in the case of the portal circulation in the 

 liver and kidney, veins do break up into smaller 

 vessels to again unite into a . larger one ; when this 

 phenomenon obtains either with an artery or with a 

 vein in any other part of the body than the organs 

 just mentioned, such a plexus of vessels is ordinarily 

 spoken of as a rete mirabile. 



The function of such retia is not far to seek, the 

 moment we remember the physical law that the 

 passage of a fluid is slower through a narrow than a 

 wide current ; in other words, the exchange of gas or 



