XXIV] RESPIRATORY SYSTEM 623 



the posterior cardinal veins have disappeared: but their hinder 

 parts remain as the renal portal veins which as usual arise by the 

 bifurcation of the caudal vein and receive on each side a femoral 

 and a sciatic vein from the leg. The renal portal pours its blood 

 into the inferior vena cava, not as in Amphibia and Reptiles through 

 a system of capillaries, but directly by a single vessel channelled 

 through the substance of the kidney. Hence in Birds the kidney 

 tubules receive blood only from the aorta and do not, as in the 

 lower Craniata, receive a double supply. From the point where the 

 caudal vein divides into the two renal portals a vein is given off 

 which descends into the mesentery and opens into the posterior 

 mesenteric branch of the portal vein, thus establishing a connection 

 between the portal and cardinal systems of veins. This vein is 

 called the coccygeomesenteric (12, Fig. 311), and is quite 

 peculiar to Birds. 



The lungs are firmly fitted in against the ribs; they do not, as 

 in most Reptiles or as in ourselves, hang freely in a cavity; their 



most remarkable feature is the possession of great thin- 

 sjStem. rat ry walled bladder- shaped outgrowths, the air-sacs, of 



which the prolongations extend even into the bones. 

 There are nine of these great air-sacs, one placed at the base of the 

 neck, and the other eight situated in pairs at the sides of the body 

 cavity under the ribs (Fig. 312). When the ribs are in their normal 

 position, the air-sacs are expanded, but when the ribs are pulled 

 backwards so as to compress the air-sacs, air is driven out; when 

 the ribs and wall of the body behind come into their natural 

 position again, the air-sacs are expanded and air rushes in. 

 Breathing out or expiration is therefore the active function 

 drawing in air is an elastic reaction, the opposite to what is the 

 case in Man and other Mammals, It must be remembered that in 

 the lungs of Birds as indeed of those of all other land animals the 

 air which is breathed in or out, the tidal air as it is termed, is only 

 a fraction of the total air contained in the lung. Oxygen and other 

 gases pass from the tidal air to the residual air and vice versa by 

 the swift process of gaseous diffusion and as the torrent of tidal air 

 rushes past the lungs into the air-sacs the limgs abstract oxygen 

 from it, both on its way in and on its way out. This double 

 oxygenation of the air in each complete set of respiratory move- 

 ments is perhaps the reason for the extraordinary activity and 

 strength of Birds 'in proportion to their size. 



The windpipe or trachea is long, and the hoops of cartilage 



