474 THE BIRDS xvi. 24- 



flight the movements of the pectoral muscles provide the ventilation, 

 the sternum moving towards and away from the vertebral column. 



25. Excretory system 



The kidneys are, of course, metanephric and are relatively large, 

 elongated, and lobulated. They are provided with venous blood by 



-thas 



obi-- 



rpr 



ms / 



st 



Fig. 285. Diagram of transverse section through the thorax of a bird. 



ec. Excurrent passage from lung to air-sac through pulmonary aponeurosis; h. heart; 

 lis. left liver-sac; Ig. lung; m. muscle; ms. mesentery below oesophagus; obi. oblique septum; 

 p. pericardial coelom; pa. pulmonary aponeurosis; pic. reduced pleural coelom; r. dorsal 

 rib; re. recurrent bronchus from sac to lung; rl. right lobe of liver; rpr. right pulmonary 

 recess; St. sternum; thas. posterior thoracic air-sac; vr. sternal rib. (From Goodrich.) 



the renal portal veins and arterial blood from the renal arteries. The 

 arrangement is essentially as in amphibia and reptiles, with the renal 

 arteries supplying the glomeruli and the portal veins, which break up 

 into inter-lobular branches, sending blood to the renal tubules, whence 

 it is collected into a central intra-lobular vein. It is not certain, how- 

 ever, exactly how the system operates, and it is possible that much of 

 the blood-flow is directly from the renal portal to the renal veins, 

 making little contact with the tubule walls. 



The excretory system is highly specialized for water-saving. For 

 this purpose the end product of nitrogenous metabolism is the rela- 

 tively insoluble uric acid, synthesized in the liver, probably from 

 ammonium lactate. After excretion by the kidney the urine is con- 

 centrated in the cloacal chambers and the uric acid precipitates as 



