752 PHYLUM CHORDATA : CLASS AVES — BIRDS 



The hepatic portal system is as usual — mesenteric veins from the 

 intestine combine in portal veins ; the blood filters through the liver, 

 and is collected in hepatic veins, which unite with the anterior end of 

 the inferior vena cava. 



A hint of a renal-portal system is represented by small branches, which 

 the femorals give off to the kidney. 



From the transverse vein formed between the two hypogastrics or by 

 the division of the caudal vein, a coccygeo-mesenteric arises, which 

 receives vessels from the cloaca and large intestine, and is continued 

 along the mesentery to join the hepatic portal system. 



As there are rarely any valves in the hypogastric veins, the blood 

 from the viscera and hind-limbs can pass freely either through the iliac 

 veins and thence to the inferior vena cava, or through the coccygeo- 

 mesenteric vein to the hepatic portal system. 



The epigastric vein of the bird takes blood from the fat-laden sheet 

 or great omentum which covers the abdominal viscera. It leads not 

 into the liver, but into one of the hepatic veins. 



Associated with the blood-vascular system there is a 

 lymphatic system with a few lymphatic glands. 



The spleen lies on the right side of the proventriculus, 

 the paired thyroid lies beside the origin of the carotids, and 

 a paired thymus is found in young birds in the neck region. 

 Small yellowish (suprarenal) glands lie on the front part of 

 the kidneys. 



Respiratory system. — The important facts are — that 

 there is no true diaphragm ; that some of the bronchial 

 branches in the lungs are continued into adjacent air- 

 sacs ; that expiration is a more active process than 

 inspiration. 



The nostrils lie at the base of the beak overlapped by 

 the cere. Only in the kiwi are they at the tip of the beak. 

 The glottis behind the root of the tongue leads into the 

 trachea, which has a voiceless larynx at its anterior end, and 

 a syrinx, with vocal chords, at its base. The trachea is 

 strengthened by bony rings, and is moved by two sterno- 

 tracheal muscles from the sternum. The bronchial tubes 

 branch irregularly, in a kind of tree-like fashion, in the 

 lungs, ending in delicate interconnections. The lungs lie 

 attached to the dorsal wall of the thorax, indented by the 

 ribs, and covered with pleural (peritoneal) membrane on 

 their ventral surface only. 



Around the lungs, and connected with the ends of the 

 main bronchial branches, are the nine air-sacs. In order 

 from behind forwards, lie the abdominals, the posterior 



