RESPIRATORY SYSTEM. 675 
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 and end in very fine air-tubes. These 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 
thoracics, the anterior thoracics, the cervicals, and the inter- 
clavicular in the middle line in front. The interclavicular 
sac is in connection with both lungs, and is continued into 
two axillary sacs in the arm-pits. The anterior and posterior 
air-sacs are continuous with air-spaces in the bones. In 
the resting bird the sternum rises and falls; in the flying 
birds the thoracic region compresses the lungs and air-sacs ; 
in either case, expiration is the more active part of the 
respiratory process. 
Excretory system.—The kidneys are three-lobed, and 
lie embedded in the pelvis. They receive blood from the 
dorsal aorta by renal arteries, and the filtered blood leaves 
them by renal veins which unite with femorals and renal 
portals to form the iliacs, or, we may almost say, the inferior 
vena cava. But the kidney also receives a little venous 
blood from branches of the femoral veins. Thus, there 
is just a hint of a renal-portal system, which does not 
occur in Mammals. The kidneys are metanephric in 
origin. 
The waste products, consisting for the most part of urates, pass im 
semi-solid form down the ureters into the median compartment of the 
cloaca, 
