116 
COMPARATIVE ZOOLOGY. 
Fig. 82.— Section through a bronchial tube, 
Lung of a Bird, magnified : a, the cavity ; 
fc, its lining membrane supporting blood¬ 
vessels ; c, perforations at the orifices of 
the lobular passages, d; e, interlobular 
spaces, containing the terminal branches 
found on the thorax, as un¬ 
der the wing of a Moth: 
such may be strangled by 
pinching the thorax. 
In Millipedes and Centi¬ 
pedes, the spiracles open 
into little sacs connected 
together by tubes; in Spi¬ 
ders and Scorpions, the 
spiracles, usually four in 
number, are the mouths of 
sacs without the tubes, and 
the interior of the sac is 
gathered into folds. Land- 
of the pulmonary vessels supplying the ~ *1 . Lo V p amp Qniimplp nr 
capillary plexus,/, to the meshes of which SrUllc5 Aia ' e 011(3 SpiiaClC, 01 
the air gets access by the lobular passages, aperture, On the left side of 
the neck, leading to a large cavity, or sac, lined with fine 
blood-vessels. These sacs represent the primitive idea of 
a lung, which is but an infolding of the skin, divided up 
into cells, and covered with capillary veins. 65 
Fig. 83.— Part of a transverse section of a Pig’s Bronchial Twig, X 240: a , outer 
fibrous layer; b, muscular layer; c, inner fibrous layer; d , epithelial layer with 
cilia; /, oue of the neighboring alveoli. 
