116 
COMPARATIVE ZOOLOGY. 
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 
„ co 0 „ . . ,. .. . number, are the mouths of 
Fig. S2.— Section through' a bronchial tube, ’ 
Lung of a Bird, magnified: a, the cavity; sacs without the tubes, aild 
ft, its lining membrane supporting blood- ... 
vessels; ^perforations at the orifices of the interior of the Sac is 
the lobular passages, d; e, interlobular » T . /d 
spaces, containing the terminal branches gatiieieG llltO IOIQS. .Lanu- 
of the pulmonary vessels supplying the ~ * 1 - L„ vp ftnp Rni * r o P l P nr 
capillary plexus,/, to the meshes of which snail& nave one SpiiaCie, Or 
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, wdiich is but an infolding of the skin, divided up 
into cells, and covered with capillary veins . 85 
Fig. S3.—Part of a transverse section of a Pig’s Bronchial Twig, x 240: «, outer 
fibrous layer; t>, muscular layer; c, inner fibrous layer; d, epithelial layer with 
cilia; /, one of the neighboring alveoli. 
