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 
.. . .. w v 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 
b, its lining membrane supporting blood- ... 
vessels ; c, perforations at the orifices of the interior of tile SBC is 
the lobular passages, d; e, interlobular j , . « , , y i 
spaces, containing the terminal branches gatliei CQ into lOlClS. .Land- 
of the pulmonary vessels supplying the ' ;] ] spiracle, Or 
capillary plexus,/, to the meshes of which I ’ 
the air gets access by the lobular passages, aperture, Oil 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 lwig, which is but an infolding of the skin, divided up 
into cells, and covered with capillary veins . 65 
Fig. S3. —Part of a transverse section of a Pig’s Bronchial Twig, x 240: a, onte- 
fibrous layer; h, muscular layer; c, inner fibrous layer; d y epithelial layer with 
cilia; /, one of the neighboring alveoli. 
