HOW ANIMALS BREATHE. 115 
little sacs connected to- 
gether 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 
Snails have one spiracle, or 
aperture, on the left side of 
the neck, leading to a large 
cavity, or sac, lined with 
fine blood-vessels. These 
sacs represent the primi- 
tive idea of a lung, which 
is but an infolding of the 
Fra. S2.—Section through a bronchial tube, 
Lung of a Bird, magnitied: a, the cavity ; 
b, its lining membran > supporting blood- 
vessels ; c, perforations at the orifices of 
the lobular passages, d; e, interlobular 
spaces, containing the terminal branches 
of the pulmonary vessels supplying the 
capillary plexus, .f, to the meshes of which 
the air gets access by the lobular passages. 
skin, divided up into cells, and covered with capillary 
veins.” 
Like the alimentary canal, the lungs of an animal are 
really an inflected portion of the outer surface; so that 
Fig. 83.— Part of a transverse section of a Pig’s Bronchial Twig, x 240: a, outer 
fibrous layer; 6b, muscular layer; e¢, inner fibrous layer; d, epithelial layer; Jf, 
one of the neighboring alveoli. 
