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 



PIG. S2.-Section though a bronchial tube, m ber, are the mOUths of 

 Lung of a Bird, magnified: a, the cavity; sacs without the tllDCS, and 

 6, its lining membrane supporting blood- 



vessels ; c, perforations at the orifices of the interior of the sac is 

 the lobular passages, d; e, interlobular , , . ,, , , 



spaces, containing the terminal branches gathered into lolds. 



V, avp m , p 

 nave OI16 



of the pulmonary vessels supplying the ~~ , avp m , p e r ar>A 

 capillary plexus,/, to the meshes of which snails nave OI16 SpliaCie, Or 

 the air gets access by the lobular passages, aperture, Ott 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 



Fro. S3. Part of a transverse section of a Pig's Bronchial Twig, x 240: a, outer 

 fibrous layer; b, muscular Inyer; c, inner fibrous layer; of, epithelial layer with 

 cilia; /, one of the neighboring alveoli. 



