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 

 P.O. 82,-sectionZIngh a bronchial tube, number, are the mouths of 



Lung of a Bird, magnified: a, the cavity; sacs without the tubes, aild 

 b, its lining membrane s pportiug blood- ... 



vessels ; c, perforations a the orifices of the interior of the Sac IS 

 the lobnlar passages, d e, interlobular * j 4. j: 1 J T J 



spaces, containing the le miual branches gathered into folds. Land- 

 of the pulmonary vesse supplying the Snfl 4]c } lavp onp cniraolp or 

 capillary plexus, /, to the meshes of which M Ut3 ' Or 



the air gets access by the lobnlar passages, aperture, OH 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 



Fio. 83. Part of a transverse section of n Pier's Bronchial Twig, x 240: a, outer 

 fibrous layer; I, muscular layer: c, inner fibrous layer; d, epithelial layer with 

 cilia; /, one of the neighboring alveoli. 



