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 

 , , number, are the mouths of 



Fig. 82.— Section through a bronchial tube, ' 

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

 fc, its lining membrane supporting blood- , 



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

 the lobular passages, d; e, interlobular , . -£1/1 T A 



spaces, containing the terminal branches gathered into IOluS. _Lanu- 

 of the pulmonary vessels supplying the 1,.^ rmp anir»f»lp nr 



capillary plexus,/, to the meshes of which SIU11& liaVe 0ne SpiiaCle, Or 

 the airgets access by the lobular passages, aperture, OU 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 



Fig. 83.— Part of a transverse section of a Pig's Bronchial Twig, X 240: a, outer 

 fibrous layer; b, muscular layer; c, inner fibrous layer; d, epithelial layer with 

 cilia; /, one of the neighboring alveoli. 



