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,-section through a bronchial tube, number, are the mouths of 



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



6, its lining membrane supporting blood- ... ., 



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



the lobnlar passages, d; e. interlobular , -, . / -, -. 



spaces, containing the terminal branches gathered into loldS. 



of the pulmonary vessels supplying the G ninU Ti9V> r\n(* m'raplp or 



capillary plexus,/, to the meshes of which Snails ' 16 > Ol 



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



Sac IS 



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

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

 cilia; /, one of the neighboring alveoli. 



