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 

 ^ on o . .V V. V K- , . ., number, are the mouths of 



Fio. 62.— Section through a broBchial tnbe, ' 



Lung of a Bird, maguified : a, the cavity; gaCS without the tubeS, and 

 &, its lining membrane snpporting blood- i . . « -, 



vessels J c, perforations at the orifices of the interior 01 the SaC IS 

 the lobular passages, d; e, interlobular ,■, A ' ¥ -f ^A T A 



spaces, containing the termiwal branches gatnereCt lUtO lOlflS. Lianu- 



of the pulmonary vessels 8"ppiy™g th« gnails have One spiracle, or 



capillary plexus,/, to the meshes of which r J 



the airgetsaccessby 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."' 



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

 fibrous layer; 6, muscular layer; c, inner fibrous layer; d, epithelial layer with 

 cilm; /, one of the neighboring alveoli. 



