LUNGS 



377 



The Lungs. 



Dipnoans. In Ceratodus the lung is a wide, unpaired sac 

 without any trace of a dividing septum : in other Dipnoans it is 

 distinctly paired (as in Polypterus, cf. p. 361) throughout the 

 greater part of its length, the anterior unpaired portion being 

 largely filled up by spongy trabecula?. 



The lung extends through the whole length of the body-cavity, 

 and is covered by peritoneum on the ventral surface only; the 

 lining mucous membrane forms bands and networks similar to 

 those seen in the swim-bladder of many Fishes (e.g. Lepidosteus, 

 Fig. 267), and smooth muscles are abundant. 



Amphibians. The lungs of Proteus and Necturus (Fig. 282), 

 as well as those of the Newt (Triton), though paired throughout, 

 remain at a lower stage than those of the 

 Dipnoi, their internal surface being per- 

 fectly smooth, and having, therefore, a 

 relatively much smaller superficial extent. 

 They consist of two delicate, elongated 

 sacs of unequal length and constricted 

 in the middle; in Proteus they extend 

 much further backwards than in Nec- 

 turus. A difference in length between 

 the two lungs is seen also in other Am- 

 phibia, such as Amphiuma and Siren, in 

 which the two cylindrical lungs lie near 

 together, close to the aorta. Their in- 

 ternal surface is raised into a network 

 enclosing numerous smooth muscle-fibres 

 and corresponding with the distribution 

 of the blood-vessels, the meshes being 

 much finer in Amphiuma, and still more 

 so in Menopoma, than in Siren. 



With the further development of the 

 blood-vessels, they gradually encircle the 

 wall of the lung like the hoops of a 

 barrel, so that septum-like structures 

 arise, between which the thin pulmonary 

 walls project -in the form of rounded The communication with the 



. , r J , ,. or ,~v ,,,, pharynx is indicated by a 



vesicular elevations (Fig. 283 j. Thus a black spot anteriorly, 

 beginning is made of a differentiation in 



the cavity of the lung into respiratory regions and air-passages 

 (intra-pulmonary bronchi) such as is seen in higher forms. 



In many other Urodela, as well as in the Anura and Gymno- 

 phiona, the muscular and elastic walls of the lung become still 

 further complicated by the development of a respiratory network 



FIG. 282. LUNGS OF PRO- 

 TEUS (A) AND NECTURUS 

 (B). 



