STRUCTURE OF THE LUNGS OF REPTILES AND AMPHIBIA. 73 



and some Perennibranchiates, as the Proteus and Menobranchus, 

 occupy the lowest grade, since in them each lung forms only a 

 simple and internally smooth saccular expansion of the branch 

 of air tube entering them. The remaining Amphibia present 

 on the inner surface of their constantly saccular lungs, which 

 hang from the bronchus like a berry upon its stalk, a plexiform 

 series of ridge-like elevations, which however are not all of 

 equal height, but project more or less into the cavity of the 

 organ. The principal and for the most part quadrangular 

 alveoli formed by the most strongly projecting ridges are sub- 

 divided into smaller depressions by less elevated septa, and 

 these again by others, till ultimately a number of polygonal, 

 and usually four or five angled, fossae or alveoli are produced, 

 the flat bases of which lie in contact with the investing mem- 

 brane of the air sac, have lateral walls formed by these vertical 

 ridges, and open directly into the general cavity of the pulmo- 

 nary sac. 



In the elongated tubular lungs of Ophidia and of Amphisbsena, 

 the thick- walled anterior segment is characterized by the depth 

 and complicated structure of the alveoli. The principal ridges, 

 springing at right angles from the pulmonary wall, are not 

 smooth-walled as in other Amphibia, but support secondary 

 ploxiform ridges on their surface, and thus alveoli are formed, 

 the bases or fundi of which are no longer constituted by the 

 proper wall of the lung, but by the principal ridges; whilst their 

 orifices, instead of being directed towards the general cavity of 

 the lung, open into the secondary cavities or alveoli bounded 

 by these ridges or septa. In the lungs of the Snakes and 

 Amphisbsena, the entire network of septa becomes more simple 

 posteriorly, the ridges being less elevated, and ultimately 

 vanishing so completely that the lung terminates in a smooth- 

 walled and simple membranous sac. 



Whilst the lungs of many Saurians (Anguis fragilis, Lacerta 

 agilis, Scincus bistriatus) do not essentially differ in the struc- 

 ture of the air sacs from the simple lungs of Amphibia, in 

 others, as for example the Chameleon, the common cavity of 

 each lung presents two or more subdivisions, though not per- 

 haps completely separated from each other, caused by the pro- 

 jection of one or several large septa which spring from the 



