258 COMPARATIVE ANATOMY. 



inasmuch as their internal surface is perfectly smooth, and has, 

 therefore, a much smaller superficial extent. They consist of two 

 delicate elongated sacs, of unequal length, and constricted in their 

 middle ; in Proteus they extend much further backwards than in 

 Menobranchus. A difference in length between the two lungs is 

 seen also in other Amphibia, such as Amphiuma and Siren 

 lacertina, in which the two cylindrical lungs lie near together, 

 close to the aorta. Their internal surface is raised into a net- 

 work, corresponding with the distribution of the blood-vessels, the 

 meshes of which are much finer in Amphiuma, and still more so in 

 Menopoma, than in Siren. 



In Salamanders the lungs as a rule are equal in size, and have 

 the form of cylindrical tubes extending backwards as far as 

 the end of the stomach ; their internal surface is more or less 

 smooth. The lungs of Gymnophiona are similar to those of Sala- 

 manders, but the right alone is fully developed, and this shows in 

 its interior a complicated trabecular network : the left is only a few 

 millimetres long. A similar abortion of one lung is also seen in 

 Snakes, and in both cases has to do with the elongated form of the 

 body (cp. the note below). The wide, elliptical lungs of Anura 

 are quite symmetrical. Their internal surface, which is partly 

 lined by ciliated epithelium, is raised up into a rich respiratory 

 network of trabecula3, and numerous smooth muscular fibres are 

 present in their walls. 



Reptiles. In Reptiles, as in all other air-breathing Verte- 

 brates, the form of the lungs is to a great extent regulated by that 

 of the body. In the higher types, such as the Chelonia and 

 C roc od ilia, their structure is much more complicated than in 

 Amphibia ; this complication finds expression in a very con- 

 siderable enlargement of the respiratory surface. With the ex- 

 ception of the thin- walled lungs of Lizards, which retain a very 

 primitive condition, we no longer meet with a large central cavity, 

 but the organ becomes penetrated by a branched system of 

 bronchi, giving it the character of a tubular and sponge -like mesh- 

 work. 1 Thus the phylo^enetic stages of development are here 

 again repeated ontogenetically. 



The kings of the Chameleon present a very curious modification. The 

 anterior portion is much more compact and spongy than the posterior, which 

 grows out into numerous sac-like processes, some of which reach as far back 

 as the pelvic region ; their form is very variable, being spindle-shaped, club- 

 shaped, or lobulated, and their walls are very thin ; they extend in amongst the 

 viscera wherever there is room. If these processes have any respiratory function, 

 it is at most only a very slight one. An indication of a similar arrangement 



1 The lung of Snakes exhibits an intermediate form, for, in spite of the finely- 

 meshed tissue arising from the periphery, it still retains a narrow central cavity. As 

 already mentioned, only the right lung is as a rule fully developed in Snakes and 

 Amphisbsenians, owing to the elongated form of the body, while the left remains 

 in a rudimentary condition, or even disappears entirely. 



