On the Structure and Functions of the Organs 0/ Respiration. 227 



(as in the earliest conditiou of air-breathing animals) to their entrance into 

 the stomach, the oesophagus, and even the mouth. From the simple un- 

 divided condition of the air bags themselves, we find them gradually more 

 complex in their structure, and more subdivided internally, (as in the frogs 

 and land salamanders,) so as to present a larger surface for the ramification 

 of the blood vessels. There is very great variety in the relative degrees 

 of aerial and aquatic respiration in the family of pereunibranchia, of course 

 depending on the comparative development of their organs. Thus in the 

 siren, the pulmonic respiration is more extensive and important than the 

 branchial ; but the reverse is the case in the proteus. 



All the amphibia are covered with a soft skin, which answers such an 

 important purpose in the function of respiration, that if it be covered with 

 oil or grease, so as entirely to prevent the access of air, the animal will 

 die ; while if, on the other hand, the lungs be removed, or the access of air 

 to them be prevented, the animal will be supported for some time by its 

 cutaneous respiration. It is also remarkable, that as none of the amphibia 

 are provided with ribs or a diaphragm, air can only be forced into the lungs 

 by a process resembling swallowing ; and a frog may thus be prevented 

 from inspiring air by holding his mouth open. 



In serpents we usually find a long cylindrical sac, only divided into cells 

 at its upper part, and generally extending along the tail. In some genera, 

 however, this sac is double ; and where there is only one, it is that on the 

 right side which is developed, the other remaining in its rudimentary state. 

 From the great capacity of the respiratory sac, the mobility of their ribs, 

 and the power of their intercostal muscles, serpents are capable of rapid 

 and extensive inspiration and expiration, which contributes to the energy 

 of all their muscular exertions. In the aquatic serpents, the large volume 

 of air contained in the lungs serves to buoy up their heavy and slow- 

 moving bodies in that dense element ; and its expulsion gives rise to the 

 long continued hissing sound by which these animals sometimes alarm 

 their prey. 



In the saurian reptiles, we still find a very imperfect subdivision of 

 the pulmonary sacs ; but they are equally developed on both sides of 

 the body. In the lower genera of this class, there is scarcely any ap- 

 pearance of cells J but when we have advanced upwards to the crocodile, 

 we find the lungs, though externally small, subdivided to a great degree of 

 minuteness by internal partitions; and we also find a rudimentary con- 

 dition of the diaphragm, which is entirely wanting in all the inferior 

 genera. In the chamelion, for instance, the lungs extend under the skin 

 of the whole trunk; and their fulness or emptiness of air give rise to the 

 plump or lean appearance, either of which this remarkable animal has the 

 power of assuming by the simple process of inspiration or expiration. How 

 far the state of tiic lungs is connected with the change of colour, is a ques- 

 tion not as yet satisfactorily determined. 



