RESPIKATION OF AMPHIBIA. 695 



vessels principally ramify : towards the hinder part of the viscus the cells 

 become larger, and the breathing-surface proportionately less extensive, 

 until in some cases, as in Serpents, the cells being quite obliterated, the 

 lung terminates posteriorly in a simple membranous bladder. 



(1946.) The air is brought into the lungs through a long trachea, 

 composed, as in other Yertebrata, of a series of cartilaginous rings ; but 

 there is this peculiarity in the construction of the Keptile lung the 

 trachea never divides into bronchial ramifications, but terminates ab- 

 ruptly by one or more orifices which open at once into the general pul- 

 monary cavity. 



(1947.) It must be evident, from the whole construction of a lung of 

 this description, that, owing to the comparatively limited surface that it 

 presents internally, it is far less adapted efficiently to expose the circu- 

 lating fluid to the influence of the atmosphere than the more complex 

 apparatus of Birds and Mammalia : the respiration of Reptiles is conse- 

 quently proportionately imperfect ; and hence that coldness of their blood 

 and feebleness of muscular movement which are so characteristic of the 

 entire class. 



(1948.) The air required for purifying the blood is, of course, con- 

 tinually "changed, being alternately taken into the lungs, and again ex- 

 pelled in a deteriorated condition, by a mechanism which will be found 

 to vary in different reptiles in accordance with the peculiarities of their 

 organization. No Reptile possesses a diaphragm ; and being destitute of 

 this important muscle, the movements whereby inspiration and expira- 

 tion are accomplished are, in such genera as are furnished with moveable 

 ribs, entirely dependent upon the mobility of the framework of the 

 chest the dilatations and contractions of the thorax, consequent upon 

 the alternate elevation and depression of the ribs, being sufficient to en- 

 sure the inhalation and expulsion of air : such is the case in the Serpent 

 and the Lizard. 



(1949.) In the AMPHIBIA, however, there are not even ribs developed, 

 or, if they exist at all, they are such mere rudiments as to be quite useless 

 as instruments of respiration ; and on the other hand, in the CHELONIAN 

 REPTILES, the large and expanded bones of the thorax are so consolidated 

 together, and so immoveably fixed to the broad and osseous sternum, 

 that respiration in the ordinary manner would be altogether imprac- 

 ticable. Under these circumstances, as a compensation for the want of 

 mobility in the chest, the os hyoides and the muscles of the throat are 

 converted into a kind of bellows, by which the air is forced mechanically 

 into the lungs, and they are thus distended at pleasure. 



(1950.) Any one who watches a Frog or a Tortoise with a little at- 

 tention will at once understand the mechanism by which this is effected. 

 The mouth is kept closely shut ; and the nostrils, which open immedi- 

 ately into its cavity, are each provided with a muscular valve, so dis- 

 posed as freely to permit the entrance of air into the mouth, but also 



