THE YOUNG NATURALIST. 



147 



Whilst the respiratory apparatus is becoming reptilian, the heart is also 

 passing from its primitive fish-like simplicity, possessing only an auricle and 

 a ventricle, and containing only venous blood, to a higher and more complex 

 form, possessing two auricles and a ventricle, in which, though a partial mix- 

 ture of pure and impure blood takes place, yet we can see traces of the 

 subsequent perfection of this organ. 



The result of this mixture of arterial aud venous blood is that frogs and 

 toads are cold-blooded. 



An absolutely distinctive character of the Batrachian as compared with the 

 reptilian skeleton is, that in the former, the skull articulates with the vertebral 

 column by means of two condyles, while in the latter only one occipital con- 

 dyle is present. 



Another character of the Batrachian skeleton is the absence of ribs, which 

 play so important a part in facilitating the respiration in all higher animals, 

 in which the breast bone and ribs are capable of moving in such a manner as 

 to alternately increase and diminish the size of the thorax, so giving rise to 

 an inspiratory and expiratory flow of air. 



Progs and Toads being destitute of ribs can only breathe by swallowing 

 the air. In this process the throat is alternately distended and contracted, 

 this curious action having no doubt given rise to the idea that the animal 

 was just getting ready to spit upon its foe the venomous fire of ancient fable. 

 The real cause of this movement is the alternate contraction and relaxation of 

 the muscles, which first depress the tongue, thus enlarging the cavity of the 

 mouth, into which the air rushes through the nostrils ; these are then closed 

 by the tongue, and the contraction of the muscles, diminishing the cavity, 

 forces the air into the lungs * for when the muscles of the throat contract, 

 appropriate muscles simultaneously close the entrance to the stomach, and as 

 the tongue prevents the air from escaping by the nostrils, it can only go to 

 the lungs. It follows that to prop open the mouth of a Batrachian is suffi- 

 cient to cause death by suffocation. 



The bones of the fore legs are short, bnt those of the hind legs are greatly 

 developed ; forming long levers for the muscles, which give to these animals 

 their great swimming, and to the frogs, their great leaping powers. Anyone 

 who has seen a frog or toad swimming, cannot fail to have noticed the 

 ludicrous resemblance which it presents to a man. 



Near the hinder part of the back of a frog or toad are two protuberances, 

 behind which the back falls suddenly, and presents a somewhat triangular 

 form, the apex of the triangle being the extremity of the body. This appear- 

 ance is due to the peculiar shape of the pelvis, as anyone who has seen a frog's 

 skeleton will know. The iliac bones which form the sides of the pelvis are 



