Mar. 12, 1 8 74 J 



NATURE 



367 



THE COMMON FROG* 

 XI. 



THE eye of the frog is a beautiful and brilliant object, 

 and relatively large. It is furnished with two eyelids, 

 Dut, unlike those of man, it is the inferior one which is the 

 more movable. In addition to these it is defended by a 

 third eyelid, called the victitating eyelid, which is similar 

 to that one which may be seen (if watched for) so fre- 

 quently and rapidly to cross the eye of birds, e.g. of a 

 hawk. 



Fig. 74. — I. The left 'side : and II. the right side of the Heart dissected. 

 I. — LA, the left auricle: PV, the four pulmonary veins; cd, a style 

 passed through the auricalo-ventrlcular aperture ; MVy the mitral valve : 

 ab, a style parsed through the left ventricle into the aorta ; RA, RV, 

 parts of the right side ot the heart ; PA, pulmonary artery. 

 II,— ;?^, the right auricle: VCS, superior vena cava; VCI, inferior 

 vena cava, the styles fe, cd, being passed through them into the auricle ; 

 ah, style passed through the auriculo-ventricular aperture ; 7* F, tricuspid 

 valve : RV, right ventricle : SL. semi-lunar valves at the base of PA, 

 the pulmonary artery, through which the style ^/^ is passed : LA, LV, 

 auricle and ventricle of the left side of the heart. 



This structure, however, is no mark of affinity to birds, 

 as it is one which reappears, when wanted, in widely dif- 

 ferent forms. Thus we find it in the whale, i.e. in the 



JFlG. 7S.— The Trog's Heart. The ventricle is below j, the aortic bulb is on the 

 left ol s, and ends in six aortic trunks, three on each side. 1 he first of 

 these (i), enrts in the carotid gland Id), whence spring the lingual (/), and 

 the carotid if), .arteries. The second trunk (2). is the root of the great 

 dorsal aorta. The third trunk (3), ends in the pul mo-cutaneous artery 

 (r), and the pulmonary artei;y (/>), which is shown sending ramifioations 

 over each lung. 



highest class of the Vertebrate sub-kingdom, and in cer- 

 tain sharks, i.e. in the lowest class of the same. 



Eyelids do not exist in all members of the frog's class. 

 Even in its order they are e.xtremely minute, in Pipa and 



* Continued from-p 307. 



Dadylelhra, which have very small eyes. In Amphittma 

 they are completely wanting, and in Proteus and in the 

 Ophioinorphti the minute eyeballs are covered with the 

 ordinary and unchanged skin of the head. 



The ear of the frog's class presents us with the inci- 

 pient condition of that part as an organ destined to re- 

 spond to sonorous vibrations conveyed to it by the 

 atmosphere. 



In man the internal ear (enclos'id in the densest bone 



Fig. 76. — Section of heart, a and b, openings of the auricles into the ven- 

 tricle : c, opening of the aortic bulb into the ventricle. 



of the skull, named, from its density, "petrous") is a very 

 comple.x organ. The aperture, surrounded by the folds 

 of the external ear, leads by a canal towards a cavity 

 called the tympanic cavity, which cavity is shut off from 

 the exterior by the tympanic membrane (or drum of the 

 ear), which stretches across the canal at a considerable 

 distance from its external aperture. On the inner side of 

 the tympanic cavity lie the convoluted tubes (richly sup- 

 plied with nerves) which constitute the real organ of 

 hearing or internal ear. 



<•»"— ^ p-'3'* 



Fig. 77.— Diagram ofsection of Frog's heart. LA, left auricle : RA, right 

 auricle ; 1^, ventricle ; s, movable septum dividing the left aortic pas- 

 sage // from the right aortic passage i/> : v, valve ; 3, 3, aortic trunks 

 leadiug to p, pulmonary artery and r, cutaneous respiratory artery ; 

 2,2, aortic trunks going to f>rm th- great dorsal aorta: cgld, carotid 

 gland interrupting the flow of blood into /, the lingual artery, and c, the 

 carotid artery. 



Although the tympanic cavity is shut off from the exte- 

 rior by the tympanum, it nevertheless is not altogether 

 shut off from the exterior, since it communicates with the 

 back of the mouth by a long and narrow canal termed 

 the Eustachian tube. 



It is the existence of these Eustachian openings into the 



