100 



AMPHIBIA. 



simple structure of these pulmonary sacs, it will 

 readily be seen that the function of respiration 

 could be only very ineffectively aided by the 

 latter organs, even were there no other diffi- 

 culty arising from the imperfect structure of the 

 apparatus which in the air-breathing amphibia 

 serves the office of conveying the air into the 

 lungs. A short description of the means by 

 which the act of inspiration is effected in the 

 frog will enable us to judge how far it may be 

 possible that the rudimentary lungs in the pro- 

 tens and siren are to be considered as performing 

 any such function. 



In the adult frog, toad, salamander, and all 

 others of the higher orders of amphibia, the 

 reception of air into the lungs is effected not 

 by the primary expansion of the pulmonic cavity 

 and the consequent rush of air into it, but by 

 the act of forcing air into the lungs, or in fact 

 by a simple act of swallowing. This is effected 

 in the following manner. The os hyoides and 

 tongue are brought downwards to a considerable 

 extent, and the cavity of the mouth being thus 

 much enlarged, the air enters by the nostrils. 

 The pharynx is then shut at the posterior part, 

 so as to prevent the passage of air into the oeso- 

 phagus, and the cavity being suddenly con- 

 tracted by means of the muscles acting on the 

 os hyoides, the air is necessarily forced through 

 the glottis and trachea into the lungs, as the 

 posterior nares are closed either by their mar- 

 gins acting as a valve, or by the pressure of the 

 tongue against them. This view of the mode 

 of inspiration explains the cause of the well- 

 known fact, that if the mouth of frogs be held 

 open they perish from actual suffocation ; for 

 the motions of the os hyoides being thus im- 

 peded, and an external passage being also 

 afforded for the air, respiration by the injection 

 of air into the lungs is obviously impossible. 

 Any other mode of inspiration, connected with 

 the primary expansion of the thoraco-abdo- 

 minal cavity is obviously impossible in the frog 

 and its congeners, from the total absence of ribs. 

 It may not be out of place to explain here the 

 mode in which the peculiar noise uttered by the 

 male frog, called croaking, is produced.* Ac- 

 cording to the observations of P. Camper, the 

 inspired air is forced against the inferior surface 

 of the tongue, the protuberance of which di- 

 vides it as it were into two currents, which pass 

 into the membranous sacs adhering to the 

 lower jaw and existing exclusively in the males. 

 From these sacs it is directed over the tongue, 

 and by its vibration the peculiar sound in ques- 

 tion is produced. 



It is an interesting question whether in the 

 perennibranchiate amphibia, the organs which 

 have just been described as rudimentary lungs, 

 do ever serve the purposes of respiration in 

 even the smallest degree ; and it is one of no 

 small difficulty. The superficial structure of 

 the nares in the siren and the proteus, in which 

 they almost exactly resemble those of fishes, 

 and which would preclude the mode of inspi- 

 ration practised by the frogs, together with the 

 slight and attenuated character of the mem- 



* Comment. Soc. Reg. Scient. Gbtting. v. ix. 



branous tube and sacs, would almost lead to 

 the conclusion, assumed by Rusconi,that in the 

 proteus at least these organs do not exercise 

 any function appertaining to respiration. If 

 these animals be confined for a considerable 

 time in the same water, the branchiae become 

 purple instead of having the florid red colour 

 which characterizes them in a healthy state, 

 and they die asphyxiated. On the other hand, 

 the very excitement of the two sacs, accom- 

 panied by tubes of such length, and opening to 

 the pharynx by a sort of simple glottis, go- 

 verned by a distinct muscular apparatus, would 

 seem to warrant the opinion that a nearer affi- 

 nity to true lungs is to be traced in these 

 organs than in the air-bag of fishes, though 

 recent observations have shewn the latter organ 

 to be analogous to the lowest rudimentary state 

 of lungs in the higher animals. The chain of 

 affinities, therefore, is here perfect, as far as re- 

 gards the pulmonary cavities. 



VII. The nervous system. The centre of 

 the nervous system offers a not less striking in- 

 stance of the progressive development of the am- 

 phibia in their passage from the pisciform to the 

 reptile state than those which we have already 

 shewn in the organs of the other functions of the 

 body. The condition of the brain in the early 

 state of the frog tadpole, the genus in which the 

 changes are most strongly marked, is almost ex- 

 actly that which it possesses in the fishes. The 

 linear arrangement of the different lobes, the 

 broad and lobed form of the medulla oblongata, 

 the small cerebellum, the large size of the op- 

 tic thalami, with the distinct ventricles which 

 they contain, and the very diminutive extent of 

 the hemispheres, all evince a low degree of 

 development, and one not yet emerged from 

 that which we find in the brain of fishes. The 

 same imperfect character is also observed in the 

 spinal marrow, which even in the frog is con- 

 tinued into numerous coccygeal vertebrae, and 

 as the extremities are not yet in existence, is 

 devoid of those enlargements which afterwards 

 take place where the nerves of the anterior and 

 posterior members are given off. The brain 

 becomes developed, however, in a very short 

 period ; the changes which take place being 

 very rapid, though at last not very considerable; 

 the hemispheres become enlarged, expanding 

 laterally and in some measure upwards, con- 

 stituting the first step towards that superiority 

 in position, as well as in size, over the other 

 lobes, which is so conspicuous a character of 

 these important portions of the brain in the 

 higher animals. Fig. 28. represents the brain 



Fig. 38. 



1, pneumogastric nerve; 

 2, ninth pair; 3, sixth 

 pair ; 4, acoustic ; 5, fa- 

 cial ; 6, the eye : 7, optic 

 nerve and its tubercle ; 8 

 and 9, base of the hemi- 

 spheres ; 10, anterior por- 

 tion of ditto; 11, pedicle 

 of olfactory lobe. 



in the common frog after Serres. As the limbs 

 begin to make their appearance, the enlarge- 



