4o6 



NATURE 



\^Mar. 26, 1874 



THE COMMON FROG* 

 XII. 



SO much for the circulation of the frog in its adult con- 

 dition. Its larval, or tadpole stage, presents us with 

 a series of changes which, though more familiar, are not 

 less wonderful. 



In the first place, however, it may be well to describe 

 shortly the condition of the circulation in fishes, where 

 the purilication of the blood is effected, not by means of 

 the exposure of the blood to the action of air taken into 

 respiratory cavities of the body, but by its subjection in 

 little plates of membrane, the gills, to the influence of air 

 mechanically mixed up with and dissolved in the water 

 in which those gills arc bathed. 



In fishes, moreover, unlike all air-breathing animals, 

 none of the oxygenated blood is returned to the heart for 

 propulsion, but is collected directly into the great dorsal 

 aorta, whence it is distributed to the whole body, only 

 being returned to the heart after such distribution, so that 

 venous blood alone enters that organ. 



This venous blood is sent out from the heart through a 

 bulbous aorta, whence arise on each side a series of 

 arteries which ascend the branchial arches, one on the 

 outer side of each such arch, decreasing in size as it 

 ascends. 



Each branchial artery gives off small gill arteries, 

 which run along one edge of each little membranous 

 leaflet or gill, and supply it with minute branches ending 

 in capillaries, in which the blood is purified. There the 

 purified blood is taken up by minute veins which open 

 mto gill veins, one of which runs along the opposite edge 

 of each gill to that occupied by the gill artery. 



The gill veins pour their contents into branchial veins, 

 one of which ascends the outer side of each branchial 

 arch, increasing in size as it ascends. The branchial 

 veins open into the great dorsal aorta, whence the blood 

 is distributed over the body. Generally the branchial 

 arteries are only connected with the branchial veins by 

 the intervention of the capillary vessels of the gills. Some- 

 times, however (as e.g. in the mud-fish, Lepidosircn), the 

 branchial veins are directly continuous with the branchial 

 arteries. 



In the tadpole, while the gills remain fully developed, a 

 condition exists quite similar to that of fishes. Minute 

 vessels, however, directly connect together, at the root of 

 each gill, the branchial artery and branchial vein of each 

 gill. Such a connecting vessel is tertned d^ductiis botalli. 



A minute vessel given off from the third branchial 

 artery, is the incipient pulmonary artery. 



As development proceeds, as the gills diminish by ab- 

 sorption, and as their respective arteries and veins de- 

 crease in size and importance, each duchis /'oA;///' increases 

 until at last we have established the six great continuous 

 vessels of the adult frog. 



We have, then, in the life-history of the frog, a com- 

 plete transition from the condition of the fish to that of 

 a true air-breathing vertebrate, as regards its circulation. 

 The various conditions herein referred to have, however, 

 an important bearing on the question of the first origin 

 of such structure. 



All higher animals, even the very highest, have the 

 great arteries, when they first appear, arranged substan- 

 tially as in fishes. 

 _ From the common aortic bulb five vessels ascend each 

 side of the neck, and more or fewer of these arteries abound 

 in diflerent classes, the permanent adult condition being 

 arrived at by this circuitous route. 



This argument has commonly been adduced as an 

 argument in favour of the descent of air-breathing animals 

 from more ancient gill-bearing forms, and it is not with- 

 out weight. 



* Continued from p. 369 



Nevertheless it must be borne in mind that the primi- 

 tive condition in Fishes is that of direct continuity between 

 the branchial arteries and veins such as we have seen 

 exists permanently in Lepidosiren. It is only as develop- 

 ment proceeds that each primitive continuous arch 



Fig. 78. — Infero-lateral view of Head and Aortic Arches of Lepidosiren (after 

 Hyrtl). a, cesophagus : b, anterior end of bulbus aortae : c, common 

 roots of the first aortic arches : d, third aortic arch : tf, first aortic arch ; 

 y, dorsal union of the first three aortic arches : ^, aorta ; /f, cceliac artery; 

 (, exit of the fifth nerve ; k, part of operculum ; /, exit of the nervus 

 vagus from the skull : ?«, branches to cesophagus ; «, nerve going to the 

 rectus abdominis ; 0, nervus lateralis ; p, first and hypertrophied rib ; g, 

 posterior part of the skull ; r, segm«ited neural spines ; J, chorda dor- 

 salis ; /, mandible ; ?/, quadrate. 



becomes broken up into an artery and a vein connected 

 by a net-work of capillaries. 



Now we can understand the series of unbroken arches 

 in higher animals as the relics of ancestral vessels which 

 divided for gill circulation and were therefore once of e.K- 

 treme functional importance and utility. But how can 

 we understand the primitive unbroken series of arches in 

 Fishes .■' Their utility was yet to come ! 



The frog when adult has, besides its skin, no breathing 

 organs but the lungs. As has been said before, other 

 members of the Frog's class retain gills and aquatic 

 respiration during the whole of life, as for example Me/io- 

 branchus. 



Every one kind, however, whether provided perma- 

 nently with gills or not, develops lungs, and it might 

 easily be imagined that similarly every giiled-creature 

 which has lungs is also a Batrachian. 



Fig. 79. — The Circulation of a Tadpoli 

 all the blood is distributed t 

 quite rudimentary, and the ve 

 ther the branchial artery and ' 

 bulbus aortx ; b, branchial 



its primitive stage, when nearly 



the gills : the pulmonary arteries being 



1 (or ductus botalli) connecting logc- 



1 at the root of each gill being minute. 



br', by', br\ the three gills (o: 



branchiae of each side) : /t', the branchial veins which bring back the 

 blood from the gills — the hindermost pair of branchial veins on each side 

 unite to form an aortic arch (^aa), which again unites with its fellow of 

 the opposite side to form da^ the descending (or dorsal) aorta. The 

 branchial veins of the foremost gills give rise to the carotid arteries, cc^ 

 o, artery going to the orbit -.pa, pulmonary artery ; i. 2, 3, anastomosing 

 branches connecting together the adjacent branchial arteries and veins. 



This, however, would be a mistake. 



The Mud-fish or Lepidosiroi, already referred to more 

 than once, is furnished with both gills and lungs through- 

 out the whole of hfe. On this account it has been 

 reckoned by some naturalists to be a Fish and not a 

 Batrachian. Its fish-nature, however, has now been tho- 



