368 



NA TURE 



[Mar. 12, 1874 



ear from the mouth which causes people when intently 

 listening to keep their mouth slightly open. 



In the frog there is no such external canal, but the 

 tympanum is plainly to be seen in the way already de- 

 scribed, on the side of the head, covered only by a slightly 

 striated portion of the skin of the body. The Eustachian 

 tube, however, still exists in the frog, though it is short 

 and wide, and the opening of each is to be seen on one 

 side of the back of the mouth. 



This condition of things, however, does not exist in all 

 the members of the frog's order, still less of his class. But 

 in Proteus, Shrn, and Mcnobranchus there is no tympanic 

 cavity whatever, and the organ of hearing is simply im- 

 bedded in the skull, and probably responds but to sonorous 

 vibrations conveyed to it by the denser aquatic medium, 

 and not at all, or but very imperfectly, to those of the 

 atmosphere. 



In the ordinary efts we still meet with an Eustachian 

 canal, but the tympanum is absent. 



In the frog's own order, as in Pelotates and Bpmbina/or, 

 we may fail to find any tympanum, while the Eustachian 

 tubes are all but obliterated, being reduced to the most 

 minute dimensions. 



Another condition, however, may be presented which 

 offers a singular contrast, and is the more remarkable from 

 the widely separated geographical situations of the forms 

 which present it. In the South American Pipa, as well 

 as in the South African Dactylcthra, the two Eustachian 

 tubes run together and open at the back of the mouth, by 

 a median and common aperture. 



Strange to say, this is the very condition which exists in 

 birds, though most certainly it cannot be taken as any 

 sign of affinity. In the crocodile these tubes have also a 

 common median opening, but, unlike birds, each tube has 

 also its own lateral opening into the throat, so that there 

 come to be three Eustachian openings. 



Can the resemblance between Pipa and Dactylethra in 

 this matter be taken as a serious indication of genetic 

 affinity, in spite of the wide, deep, and probably ancient 

 Atlantic which rolls between the two species now ? 



This is a question which cannot be confidently .an- 

 swered, seeing in how many other instances structural 

 peculiarities have evidently had an independent origin. 

 Nevertheless, the fact that these two genera agree also in 

 the small size of the eyes, rudimentary eyelids, and vastly 

 expanded sacral transverse process would seem to point to 

 some ancestral and fundamental relationship. If so, how- 

 ever, it is remarkable that no other such forms, or no 

 intermediate ones should have been preserved, seeing that 

 neither kind can be suspected of having migrated to its 

 own habitat from the existing habitat of the other ; and 

 therefore that forms similar to that from which we may, if 

 we please, conceive both to have been derived must have 

 had a more or less widely extended geographical distribu- 

 tion and have been numerous in order to have given origin 

 to genera in many respects so different as the two in 

 question. 



The Circulation of the Frog 



Not only every animal, but every living being, requires, 

 in order to carry on the functions of life, to interchange 

 some of the gaseous elements of its body with gases of 

 the medium (air or water) in which it happens to live. 



Another function of extreme generality is that of con- 

 veying to all the paits and organs of the body nutritious 

 matter for their growth or for the repair of those destruc- 

 tive effects Wiiich the processes of life inevitably produce 

 in them. 



In all members of the highest sub-kingdom of animals 

 (i.e. in all Vertebrata) these processes of gaseous inter- 

 change and nutrition are effected by means of closed 

 vessels, along which the stream of nutritious fluid (the 

 blood) is continually carried in a definite and constant 

 course. During some or other part of that course the 



blood becomes exposed to conditions specially favourable 

 to the gaseous interchange, the bl od parting with car- 

 bonic acid gas and ootaining in its place an increased 

 supply of oxygen. 



This process of blood oxygenation is termed respira- 

 tion, and the organs which subserve it are termed respira- 

 tory or breathing organs. Such organs in man are the 

 lungs. The central organ of circulation in man is, as all 

 know, the heart, which is a muscular organ, divided into 

 four chambers, or cavities. 



These chamlsers are called " auricles " and " ventricles," 

 and there are two of each — there being an auricle and a 

 ventricle on the right side and also on the left. 



Blood that has performed its nutritive functions, and is 

 therefore charged with carbonic acid gas, is called venous 

 blood, and is conveyed by the veins to the right auricle, 

 whence it passes into the right ventricle, which sends it 

 to the lungs for purification. 



The oxygenated, or arterial blood, is returned from the 

 lungs to the left auricle, and hence it is directly trans- 

 mitted to the left ventricle, whence it is driven through 

 the great artery (the aorta) into other arteries, and so dis- 

 tributed all over the body. The aorta passes downwards 

 in front of the backbone, when it is called the descending 

 aorta. Before turning downwards, however, it gives off 

 great arteries to the arms and head, the carotid arteries 

 carrying blood to the latter. 



Now It is very important that the blood should not 

 proceed in a direction the reverse of that indicated, and 

 to prevent such misdirection, or regurgitation, special 

 valves are placed at different openings ; these valves 

 freely allowing the blood to flow in the proper direction, 

 but instantly opposing an effectual obstacle to a contrary 

 flu.x. 



The openings of the auricles into the ventricles are 

 guarded by valves, as also is the opening of the left ven- 

 tricle into the aorta, and that of the right ventricle into 

 the artery going to the lungs. 



The valve which guards the entrance into the right 

 ventricle is called tricuspid, and consists of three flaps 

 attached by delicate tendinous cords in such a way as to 

 hinder the tending backwards of the flaps into the right 

 auricle, and so allowing the blood to flow back into that 

 chamber. 



The valve which guards the entrance into the left ven- 

 tricle is called mitral (from a fancied resemblance to a 

 bishop's mitre), and consists of two flaps. The aperture 

 leading from the left ventricle to the aorta is guarded by 

 three crescentric flaps — called the " semilunar " valves of 

 the aorta. 



In man the whole of the blood is sent to the lungs for 

 purification during each circuit of this most important 

 fluid, and every organ is supplied with o.xygenated blood. 



If in any animals the process of purification is incom- 

 plete it is manifestly desirable that these organs of the 

 body, the functions of whicli are the most important, 

 should be supplied with that part of the blood which is 

 pure. This consideration eminently applies to the brain, 

 the director and controller of the entire body. 



Now all birds and beasts without exception, share with 

 man this perfect aeration of the entire blood, the whole of 

 the blood m the classes Mammalia and ^Ivcs being purified 

 in the lungs before being distributed to the body. 



The conoiiions by which the frog, at the various stages 

 of iis existence, oxygenates its olood and directs the puri- 

 fied stream in the most desirable manner, are curious and 

 instructive. 



It is generally known that the lower air-breathing Ver- 

 tebrates (Reptiles and Batrachians) have the heart less 

 completely divided than in the higher classes, so that the 

 oxygenaicu (or arterial) blood and the unoxygenated (or 

 venous) blood become mixed in the single or imperfectly 

 divided ventricle. 



It might well be supposed, and in fact has generally 



