i^j APPENMX 



ganization of the acephalous class of animals, in which the veins of the intestinal cana'i, 

 and Consequently the vena porta, also, open immediately into the vena cava inferior. 

 5th, The lungs are not formed until a more advanced period. 



In man, manifest traces of them cannot be seen before the sixth or seventh week. Then 

 they advance beneath the heart, at the two sides of the inferior extremity of the pectoral 

 portion of the aorta. At the period of their appearance, and even for some time afterwards, 

 they are so small, in proportion to the heart and the other organs, that it requires the great- 

 est attention in following their progressive development, to be convinced that they are ia 

 reality the rudiments of the respiratory organ. 



At first the lungs closely approach one another: they are flat and of a whitish colour. 

 Their surface is perfectly united ; but, on their external border, may be observed, at an 

 early period, indentations, which are the traces of the approaching separation of the lobes, 

 notwithstanding that these lobes are not yet in existence. At a farther advanced period 

 the lobes appear to be composed of lobules. These latter are at first larger and much less 

 numerous, in proportion, than at subsequent periods, but they separate by degrees into 

 others much smaller. At the period when they are first observed, they are as much more 

 apparent, and as much less intimately united, by cellular tissue, as the embryo itself is 

 younger. 



6th, As the lungs become developed, in the reptiles and leeches, in the form of an empty 

 sack, it is natural to suppose that their production, in animals of a higher order, takes place 

 according to the same manner and law. M. Meckel endeavoured to ascertain whether or 

 no this was actually the case. But, under whatever aspect he viewed the lungs at the ear- 

 ly stages of their formation, even with the assistance of the microscope, he always found 

 the slices which were removed from them completely solid; if they are really so during this 

 epoch, it would seem as if they had some analogy of structure with the Branchiae of fishes. 

 7th, The branches of the pulmonary artery, which proceed from the right or pulmonary 

 aorta, are at first certainly wanting. It must therefore be admitted that at this epoch, 

 their places are supplied by the bronchial arteries, especially by the inferior, since the lungs 

 are at first placed low in the inferior part of the chest. Moreover, this depending situation 

 of the respiratory organ, at the commencement of its development, is remarkable under two 

 points of view: 



Jl. Since, amongst reptiles, and many of the mammiferi, the lungs are placed much lower 

 than in man, and below the heart, in every respect like the fishes, the swimming bladder 

 is placed below this organ. 



B. Because it seems that the lungs and the thymus gland correspond in their functions, 

 the development of the one being in direct proportion to the decrease of the other. 



L F. Meckel concludes from the researches of which we have given an abridged outline, 

 that the general results confirm it to be a grand law of the animal economy, that the em- 

 bryo, from the instant of its formation until that of its maturity, rises successively through 

 many inferior grades of organization, and that the principal monstrosities of the heart and 

 large blood-vessels depend upon these organs being arrested at some one grade or degree of 

 organization, instead of following the progress of the others towards perfection. 



Respiration of the Foetus. The thymus gland appears to assist the placenta, the liver, 

 and the secretion cf fat, in the respiration of the foetus, or rather in purifying the blood of 

 the foetus. It seems to form a nidus for the reception of those elements of the blood ^-car- 

 bon and hydrogen, which are secreted in a state approaching to fat, and which if too abun- 

 dant in this fluid, would endanger the existence of the foetus. These materials on the com- 

 mencement of active respiration are again absorped, to be discharged from the economy by 

 the lungs, liver, or intestinal canal. The thymus gland in the human foetus, in the ninth 

 month, generally weighs from 160 to 180 grains; at 28 years of age only 90 grains. 

 In the calf it weighs 16 ounces, in the cow 9 ounces. 



*' Etenim placenta, hepar, adipis aucta secretio respirationi, sed aliud aliomodo, inser- 

 viunt. Quoe naturre institutio, ut in foetu organon alterum alterius vices obtinere possit, 

 pulcherrima et prsestantissima; quo fit, ut foetus vita nondum autonomica, a noxiis qtiibus- 

 cunque momentis, quae vim in ipsum habere possunt, tueatur conserveturque, donee ex 

 asylo matris in lucem aeremque editus vim innatam exerceat. 



Vena umbilicalis illo principio (oxygenio) gravida partim in hepar, partim in venam ca- 

 vam inferiorem sanguinem reducit a partibus phlogisticis liberatum. Itaque vepa cava in- 

 ferior, postquam sangninis partem ex vena umbilicali et venis hepaticis excepit, prater san- 

 guinem oxydatum, venosum quoque sanguinem ex corporis partibus reducem continet, cujus 

 tamen pars satis magna, sanguis lienalis ac meseraicns, in hepate jam carbone relicto, mera 

 existit. Quoniam vero vena' cava inferior prima vitae foetalis parte magis in sinistrum quam 

 in dextrum cordis atrium aperitur, sanguis autem venre cavae superioris, nil minus quam 

 oxygenium ducens ex dextro cordis atrio per arteriam pulmonalera, hinc per ductum Botal- 

 licura, demum, postquam jam arteriac superiorum partlum ex aorta oxcreverun 1 -, in ipsaia 

 perfunditur; -sequitur caput atque extremitates superiores sanguinem magis oxydaturn, 

 sen, si mavis, dephlogisticatum, in atrio sinistro ventriculcque congestum, accipere: aor- 



