Note 36 to p. [61].
As these views have a social and economical importance quite independent of their physiological value, I will endeavour, in this note, to fortify them still further, by showing that the connexion between carbonized food and the respiratory functions may be illustrated by a wider survey of the animal kingdom.
The gland most universal among the different classes of animals is the liver;[a] and its principal business is to relieve the system of its superfluous carbon, which it accomplishes by secreting bile, a highly carbonized fluid.[] Now, the connexion between this process and the respiratory functions is highly curious. For, if we take a general view of animal life, we shall find that the liver and lungs are nearly always compensatory; that is to say, when one organ is small and inert, the other is large and active. Thus, reptiles have feeble lungs, but a considerable liver;[c] and thus, too, in fishes, which have no lungs, in the ordinary sense of the word, the size of the liver is often enormous.[d] On the other hand, insects have a very large and complicated system of air tubes; but their liver is minute, and its functions are habitually sluggish.([e]) If, instead of comparing the different classes of animals, we compare the different stages through which the same animal passes, we shall find further confirmation of this wide and striking principle. For the law holds good even before birth; since in the unborn infant the lungs have scarcely any activity, but there is an immense liver, which is full of energy and pours out bile in profusion.[f] And so invariable is this relation, that in man the liver is the first organ which is formed: it is preponderant during the whole period of fœtal life; but it rapidly diminishes when, after birth, the lungs come into play, and a new scheme of compensation is established in the system.[g]
Footnotes:
[a]‘The most constant gland in the animal kingdom is the liver.’ Grant's Comp. Anat. p. 576. See also Béclard, Anat. Gén. p. 18, and Burdach, Traité de Physiol. vol. ix. p. 580. Burdach says, ‘Il existe dans presque tout le règne animal;’ and the latest researches have detected the rudiments of a liver even in the Entozoa and Rotifera. Rymer Jones's Animal Kingdom, 1855, p. 183, and Owen's Invertebrata, 1855, p. 104.
[]Until the analysis made by Demarçay in 1837, hardly any thing was known of the composition of bile; but this accomplished chemist ascertained that its essential constituent is choleate of soda, and that the choleic acid contains nearly sixty-three per cent. of carbon. Compare Thomson's Animal Chemistry, pp. 59, 60, 412, 602, with Simon's Chemistry, vol. ii. pp. 17–21.
[c]‘The size of the liver and the quantity of the bile are not proportionate to the quantity of the food and frequency of eating; but inversely to the size and perfection of the lungs…. The liver is proportionately larger in reptiles, which have lungs with large cells incapable of rapidly decarbonizing the blood.’ Good's Study of Medicine, 1829, vol. i. pp. 32, 33. See Cuvier, Règne Animal, vol. ii. p. 2, on ‘la petitesse des vaisseaux pulmonaires’ of reptiles.
[d]Carus's Comparative Anatomy, vol. ii. p. 230; Grant's Comp. Anat. pp. 385, 596; Rymer Jones's Animal Kingdom, p. 646.
[e]Indeed it has been supposed by M. Gaëde that the ‘vaisseaux biliares’ of some insects were not ‘sécréteurs;’ but this opinion appears to be erroneous. See Latreille, in Cuvier, Règne Animal, vol. iv. pp. 297, 298.
[f]‘La prédominance du foie avant la naissance’ is noticed by Bichat (Anatomie Générale, vol. ii. p. 272), and by many other physiologists; but Dr. Elliotson appears to have been one of the first to understand a fact, the explanation of which we might vainly seek for in the earlier writers. ‘The hypothesis, that one great use of the liver was, like that of the lungs, to remove carbon from the system, with this difference, that the alteration of the capacity of the air caused a reception of caloric into the blood, in the case of the lungs, while the hepatic excretion takes place without introduction of caloric, was, I recollect, a great favourite with me when a student…. The Heidelberg professors have adduced many arguments to the same effect. In the fœtus, for whose temperature the mother's heat must be sufficient, the lungs perform no function; but the liver is of great size, and bile is secreted abundantly, so that the meconium accumulates considerably during the latter months of pregnancy.’ Elliotson's Human Physiology, 1840, p. 102. In Lepelletier's Physiologie Médicale, vol. i. p. 466, vol. ii. pp. 14, 546, 550, all this is sadly confused.