We have just now seen that the animal life, which is inactive in the fœtus, is developed after birth: we have also followed up the particular laws of its development. On the contrary, the organic life comes into action almost as soon as the fœtus is conceived; for as soon as the least organization is apparent, the little heart will be seen protruding its blood on all sides. The heart is the first formed part, the first in action: now, as all the organic phenomena depend upon it, we may readily conceive in what way the functions of the inward life are thrown into exercise.
I. Of the mode of the organic life in the fœtus.
Nevertheless, the organic life of the fœtus, is not the same as that which the adult is destined to enjoy. Let us enquire into the reason of this difference.
We have said that the organic life is the result of two great orders of functions, of those namely of assimilation and decomposition, so as to form an habitual circle of creation and destruction. Now in the fœtus this circle is singularly contracted.
For in the first place, the functions of assimilation are much fewer in number; the molecules before they arrive within the organs which they are destined to create, are not submitted to so many actions; they penetrate the fœtus already elaborated by the digestion, circulation, and respiration of the mother. Instead of traversing the apparatus of the digestive organs, which at this age appear to be almost inactive, they enter at once into the system of the circulation; the road which they have travelled is less, it is not requisite that they should be presented to the influence of respiration; and accordingly the fœtus of the mammalia has in its preliminary organization a near analogy with that of the adult reptile, in which but a small part of the blood at its issuing from the heart, is sent into the vessels of the lungs.[48]
The molecules of nourishment in this way pass almost directly from the circulating torrent into the nutritive system. The general process of assimilation, then, is much less complicated than that of the following age.
On the other hand, those functions which habitually decompose the organs, which clear the system of substances already become injurious and foreign to its nature, are at this age but very inactive. Neither the pulmonary exhalation, nor sweating, nor transpiration have as yet commenced: the bile, urine, and saliva are but small in quantity, if compared with what they are destined at a future time to be, so that the portion of blood from which they are to be made in the adult, in the fœtus is almost entirely expended on the system of the nutritive organs.
The organic system of the fœtus, then, is remarkable—on the one hand, for the extreme promptitude of its assimilation, a promptitude depending on the very small number of the functions concurring to that end; and on the other, for the extreme inertia of its decomposition, an inertia depending on the little activity of the different functions, which are the agents of this great process.