ARTICLE FOURTH.
DEVELOPMENT OF THE SEROUS SYSTEM.
I. State of this System in the First Age.
All the serous surfaces are extremely delicate in the fœtus. In opening the thorax by a longitudinal section of the sternum and examining the pleura in the mediastinum where it is free on both sides, it is found to have less thickness than the transparent layers of the omentum or the arachnoides in the adult. The peritoneum is a little thicker in proportion, but yet its delicacy is very great. The comparison of soap bubbles is hardly sufficient to convey an idea of the fineness of the texture of the omentum and the arachnoides.
At this period the fluid that lubricates the serous surfaces is much more unctuous and viscid than it is afterwards; by carrying the fingers over these surfaces at the different ages, the difference is easily perceived. It might almost be said that the tangible qualities of the serous fluids then approximate those of the synovia. I know not to what this difference belongs.
Besides, the quantity of these fluids does not appear to be so great in proportion as that of the cellular fluids, with which they have however so much analogy; which is probably owing to this, that the internal motions being less numerous, on account of the inaction of most of the organic muscles, less fluid is necessary to lubricate the surfaces.
The growth of the serous system is always in proportion to that of the organs which it covers. The arachnoides is larger in proportion than it will be in the adult; it seems even, like the brain, to become then the seat of a more active labour; thus diseases are more frequent in it. The increase of exhalation is more common in it than in all the other serous sacs; hence hydrocephalus.
At birth, when the internal motions become suddenly very numerous, on account of respiration, digestion and the excretions, I presume that the serous surfaces become the seat of a more active exhalation. Besides, as very little blood penetrates them, the sudden production of the red blood and its entrance by the arterial system, where it succeeds the black blood, produces less changes upon them than upon the mucous surfaces and the muscular system.
The serous membranes grow like the other organs; for a long time delicate and diaphanous, they gradually thicken as we advance in age, and become of a dull white. Their suppleness diminishes as their density increases; they resist the different reagents so much the less as the subjects are younger. In infants, maceration and ebullition reduce them much more quickly to a homogeneous pulp.
I have observed that in the fœtus which has become putrid, there is often collected different gases in the serous cavities, as may be proved by opening these cavities under water; a phenomenon much less evident in the adult, in whom the cellular texture is often wholly emphysematous by the putrefactive motion, without the escape of any thing by the canula of a trochar which is plunged into the peritoneal cavity or into that of the pleura, as I have many times ascertained. In general, there is disengaged much more aeriform fluid from the organs of the fœtus, than from those of the adult, in the experiments of maceration.