The vessels of the umbilical cord are imbedded in a thick viscid substance; upon minute examination, it will be found to consist of a very fine cellular tissue, containing an albuminous matter which slowly exudes, when pressed between the fingers. This cellular tissue itself may be demonstrated by the inflation of air or injection with mercury: it seems to accompany the umbilical vessels as far as the posterior surface of the peritoneum; and Lobstein is of opinion that it is a continuation of the cellular tissue, which covers this membrane. (Lobstein, sur la Nutrition du Fœtus. § 75.)
Externally, the umbilical cord is covered by a continuation of the amnion, which, although it be the inner membrane of the ovum, is the outer covering of the cord: in some places it is very thick and strong, and not easily ruptured. From repeated observations, the weakest part of the cord seems to be at about three or four inches distant from the umbilicus, this being the spot where it has invariably given way in every case we have seen, where the cord has been broken at the moment of the child’s birth.
From the time of the commencement to the full time of utero-gestation, the cord becomes gradually longer, so that it attains an average length of from eighteen to twenty inches; this, however, varies remarkably. We have known the cord exceed forty inches; and a case is described by Baudelocque, where it was actually fifty-seven inches long: on the other hand, it is sometimes not more than four or five inches in length.
It is remarkable that the cord, which at the end of pregnancy is usually of about the same length as the fœtus, is relatively much longer during the sixth month; hence we may conclude, that in those cases where knots have been found upon the cord, the knot must have been formed at this period when the fœtus was small enough to pass through a coil of it.
Neither blood-vessels nor lymphatics have as yet been found in the structure of the cord itself. A filament of nerve from the solar plexus has been occasionally seen passing through the umbilical ring, and extending to a distance down the cord.
The vesicula umbilicalis and allantois, being essentially connected with the earliest grades of fœtal development, will be considered under that head.
Embryo. There is, perhaps, no department of physiology which has been so remarkably enriched by recent discoveries, as that which relates to the primitive development of the ovum and its embryo. The researches of Baer, Rathke, Purkinje, Valentin, &c. in Germany; of Dutrochet, Prevost, Dumas, and Coste, &c. in France; and of Owen, Sharpey, Allen Thomson, Jones, and Martin Barry in England, but more especially those of the celebrated Baer, have greatly advanced our knowledge of these subjects, and led us deeply into those mysterious processes of Nature which relate to our first origin and formation.
These researches have all tended to establish one great law, connected with the early development of the human embryo, and that of other mammiferous animals, viz, that it at first possesses a structure and arrangement analogous to that of animals in a much lower scale of formation: this observation also applies of course to the ovum itself, since a variety of changes take place in it after impregnation, before a trace of the embryo can be detected.
At the earliest periods, the human ovum bears a perfect analogy to the eggs of fishes, amphibia, and birds; and it is only by carefully examining the changes produced by impregnation in the ova of these lower classes of animals, that we have been enabled to discover them in the mammalia and human subject.
As the bird’s egg, from its size, best affords us the means of investigating these changes, and as in all essential respects they are the same in the human ovum, it will be necessary for us to lay before our readers a short account of its structure and contents, and also of the changes which they undergo, after impregnation. In doing this we shall merely confine ourselves to the description of what is applicable to the human ovum.