Fig. 183.—Position of the abdominal viscera in a new-born animal: Ru, rumen; E, epiploon; Rg, left kidney; Ig, small intestine; C, abomasum; U, ureter; O, urachus; R, rectum. Umbilical cord: Vo, Umbilical vein; Aa, allantoid arteries; Va, allantoid veins; O, the urachus.
It is composed of four principal structures—the umbilical arteries, the umbilical vein, the urachus, and the interstitial mucous tissue.
The umbilical arteries and vein consist of two parts—the extra-fœtal part, which co-operates in forming the cord, and the intra-fœtal part.
The first is formed of two arteries and two veins, in contra-distinction to the condition in solipeds, where the cord only contains one vein. In the second, the arrangement is as follows: The two umbilical allantoid arteries on entering the abdomen curve backwards towards the entry of the pelvis, passing over the sides of the bladder enveloped in the lateral ligaments, and extend upwards towards the bifurcation of the aorta, finally pouring their contents into the internal iliac arteries. In the adult they may still be traced as annexes of these latter vessels. The two umbilical veins on passing through the ring unite to form one within the abdomen. This vessel passes forwards, rising along the lower abdominal wall, then becomes lodged in the thickness of the inferior middle ligament of the liver, and finally penetrates that organ where it unites with the portal vein. It is also connected with a vessel known as “the vein of Arantius,” which places it in communication with the posterior vena cava, a vein not found in solipeds.
The fœtal blood is purified by exchanges between it and that circulating in the maternal placenta, and when re-arterialised it returns by the umbilical vein.
The urachus, found in the embryo and fœtus, eventually gives rise to the bladder. In new-born animals this viscus is therefore open at its base, and communicates with the allantoid cavity through the urachus. The urachus starts from the base of the bladder, and, extending along the median plane of the lower abdominal wall between the two umbilical arteries as far as the umbilical opening, takes its place in the cord alongside the vessels. Through it the secretions of the fœtal kidneys drain into the allantoid cavity. The interstitial mucous tissue, also called “Wharton’s jelly,” is a gelatinous material which unites these different vessels and helps to support and protect them in the umbilical cord. It is particularly abundant opposite the umbilicus.
Immediately after birth the umbilical cord ruptures of itself as a result of the fall which the young animal experiences or of movements made by the mother, as for instance when she attempts to rise. In certain other cases it is divided by the mother biting it, or it may be ligatured by some person present. However the rupture may be brought about, it always occurs at a distance of 2 to 4 inches from the umbilicus. The immediate result is to produce thrombosis of the umbilical vessels and obstruction of the urachus. The two umbilical arteries rarely bleed, for hæmostasis is brought about by stretching, and these arteries, being very elastic, almost immediately retract and close. The umbilical veins simultaneously become blocked, and the single intra-abdominal vein having no further raison d’être, gradually becomes obliterated. The urachus should normally be obliterated at the moment of delivery (Colin and Saint-Cyr), or at any rate soon afterwards, as a consequence of rupture of the cord (Chauveau and Zundel).
Immediately after delivery another change sets in. The extra-fœtal portion of the cord, which remains attached to the umbilicus, dries on contact with the air, the Wharton’s jelly retracts, the whole undergoes a kind of necrosis, assumes the appearance of a dry scab, and in eight or ten days falls away, leaving in its place the umbilicus, which should be half cicatrised on the fall of the cord. Thus the umbilical cord presents an extra-fœtal degenerated portion and a persistent portion about ½ to 1 inch only in length, buried in the cutaneous ring of the umbilical region.
If all the changes indicated occur normally and physiologically, the little wound in the region of the umbilicus cicatrises in a perfectly regular way. But unfortunately this is not always the case. At times the cicatrix becomes contaminated by manure, urine or dust, suppurates, and may then become the seat of various complications, such as umbilical phlebitis, omphalitis or persistence of the canal of the urachus.